BIOCHEMISTRY AND PATHOBIOCHEMISTRY OF BLOOD. RESPIRATORY FUNCTION OF ERYTHROCYTES. PATHOLOGICAL FORMS OF HEMOGLOBIN.
Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
Biological functions of the blood
The blood is the most specialized fluid tissue which circulates in vascular system and together with lymph and intercellular space compounds an internal environment of an organism.
The blood executes such functions:
1. Transport of gases – oxygen from lungs is carried to tissues and carbon dioxide from tissues to lungs.
2. Transport of nutrients to all cells of organism (glucose, amino acids, fatty acids, vitamins, ketone bodies, trace substances and others). Substances such as urea, uric acid, bilirubin and creatinine are taken away from the different organs for ultimate excretion.
3. Regulatory or hormonal function – hormones are secreted in to blood and they are transported by blood to their target cells.
4. Thermoregulation function - an exchange of heat between tissues and blood.
5. Osmotic function- sustains osmotic pressure in vessels.
6. Protective function- by the phagocytic action of leucocytes and by the actions of antibodies, the blood provides the most important defense mechanism.
7. Detoxification function - neutralization of toxic substances which is connected with their decomposition by the help of blood enzymes.
Blood performs two major functions:
· transport through the body of
o oxygen and carbon dioxide
o food molecules (glucose, lipids, amino acids)
o ions (e.g., Na+, Ca2+, HCO3−)
o wastes (e.g., urea)
· defense of the body against infections and other foreign materials. All the WBCs participate in these defenses.
All the various types of blood cells
Are produced in the bone marrow (some 1011 of them each day in an adult human!). Arise from a single type of cell called a hematopoietic stem cell — an "adult" multipotent stem cell.
These stem cells
• are very rare (only about one in 10,000 bone marrow cells);
• are attached (probably by adherens junctions) to osteoblasts lining the inner surface of bone cavities;
• express a cell-surface protein designated CD34;
•produce, by mitosis, two kinds of progeny:
o more stem cells (A mouse that has had all its blood stem cells killed by a lethal dose of radiation can be saved by the injection of a single living stem cell!).
o cells that begin to differentiate along the paths leading to the various kinds of blood cells.
Which path is taken is regulated by
• the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones.
Interleukin-7 (IL-7) is the major cytokine in stimulating bone marrow stem cells to start down the path leading to the various lymphocytes (mostly B cells andT
Erythropoietin (EPO), produced by the kidneys, enhances the production of red blood cells (RBCs).
Thrombopoietin (TPO), assisted by Interleukin-11 (IL-11), stimulates the production of megakaryocytes. Their fragmentation produces platelets.
Granulocyte-macrophage colony-stimulating factor (GM-CSF), as its name suggests, sends cells down the path leading to both those cell types. In due course, one path or the other is taken.
Under the influence of granulocyte colony-stimulating factor (G-CSF), they differentiate into neutrophils.
Further stimulated by interleukin-5 (IL-5) they develop into eosinophils.
Interleukin-3 (IL-3) participates in the differentiation of most of the white blood cells but plays a particularly prominent role in the formation of basophils(responsible for some allergies).
Stimulated by macrophage colony-stimulating factor (M-CSF) the granulocyte/macrophage progenitor cells differentiate into monocytes, macrophages, and dendritic cells (DCs).
Biological chemistry of blood cells
Two types of blood cells can be distinguished - white and red blood cells. White blood cells are called leucocytes. Their quantity in adult is 4-9 x 109/L. Red blood cells are called erythrocytes. Their quantity in peripheral blood is 4,5-5 x 1012/L. Besides that, there are also thrombocytes or platelets in blood.
White Blood Cells (leukocytes)
Leucocytes (white blood cells) protect an organism from microorganisms, viruses and foreign substances, that provides the immune status of an organism.
• are much less numerous than red (the ratio between the two is around 1:700),
• have nuclei,
• participate in protecting the body from infection,
• consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules.
Leucocytes are divided into two groups: Granulocytes and agranulocytes. Granulocytes consist of neutrophils, eosinophils and basophils. Agranulocytes consist of monocytes and lymphocytes.
Neutrophils comprise of 60-70 % from all leucocytes. Their main function is to protect organisms from microorganisms and viruses. Neutrophils have segmented nucleus, endoplasmic reticulum (underdeveloped) which does not contain ribosomes, insufficient amount of mitochondria, well-developed Golgi apparatus and hundreds of different vesicles which contain peroxidases and hydrolases. Optimum condition for their activity is acidic pH. There are also small vesicles which contain alkaline phosphatases, lysozymes, lactopherins and proteins of cationic origin.
Glucose is the main source of energy for neutrophils. It is directly utilized or converted into glycogen. 90 % of energy is formed in glycolysis, a small amount of glucose is converted in pentosophosphate pathway. Activation of proteolysis during phagocytosis as well as reduction of phosphatidic acid and phosphoglycerols are also observed. The englobement is accompanied by intensifying of a glycolysis and pentosophosphate pathway. But especially intensity of absorption of oxygen for neutrophils - so-called flashout of respiration grows. Absorbed oxygen is spent for formation of its fissile forms that is carried out with participation enzymes:
1. NADP*Í -OXYDASE catalyzes formation of super oxide anion
2. An enzyme NADH- OXYDASE is responsible for formation of hydrogen peroxide
3. Ěyeloperoxydase catalyzes formation of hypochloric acid from chloride and hydrogen peroxide
Neutrophils are motile phagocyte cells that play a key role in acute inflammation. When bacteria enter tissues, a number of phenomena occur that are collectively known as acute inflammatory response. When neutrophils and other phagocyte cells engulf bacteria, they exhibit a rapid increase in oxygen consumption known as the respiratory burst. This phenomenon reflects the rapid utilization of oxygen (following a lag of 15-60 seconds) and production from it of large amounts of reactive derivates, such as O2-, H2O2, OH. and OCl- (hypochlorite ion). Some of these products are potent microbicidal agents. The electron transport chain system responsible for the respiratory burst contains several components, including a flavoprotein NADPH:O2-oxidoreductase (often called NADPH-oxidase) and a b-type cytochrome.
This scanning electron micrograph (courtesy of Dr. Marion J. Barnhart) shows the characteristic biconcave shape of red blood cells.
Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded intobile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.
Red blood cells are responsible for the transport of oxygen and carbon dioxide.
In adult humans the hemoglobin (Hb) molecule
• consists of four polypeptides:
o two alpha (α) chains of 141 amino acids and
o two beta (β) chains of 146 amino acids
• Each of these is attached the prosthetic group heme.
• There is one atom of iron at the center of each heme.
• One molecule of oxygen can bind to each heme.
The reaction is reversible.
• Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
• Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.
Carbon Dioxide Transport
95% of the CO2 generated in the tissues is carried in the red blood cells:
• It probably enters (and leaves) the cell by diffusing through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is theD antigen that is the most important factor in the Rh system of blood groups.)
• Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen).
• The rest is converted — following the equation above — by the enzyme carbonic anhydrase into
o bicarbonate ions that diffuse back out into the plasma and
o hydrogen ions (H+) that bind to the protein portion of the hemoglobin (thus having no effect on pH).
Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)
Anemia is a shortage of
• RBCs and/or
• the amount of hemoglobin in them.
Red blood cells have surface antigens that differ between people and that create the so-called blood groups such as the ABO system and the Rh system.
1 is a model of human deoxyhemoglobin.
It was created in RasMol version 2.6 by Roger Sayle using the pdb
coordinates from the pdb file 4hhb. The
3D coordinates were determed from x-ray crystallography by Fermi, G., Perutz,
M. F., Shaanan, B., Fourme, R.: The crystal structure of human deoxyhaemoglobin
Hemoglobin is the protein that carries oxygen from the lungs to the tissues and carries carbon dioxide from the tissues back to the lungs. In order to function most efficiently, hemoglobin needs to bind to oxygen tightly in the oxygen-rich atmosphere of the lungs and be able to release oxygen rapidly in the relatively oxygen-poor environment of the tissues. It does this in a most elegant and intricately coordinated way. The story of hemoglobin is the prototype example of the relationship between structure and function of a protein molecule.
A hemoglobin molecule consists of four polypeptide chains: two alpha chains, each with 141 amino acids and two beta chains, each with 146 amino acids. The protein portion of each of these chains is called "globin". The a and b globin chains are very similar in structure. In this case, a and b refer to the two types of globin. Students often confuse this with the concept of a helix and b sheet secondary structures. But, in fact, both the a and b globin chains contain primarily a helix secondary structure with no b sheets.
Figure 2 is a close up view of one of the heme groups of the human a chain from dexoyhemoglobin. In this view, the iron is coordinated by a histidine side chain from amino acid 87 (shown in green.)
Each a or b globin chain folds into
A heme group is a flat ring molecule containing
carbon, nitrogen and hydrogen atoms, with a single Fe2+ ion at the center.
Without the iron, the ring is called a porphyrin. In a heme molecule, the iron
is held within the flat plane by four nitrogen ligands from the porphyrin ring.
The iron ion makes a fifth bond to a histidine side chain from one of the
helices that form the heme pocket. This fifth coordination bond is to histidine
The Bohr Effect
The ability of hemoglobin to release oxygen, is affected by pH, CO2 and by the differences in the oxygen-rich environment of the lungs and the oxygen-poor environment of the tissues. The pH in the tissues is considerably lower (more acidic) than in the lungs. Protons are generated from the reaction between carbon dioxide and water to form bicarbonate:
CO2 + H20 -----------------> HCO3- + H+
This increased acidity serves a twofold purpose. First, protons lower the affinity of hemoglobin for oxygen, allowing easier release into the tissues. As all four oxygens are released, hemoglobin binds to two protons. This helps to maintain equilibrium towards the right side of the equation. This is known as the Bohr effect, and is vital in the removal of carbon dioxide as waste because CO2 is insoluble in the bloodstream. The bicarbonate ion is much more soluble, and can thereby be transported back to the lungs after being bound to hemoglobin. If hemoglobin couldn’t absorb the excess protons, the equilibrium would shift to the left, and carbon dioxide couldn’t be removed.
In the lungs, this effect works in the reverse direction. In the presence of the high oxygen concentration in the lungs, the proton affinity decreases. As protons are shed, the reaction is driven to the left, and CO2 forms as an insoluble gas to be expelled from the lungs. The proton poor hemoglobin now has a greater affinity for oxygen, and the cycle continues.
Haemoglobin or hemoglobin (frequently abbreviated as Hb or Hgb) is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebratesand other animals; in mammals the protein makes up about 97% of the red cell’s dry content, and around 35% of the total content including water. Hemoglobin transports oxygen from the lungs or gills to the rest of the body, such as to the muscles, where it releases the oxygen load. Hemoglobin also has a variety of other gas-transport and effect-modulation duties, which vary from species to species, and which in invertebrates may be quite diverse.
The name hemoglobin is the concatenation of heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme (or haem) group; each heme group contains an iron atom, and this is responsible for the binding of oxygen. The most common type of hemoglobin in mammals contains four such subunits, each with one heme group.
Mutations in the genes for the hemoglobin protein in humans result in a group of hereditary diseases termed the hemoglobinopathies, the most common members of which are sickle-cell disease and thalassemia. Historically in human medicine, the hemoglobinopathy of sickle-cell disease was the first disease to be understood in its mechanism of dysfunction, completely down to the molecular level. However, not all of such mutations produce disease states, and are formally recognized as hemoglobin variants (not diseases).
Hemoglobin (Hb) is synthesized in a complex series of steps. The heme portion is sythesized in both the the mitochondria and cytosol of the immature red blood cell, while the globin protein portions of the molecule are sythesized by ribosomes in the cytosol . Production of Hb continues in the cell throughout its early development from theproerythroblast to the reticulocyte in the bone marrow. At this point, the nucleus is lost in mammals, but not in birds and many other species. Even after the loss of the nucleus in mammals, however, residual ribosomal RNA allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the vasculature (this hemoglobin-synthetic RNA in fact gives the reticulocyte its reticulated appearance and name).
The empirical chemical formula of the most common human hemoglobin is C2952H4664N812O832S8Fe4, but as noted above, hemoglobins vary widely across species, and even (through common mutations) slightly among subgroups of humans.
In humans, the hemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-proteinheme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement, so called because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin. This folding pattern contains a pocket which strongly binds the heme group.
A heme group consists of an iron (Fe) atom held in a heterocyclic ring, known as a porphyrin. The iron atom, which is the site of oxygen binding, bonds with the fournitrogens in the center of the ring, which all lie in one plane. The iron is also bound strongly to the globular protein via the imidazole ring of a histidine residue below the porphyrin ring. A sixth position can reversibly bind oxygen, completing the octahedral group of six ligands. Oxygen binds in an "end-on bent" geometry where one oxygen atom binds Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.
The iron atom may either be in the Fe2+ or Fe3+ state, but ferrihemoglobin (methemoglobin) (Fe3+) cannot bind oxygen. In binding, oxygen temporarily oxidizes Fe to (Fe3+), so iron must exist in the +2 oxidation state in order to bind oxygen. The body reactivates hemoglobin found in the inactive (Fe3+) state by reducing the iron center.
In adult humans, the most common hemoglobin type is a tetramer (which contains 4 subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as α2β2. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 17,000 daltons, for a total molecular weight of the tetramer of about 68,000 daltons. Hemoglobin A is the most intensively studied of the hemoglobin molecules.
The four polypeptide chains are bound to each other by salt bridges, hydrogen bonds, and hydrophobic interactions. There are two kinds of contacts between the α and β chains: α1β1 and α1β2.
Oxyhemoglobin is formed during respiration when oxygen binds to the heme component of the protein hemoglobin in red blood cells. This process occurs in the pulmonary capillaries adjacent to the alveoli of the lungs. The oxygen then travels through the blood stream to be dropped off at cells where it is utilized in aerobic glycolysis and in the production of ATP by the process of oxidative phosphorylation. It doesn't however help to counteract a decrease in blood pH. Ventilation, or breathing, may reverse this condition by removal of carbon dioxide, thus causing a shift up in pH.
Deoxyhemoglobin is the form of hemoglobin without the bound oxygen. The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ. The oxyhemoglobine has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher. This difference is used for measurement of the amount of oxygen in patient's blood by an instrument called pulse oximeter.
Iron's oxidation state in oxyhemoglobin
The oxidation state of iron in hemoglobin is always +2. It does not change when oxygen binds to the deoxy- form.
Assigning oxygenated hemoglobin's oxidation state is difficult because oxyhemoglobin is diamagnetic (no net unpaired electrons), but the low-energy electron configurations in both oxygen and iron are paramagnetic. Triplet oxygen, the lowest energy oxygen species, has two unpaired electrons in antibonding π* molecular orbitals. Iron(II) tends to be in a high-spin configuration where unpaired electrons exist in eg antibonding orbitals. Iron(III) has an odd number of electrons and necessarily has unpaired electrons. All of these molecules are paramagnetic (have unpaired electrons), not diamagnetic, so an unintuitive distribution of electrons must exist to induce diamagnetism.
The three logical possibilities are:
1) Low-spin Fe2+ binds to high-energy singlet oxygen. Both low-spin iron and singlet oxygen are diamagnetic.
2) High-spin Fe3+ binds to .O2- (the superoxide ion) and antiferromagnetism oppositely aligns the two unpaired electrons, giving diamagnetic properties.
3) Low-spin Fe4+ binds to O22-. Both are diamagnetic.
X-ray photoelectron spectroscopy suggests that iron has an oxidation state of approximately 3.2 and infrared stretching frequencies of the O-O bond suggests a bond length fitting with superoxide. The correct oxidation state of iron is thus the +3 state with oxygen in the -1 state. The diamagnetism in this configuration arises from the unpaired electron on superoxide aligning antiferromagnetically in the opposite direction from the unpaired electron on iron. The second choice being correct is not surprising because singlet oxygen and large separations of charge are both unfavorably high-energy states. Iron's shift to a higher oxidation state decreases the atom's size and allows it into the plane of the porphyrin ring, pulling on the coordinated histidine residue and initiating the allosteric changes seen in the globulins. The assignment of oxidation state, however, is only a formalism so all three models may contribute to some small degree.
Early postulates by bioinorganic chemists claimed that possibility (1) (above) was correct and that iron should exist in oxidation state II (indeed iron oxidation state III as methemoglobin, when not accompanied by superoxide .O2- to "hold" the oxidation electron, is incapable of binding O2). The iron chemistry in this model was elegant, but the presence of singlet oxygen was never explained. It was argued that the binding of an oxygen molecule placed high-spin iron(II) in an octahedral field of strong-field ligands; this change in field would increase the crystal field splitting energy, causing iron's electrons to pair into the diamagnetic low-spin configuration.
Binding of ligands
Binding and release of ligands induces a conformational (structural) change in hemoglobin. Here, the binding and release of oxygen illustrates the structural differences between oxy- and deoxyhemoglobin, respectively. Only one of the four heme groups is shown.
As discussed above, when oxygen binds to the iron center it causes contraction of the iron atom, and causes it to move back into the center of the porphyrin ring plane (see moving diagram). At the same time, the porphyrin ring plane itself is pushed away from the oxygen and toward the imidizole side chain of the histidine residue interacting at the other pole of the iron. The interaction here forces the ring plane sideways toward the outside of the tetramer, and also induces a strain on the protein helix containing the histidine, as it moves nearer the iron. This causes a tug on this peptide strand which tends to open up heme units in the remainder of the molecule, so that there is more room for oxygen to bind at their heme sites.
In the tetrameric form of normal adult hemoglobin, the binding of oxygen is thus a cooperative process. The binding affinity of hemoglobin for oxygen is increased by the oxygen saturation of the molecule, with the first oxygens bound influencing the shape of the binding sites for the next oxygens, in a way favorable for binding. This positive cooperative binding is achieved through steric conformational changes of the hemoglobin protein complex as discussed above, i.e. when one subunit protein in hemoglobin becomes oxygenated, this induces a conformational or structural change in the whole complex, causing the other subunits to gain an increased affinity for oxygen. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.
Hemoglobin's oxygen-binding capacity is decreased in the presence of carbon monoxide because both gases compete for the same binding sites on hemoglobin, carbon monoxide binding preferentially in place of oxygen. Carbon dioxide occupies a different binding site on the hemoglobin. Through the enzyme carbonic anhydrase, carbon dioxide reacts with water to give carbonic acid, which decomposes into bicarbonate and protons:
The sigmoidal shape of hemoglobin's oxygen-dissociation curve results from cooperative binding of oxygen to hemoglobin.
Hence blood with high carbon dioxide levels is also lower in pH (more acidic). Hemoglobin can bind protons and carbon dioxide which causes a conformational change in the protein and facilitates the release of oxygen. Protons bind at various places along the protein, and carbon dioxide binds at the alpha-amino group forming carbamate. Conversely, when the carbon dioxide levels in the blood decrease (i.e., in the lung capillaries), carbon dioxide and protons are released from hemoglobin, increasing the oxygen affinity of the protein. This control of hemoglobin's affinity for oxygen by the binding and release of carbon dioxide and acid, is known as the Bohr effect.
The binding of oxygen is affected by molecules such as carbon monoxide (CO) (for example from tobacco smoking, cars and furnaces). CO competes with oxygen at the heme binding site. Hemoglobin binding affinity for CO is 200 times greater than its affinity for oxygen, meaning that small amounts of CO dramatically reduces hemoglobin's ability to transport oxygen. When hemoglobin combines with CO, it forms a very bright red compound called carboxyhemoglobin. When inspired air contains CO levels as low as 0.02%, headache and nausea occur; if the CO concentration is increased to 0.1%, unconsciousness will follow. In heavy smokers, up to 20% of the oxygen-active sites can be blocked by CO.
In similar fashion, hemoglobin also has competitive binding affinity for cyanide (CN-), sulfur monoxide (SO), nitrogen dioxide (NO2), and sulfide (S2-), including hydrogen sulfide (H2S). All of these bind to iron in heme without changing its oxidation state, but they nevertheless inhibit oxygen-binding, causing grave toxicity.
The iron atom in the heme group must be in the Fe2+ oxidation state to support oxygen and other gases' binding and transport. Oxidation to Fe3+ state converts hemoglobin into hemiglobin or methemoglobin (pronounced "MET-hemoglobin"), which cannot bind oxygen. Hemoglobin in normal red blood cells is protected by a reduction system to keep this from happening. Nitrogen dioxide and nitrous oxide are capable of converting a small fraction of hemoglobin to methemoglobin, however this is not usually of medical importance (nitrogen dioxide is poisonous by other mechanisms, and nitrous oxide is routinely used in surgical anesthesia in most people without undue methemoglobin buildup).
In people acclimated to high altitudes, the concentration of 2,3-bisphosphoglycerate (2,3-BPG) in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule Z, is called a heterotropic allosteric effect.
A variant hemoglobin, called fetal hemoglobin (HbF, α2γ2), is found in the developing fetus, and binds oxygen with greater affinity than adult hemoglobin. This means that the oxygen binding curve for fetal hemoglobin is left-shifted (i.e., a higher percentage of hemoglobin has oxygen bound to it at lower oxygen tension), in comparison to that of adult hemoglobin. As a result, fetal blood in the placenta is able to take oxygen from maternal blood.
Hemoglobin also carries nitric oxide in the globin part of the molecule. This improves oxygen delivery in the periphery and contributes to the control of respiration. NO binds reversibly to a specific cystein residue in globin; the binding depends on the state (R or T) of the hemoglobin. The resulting S-nitrosylated hemoglobin influences various NO-related activities such as the control of vascular resistance, blood pressure and respiration. NO is released not in the cytoplasm of erythrocytes but is transported by an anion exchanger called AE1 out of them.
Degradation of hemoglobin in vertebrate animals
When red cells reach the end of their life due to aging or defects, they are broken down, the hemoglobin molecule is broken up and the iron gets recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. This process also produces one molecule of carbon monoxide for every molecule of heme degraded ; this is one of the few natural sources of carbon monoxide production in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage
Role in disease
Decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiencyand its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency anemia are hypochromic (lacking the red hemoglobin pigment) and microcytic (smaller than normal). Other anemias are rarer. In hemolysis (accelerated breakdown of red blood cells), associated jaundice is caused by the hemoglobin metabolite bilirubin, and the circulating hemoglobin can cause renal failure.
Some mutations in the globin chain are associated with the hemoglobinopathies, such as sickle-cell disease and thalassemia. Other mutations, as discussed at the beginning of the article, are benign and are referred to merely as hemoglobin variants.
There is a group of genetic disorders, known as the porphyrias that are
characterized by errors in metabolic pathways of heme synthesis. King George
III of the
To a small extent, hemoglobin A slowly combines with glucose at a certain location in the molecule. The resulting molecule is often referred to as Hb A1c. As theconcentration of glucose in the blood increases, the percentage of Hb A that turns into Hb A1c increases. In diabetics whose glucose usually runs high, the percent Hb A1c also runs high. Because of the slow rate of Hb A combination with glucose, the Hb A1c percentage is representative of glucose level in the blood averaged over a longer time (the half-life of red blood cells, which is typically 50-55 days).
Hemoglobin levels are amongst the most commonly performed blood tests, usually as part of a full blood count or complete blood count. Results are reported in g/L, g/dLor mol/L. For conversion, 1 g/dL is 0.621 mmol/L. If the total hemoglobin concentration in the blood falls below a set point, this is called anemia. Normal values for hemoglobin levels are:
• Women: 12.1 to 15.1 g/dl
• Men: 13.8 to 17.2 g/dl
• Children: 11 to 16 g/dl
• Pregnant women: 11 to 12 g/dl
Anemias are further subclassified by the size of the red blood cells, which are the cells which contain hemoglobin in vertebrates. They can be classified as microcytic (small sized red blood cells), normocytic (normal sized red blood cells), or macrocytic (large sized red blood cells). The hemaglobin is the typical test used for blood donation. A comparison with the hematocrit can be made by multiplying the hemaglobin by three. For example, if the hemaglobin is measured at 17, that compares with a hematocrit of .51
Glucose levels in blood can vary widely each hour, so one or only a few samples from a patient analyzed for glucose may not be representative of glucose control in the long run. For this reason a blood sample may be analyzed for Hb A1c level, which is more representative of glucose control averaged over a longer time period (determined by the half-life of the individual's red blood cells, which is typically 50-55 days). People whose Hb A1c runs 6.0% or less show good longer-term glucose control. Hb A1c values which are more than 7.0% are elevated. This test is especially useful for diabetics.
BIOLOGICAL BUFFERS OF BLOOD
Acid–base homeostasis is the part of human homeostasis concerning the proper balance between acids and bases, also called body pH. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it. Outside the acceptable range of pH, proteins are denatured and digested, enzymes lose their ability to function, and death may occur.
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.
Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. One example of a buffer solution found in nature is blood. The body's acid–base balance is normally tightly regulated, keeping the arterial blood pH between 7.38 and 7.42. Several buffering agents that reversibly bind hydrogen ions and impede any change in pH exist. Extracellular buffers include bicarbonate and ammonia, whereas proteins and phosphate act as intracellular buffers. The bicarbonate buffering system is especially key, as carbon dioxide (CO2) can be shifted through carbonic acid (H2CO3) to hydrogen ions and bicarbonate (HCO3-):
Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier's principle, which in turn alters the pH.
The kidneys are slower to compensate, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In response to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer. In responses to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonium excretion.
Buffering hides from view the real change in H+ that occurs.
This huge buffer capacity has another not immediately obvious implication for how we think about the severity of an acid-base disorder. You would think that the magnitude of an acid-base disturbance could be quantified merely by looking at the change in [H+] - BUT this is not so.
Because of the large buffering capacity, the actual change in [H+] is so small it can be ignored in any quantitative assessment, and instead, the magnitude of a disorder has to be estimated indirectly from the decrease in the total concentration of the anions involved in the buffering. The buffer anions, represented as A-, decrease because they combine stoichiometrically with H+ to produce HA. A decrease in A- by 1 mmol/l represents a 1,000,000 nano-mol/l amount of H+ that is hidden from view and this is several orders of magnitude higher than the visible few nanomoles/l change in [H+] that is visible.) - As noted above in the comments about the Swan & Pitts experiment, 13,999,994 out of 14,000,000 nano-moles/l of H+ were hidden on buffers and just to count the 36 that were on view would give a false impression of the magnitude of the disorder.
The three pKa values are sufficiently different so that at any one pH only the members of a single conjugate pair are present in significant concentrations.
At the prevailing pH values in most biological systems, monohydrogen phosphate (HPO4-2) and dihydrogen phosphate (H2PO4-) are the two species present. The pKa2 is 6.8 and this makes the closed phosphate buffer system a good buffer intracellularly and in urine. The pH of glomerular ultrafiltrate is 7.4 and this means that phosphate will initially be predominantly in the monohydrogen form and so can combine with more H+ in the renal tubules. This makes the phosphate buffer more effective in buffering against a drop in pH than a rise in pH.
Note: The ‘true’ pKa2 value is actually 7.2 if measured at zero ionic strength but at the typical ionic strength found in the body its apparent value is 6.8. The other factor which makes phosphate a more effective buffer intracellularly and in urine is that its concentration is much higher here than in extracellular fluid.
Haemoglobin is an important blood buffer particularly for buffering CO2
Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l). Buffering is by the imidazole group of the histidine residues which has a pKa of about 6.8. This is suitable for effective buffering at physiological pH. Haemoglobin is quantitatively about 6 times more important then the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule. For example if blood pH changed from 7.5 to 6.5, haemoglobin would buffer 27.5 mmol/l of H+ and total plasma protein buffering would account for only 4.2 mmol/l of H+.
Deoxyhaemoglobin is a more effective buffer than oxyhaemoglobin and this change in buffer capacity contributes about 30% of the Haldane effect. The major factor accounting for the Haldane effect in CO2 transport is the much greater ability of deoxyhaemoglobin to form carbamino compounds.
This buffer functions in exactly the same way as the phosphate buffer.
Additional H+ is consumed by HCO3- and additional
The concentrations of hydrogen carbonate ions and of carbonic acid are controlled by two independent physiological systems. Carbonic acid concentration is controlled by respiration, that is through the lungs. Carbonic acid is in equilibrium with dissolved carbon dioxide gas.
H2CO3(aq) CO2(aq) + H2O(l)
An enzyme called carbonic anhydrase catalyzes the conversion of carbonic acid to dissolved carbon dioxide. In the lungs, excess dissolved carbon dioxide is exhaled as carbon dioxide gas.
The concentration of hydrogen carbonate ions is controlled through the kidneys. Excess hydrogen carbonate ions are excreted in the urine.
The much higher concentration of hydrogen carbonate ion over that of carbonic acid in blood plasma allows the buffer to respond effectively to the most common materials that are released into the blood. Normal metabolism releases mainly acidic materials: carboxylic acids such as lactic acid (HLac). These acids react with hydrogen carbonate ion and form carbonic acid.
HLac(aq) + HCO3-(aq) Lac-(aq) + H2CO3(aq)
The carbonic acid is converted through the action of the enzyme carbonic anhydrase into aqueous carbon dioxide.
H2CO3(aq) CO2(aq) + H2O(l)
An increase in CO2(aq) concentration stimulates increased breathing, and the excess carbon dioxide is released into the air in the lungs.
The condition called respiratory acidosis occurs when blood pH falls as a result of decreased respiration. When respiration is restricted, the concentration of dissolved carbon dioxide in the blood increases, making the blood too acidic. Such a condition can be produced by asthma, pneumonia, emphysema, or inhaling smoke.
Metabolic acidosis is the decrease in blood pH that results when excessive amounts of acidic substances are released into the blood. This can happen through prolonged physical exertion, by diabetes, or restricted food intake. The normal body response to this condition is increases breathing to reduce the amount of dissolved carbon dioxide in the blood. This is why we breathe more heavily after climbing several flights of stairs.
Respiratory alkalosis results from excessive breathing that produces an increase in blood pH. Hyperventilation causes too much dissolved carbon dioxide to be removed from the blood, which decreases the carbonic acid concentration, which raises the blood pH. Often, the body of a hyperventilating person will react by fainting, which slows the breathing.
Metabolic alkalosis is an increase in blood pH resulting from the release of alkaline materials into the blood. This can result from the ingestion of alkaline materials, and through overuse of diuretics. Again, the body usually responds to this condition by slowing breathing, possibly through fainting.
The carbonic acid-hydrogen carbonate ion buffer works throughout the body to maintain the pH of blood plasma close to 7.40. The body maintains the buffer by eliminating either the acid (carbonic acid) or the base (hydrogen carbonate ions). Changes in carbonic acid concentration can be effected within seconds through increased or decreased respiration. Changes in hydrogen carbonate ion concentration, however, require hours through the relatively slow elimination through the kidneys
Plasma transports materials needed by cells and materials that must be removed from cells:
• various ions (Na+, Ca2+, HCO3−, etc.
• glucose and traces of other sugars
• amino acids
• other organic acids
• cholesterol and other lipids
• urea and other wastes
Most of these materials are in transit from a place where they are added to the blood (a "source")
• exchange organs like the intestine
• depots of materials like the liver
to places ("sinks") where they will be removed from the blood.
• every cell
• exchange organs like the kidney, and skin.
Proteins make up 6–8% of the blood. They are about equally divided between serum albumin and a great variety of serum globulins.
After blood is withdrawn from a vein and allowed to clot, the clot slowly shrinks. As it does so, a clear fluid called serum is squeezed out. Thus:
Serum is blood plasma without fibrinogen and other clotting factors.
The serum proteins can be separated by electrophoresis.
• A drop of serum is applied in a band to a thin sheet of supporting material, like paper, that has been soaked in a slightly-alkaline salt solution.
• At pH 8.6, which is commonly used, all the proteins are negatively charged, but some more strongly than others.
• A direct current can flow through the paper because of the conductivity of the buffer with which it is moistened.
• As the current flows, the serum proteins move toward the positive electrode.
• The stronger the negative charge on a protein, the faster it migrates.
• After a time (typically 20 min), the current is turned off and the proteins stained to make them visible (most are otherwise colorless).
• The separated proteins appear as distinct bands.
• The most prominent of these and the one that moves closest to the positive electrode is serum albumin.
• Serum albumin
o is made in the liver
o binds many small molecules for transport through the blood
o helps maintain the osmotic pressure of the blood
• The other proteins are the various serum globulins.
• They migrate in the order
o alpha globulins (e.g., the proteins that transport thyroxine and retinol [vitamin A])
o beta globulins (e.g., the iron-transporting protein transferrin)
o gamma globulins.
§ Gamma globulins are the least negatively-charged serum proteins. (They are so weakly charged, in fact, that some are swept in the flow of buffer back toward the negative electrode.)
§ Most antibodies are gamma globulins.
§ Therefore gamma globulins become more abundant following infections or immunizations.
Albumins – multidispersed fraction of blood plasma which are characterized by the high electrophoretic mobility and mild dissolubility in water and saline solutions. Molecular weight of albumins is about 60000. Due to high hydrophilic properties albumins bind a significant amount of water, and the volume of their molecule under hydratation is doubled. Hydrative layer formed around the serum albumins provides to 70-80 % of oncotic pressure of blood plasma proteins, that can be applied in clinical practice at albumins transfusion to patients with tissue edemas. The decreasing of albumins concentration in blood plasma, for example under disturbance of their synthesis in hepatocytes at liverfailure, can cause the water transition from a vessels into the tissues and development of oncotic edemas.
Albumins execute also important physiological function as transporters of a lot of metabolites and diverse low molecular weight structures. The molecules of albumins have several sites with centers of linkage for molecules of organic ligands, which are affixed by the electrostatic and hydrophobic bonds. Serum albumins can affix and convey fatty acids, cholesterol, cholic pigments (bilirubin and that similar), vitamins, hormones, some amino acids, toxins and medicines.
Albumins also execute the buffer function. Due to the availability in their structure amino and carboxylic groups albumins can react both as acids and as alkaline.
Albumins can bound different toxins in blood plasma (bilirubin, foreign substances et c.). This is the desintoxicative function of albumins.
Albumins also play role of amino acids depot in the organism. They can supply amino acids for the building of another proteins, for example enzymes.
Globulins - heterogeneous fraction of blood proteins which execute transport (a1-globulins – transport of lipids, thyroxin, corticosteroid hormones; a2-globulins - transport of lipids, copper ions; b-globulins - transport of lipids, iron) and protective (participation of b-globulins in immune reactions as antitoxins; g-globulins as immunoglobulins) functions. They also support the blood oncotic pressure and acid-alkaline balance, provide amino acids for the organism requirements. The molecular weight of globulins is approximately 150000-300000.
The globulin level in blood plasma is 20-40 g/l. A ratio between concentrations of albumins and globulins (so called “protein coefficient”) in blood plasma is often determined in clinical practice. In healthy people this coefficient is 1,5-2,0.
Fibrinogen – important protein of blood plasma, precursor of fibrin, the structural element of blood clots. Fibrinogen participates in blood clotting and thus prevents the loss of blood from the vascular system of vertebrates. The approximate molecular weight of fibrinogen is 340000. It is the complex protein, it contains the carbohydrate as prosthetic group. The content of firinogen in blood is 3-4 g/l.
Subfractions of a1, a2, b and g globulins, their structure and functions.
Immunoglobulins (Ig A, Ig G, Ig E, Ig M) - proteins of g-globulin fraction of blood plasma executing the functions of antibodies which are the main effectors of humoral immunity. They appear in the blood serum and certain cells of a vertebrate in response to the introduction of a protein or some other macromolecule foreign to that species.
Immunoglobulin molecules have bindind sites that are specific for and complementary to the structural features of the antigen that induced their formation. Antibodies are highly specific for the foreign proteins that evoke their formation.
Molecules of immunoglobulins are glycoproteins. The protein part of immunoglobulins contain four polipeptide chains: two heavy H-chains and two light L-chains.
The acute phase response develops in a wide range of acute and chronic inflammatory conditions like bacterial, viral, or fungal infections; rheumatic and other inflammatory diseases; malignancy; and tissue injury or necrosis. These conditions cause release of interleukin-6 and other cytokines that trigger the synthesis of CRP and fibrinogen by the liver. During the acute phase response, levels of CRP rapidly increase within 2 hours of acute insult, reaching a peak at 48 hours. With resolution of the acute phase response, CRP declines with a relatively short half-life of 18 hours. Measuring CRP level is a screen for infectious and inflammatory diseases. Rapid, marked increases in CRP occur with inflammation, infection, trauma and tissue necrosis, malignancies, and autoimmune disorders. Because there are a large number of disparate conditions that can increase CRP production, an elevated CRP level does not diagnose a specific disease. An elevated CRP level can provide support for the presence of an inflammatory disease, such as rheumatoid arthritis, polymyalgia rheumatica or giant-cell arteritis.
The physiological role of CRP is to bind to phosphocholine expressed on the surface of dead or dying cells (and some types of bacteria) in order to activate the complement system. CRP binds to phosphocholine on microbes and damaged cells and enhances phagocytosis by macrophages. Thus, CRP participates in the clearance of necrotic and apoptotic cells.
CRP is a member of the class of acute-phase reactants, as its levels rise dramatically during inflammatory processes occurring in the body. This increment is due to a rise in the plasma concentration of IL-6, which is produced predominantly by macrophages as well asadipocytes. CRP binds to phosphocholine on microbes. It is thought to assist in complement binding to foreign and damaged cells and enhances phagocytosis by macrophages (opsonin mediated phagocytosis), which express a receptor for CRP. It is also believed to play another important role in innate immunity, as an early defense system against infections.
CRP rises up to 50,000-fold in acute inflammation, such as infection. It rises above normal limits within 6 hours, and peaks at 48 hours. Its half-life is constant, and therefore its level is mainly determined by the rate of production (and hence the severity of the precipitating cause).
Serum amyloid A is a related acute-phase marker that responds rapidly in similar circumstances.
In blood plasma of healthy people the C-reactive protein is absent but it occurs at pathological states accompanied by an inflammation and necrosis of tissues. The availability of C-reactive protein is characteristic for the acute period of diseases – “protein of an acute phase”. The determination of C-reactive protein has diagnostic value in an acute phase of rheumatic disease, at a myocardial infarction, pneumococcal, streptococcal, staphylococcal infections.
CRP is used mainly as a marker of inflammation. Apart from liver failure, there are few known factors that interfere with CRP production.
Measuring and charting CRP values can prove useful in determining disease progress or the effectiveness of treatments. Blood, usually collected in a serum-separating tube, is analysed in amedical laboratory or at the point of care. Various analytical methods are available for CRP determination, such as ELISA, immunoturbidimetry, rapid immunodiffusion, and visual agglutination.
Reference ranges for blood tests, showing C-reactive protein in brown-yellow in center.
A high-sensitivity CRP (hs-CRP) test measures low levels of CRP using laser nephelometry. The test gives results in 25 minutes with a sensitivity down to 0.04 mg/L.
Normal concentration in healthy human serum is usually lower than 10 mg/L, slightly increasing with aging. Higher levels are found in late pregnant women, mild inflammation and viral infections (10–40 mg/L), active inflammation, bacterial infection (40–200 mg/L), severe bacterial infections and burns (>200 mg/L).
CRP is a more sensitive and accurate reflection of the acute phase response than the ESR (Erythrocyte Sedimentation Rate). The half-life of CRP is constant. Therefore, CRP level is mainly determined by the rate of production (and hence the severity of the precipitating cause). In the first 24 h, ESR may be normal and CRP elevated. CRP returns to normal more quickly than ESR in response to therapy.
Cardiology diagnostic test
Arterial damage results from white blood cell invasion and inflammation within the wall. CRP is a general marker for inflammation and infection, so it can be used as a very rough proxy for heart disease risk. Since many things can cause elevated CRP, this is not a very specific prognostic indicator. Nevertheless, a level above 2.4 mg/L has been associated with a doubled risk of a coronary event compared to levels below 1 mg/L; however, the study group in this case consisted of patients who had been diagnosed with unstable angina pectoris; whether elevated CRP has any predictive value of acute coronary events in the general population of all age ranges remains unclear.
Crioglobulin - the protein of the g-globulin fraction. Like to the C-reactive protein crioglobulin absent in blood plasma of the healthy people and occurs at leukoses, rheumatic disease, liver cirrhosis, nephroses. The characteristic physico-chemical feature of crioglobulin is its dissolubility at standard body temperature (37 oC) and capacity to form the sediment at cooling of a blood plasma up to 4 oC.
a2-macroglobulin - protein of a2-globulin fraction, universal serum proteinase inhibitor. Its contents (2,5 g/l) in blood plasma is highest comparing to another proteinase inhibitors.
The biological role of a2-macroglobulin consists in regulation of the tissue proteolysis systems which are very important in such physiological and pathological processes as blood clotting, fibrinolysis, processes of immunodefence, functionality of a complement system, inflammation, regulation of vascular tone (kinine and renin-angiothensine system).
a1-antitrypsin (a1-globulin) – glycoprotein with a molecular weight 55 kDa. Its concentration in blood plasma is 2-3 ă/ë. The main biological property of this inhibitor is its capacity to form complexes with proteinases oppressing proteolitic activity of such enzymes as trypsin, chemotrypsin, plasmin, trombin. The content of a1-antitrypsin is markedly increased in inflammatory processes. The inhibitory activity of a1-antitrypsin is very important in pancreas necrosis and acute pancreatitis because in these conditions the proteinase level in blood and tissues is sharply increased. The congenital deficiency of a1-antitrypsin results in the lung emphysema.
Fibronectin – glycoprotein of blood plasma that is synthesized and secreted in intercellular space by different cells. Fibronectin present on a surface of cells, on the basalmembranes, in connective tissue and in blood. Fibronectin has properties of a «sticking» protein and contacts with the carbohydrate groups of gangliosides on a surface of plasma membranes executing the integrative function in intercellular interplay. Fibronectin also plays important role in the formation of the pericellular matrix.
Haptoglobin - protein of a2-globulin fraction of blood plasma. Haptoglobin has capacity to bind a free haemoglobin forming a complex that refer to b-globulins electrophoretic fraction. Normal concentration in blood plasma - 0,10-0,35 g/l.
Haptoglobin-hemoglobin complexes are absorbed by the cells of reticulo-endothelial system, in particular in a liver, and oxidized to cholic pigments. Such haptoglobinfunction promotes the preservation of iron ions in an organism under conditions of a physiological and pathological erythrocytolysis.
Transferrin - glycoprotein belonging to the b-globulin fraction. It binds in a blood plasma iron ions (Fe3+). The protein has on the surface two centers of linkage of iron.Transferrin is a transport form of iron delivering its to places of accumulation and usage.
Ceruloplasmin - glycoprotein of the a2-globulin fraction. It can bind the copper ions in blood plasma. Up to 3 % of all copper contents in an organism and more than 90 % copper contents in plasma is included in ceruloplasmin. Ceruloplasmin has properties of ferroxidase oxidizing the iron ions. The decrease of ceruloplasmin in organism (Wilson disease) results in exit of copper ions from vessels and its accumulation in the connective tissue that shows by pathological changes in a liver, main brain, cornea.
The place of synthesis of each fraction and subfruction of blood plasma proteins.
Albumins, a1-globulins, fibrinogen are fully synthesized in hepatocytes. Immunoglobulins are produced by plasmocytes (immune cells). In liver cryoglobulins and some other g-globulins are produced too. a2-globulins and b-globulins are partly synthesized in liver and partly in reticuloendothelial cells.
Causes and consequences of protein content changes in blood plasma.
Hypoproteinemia - decrease of the total contents of proteins in blood plasma. This state occurs in old people as well as in pathological states accompanying with the oppressing of protein synthesis (liver diseases) and activation of decomposition of tissue proteins (starvation, hard infectious diseases, state after hard trauma and operations, cancer). Hypoproteinemia (hypoalbuminemia) also occurs in kidney diseases, when the increased excretion of proteins via the urine takes place.
Hyperproteinemia - increase of the total contents of proteins in blood plasma. There are two types of hyperproteinemia - absolute and relative.
Absolute hyperproteinemia – accumulation of the proteins in blood. It occurs in infection and inflammatory diseases (hyperproduction of immunoglobulins), rheumatic diseases (hyperproduction of C-reactive protein), some malignant tumors (myeloma) and others.
Relative hyperproteinemia – the increase of the protein concentration but not the absolute amount of proteins. It occurs when organism loses water (diarrhea, vomiting, fever, intensive physical activity etc.).
The principle of the measurement of protein fractions by electrophoresis method.
Electrophoresis is the separation of proteins on the basis of their electric charge. It depends ultimately on their base-acid properties, which are largely determined by the number and types of ionizable R groups in their polipeptide chains. Since proteins differ in amino acid composition and sequence, each protein has distinctive acid-base properties. There are a number of different forms of electroforesis useful for analyzing and separating mixtures of proteins
If a precursor of an antibody-secreting cell becomes cancerous, it divides uncontrollably to generate a clone of plasma cellssecreting a single kind of antibody molecule. The image (courtesy of Beckman Instruments, Inc.) shows — from left to right — the electrophoretic separation of:
1. normal human serum with its diffuse band of gamma globulins;
2. serum from a patient with multiple myeloma producing an IgG myeloma protein;
3. serum from a patient with Waldenström's macroglobulinemia where the cancerous clone secretes an IgM antibody;
4. serum with an IgA myeloma protein.
§ Gamma globulins can be harvested from donated blood (usually pooled from several thousand donors) and injected into persons exposed to certain diseases such as chicken pox and hepatitis. Because such preparations of immune globulin contain antibodies against most common infectious diseases, the patient gains temporary protection against the disease.
Because of their relationship to cardiovascular disease, the analysis of serum lipids has become an important health measure.
The table shows the range of typical values as well as the values above (or below) which the subject may be at increased risk of developing atherosclerosis.
• Total cholesterol is the serum of blood
o HDL cholesterol
o LDL cholesterol and
o 20% of the triglyceride value
• Note that
o high LDL values are bad, but
o high HDL values are good.
• Using the various values, one can calculate a
cardiac risk ratio = total cholesterol divided by HDL cholesterol
A cardiac risk ratio greater than 7 is considered a warning
Residual nitrogen, its components, ways of their formation, blood content
The state of protein nutrition can be determined by measuring the dietary intake and output of nitrogenous compounds from the body. Although nucleic acids also contain nitrogen, protein is the major dietary source of nitrogen and measurement of total nitrogen intake gives a good estimate of protein intake (mg N × 6.25 = mg protein, as nitrogen is 16% of most proteins). The output of nitrogen from the body is mainly in urea and smaller quantities of other compounds in urine and undigested protein in feces, and significant amounts may also be lost in sweat and shed skin.
The difference between intake and output of nitrogenous compounds is known as nitrogen balance. Three states can be defined: In a healthy adult, nitrogen balance is inequilibrium when intake equals output, and there is no change in the total body content of protein. In a growing child, a pregnant woman, or in recovery from protein loss, the excretion of nitrogenous compounds is less than the dietary intake and there is net retention of nitrogen in the body as protein, ie, positive nitrogen balance. In response to trauma or infection or if the intake of protein is inadequate to meet requirements there is net loss of protein nitrogen from the body, ie, negative nitrogen balance. The continual catabolism of tissue proteins creates the requirement for dietary protein even in an adult who is not growing, though some of the amino acids released can be reutilized.
Nitrogen balance studies show that the average
daily requirement is
Residual nitrogen – nonprotein nitrogen, that is nitrogen of organic and inorganic compounds that remain in blood after protein sedimentation.
Organic and inorganic compounds of residual nitrogen are as follows: urea (50 % of the residual nitrogen), amino acids (25 %), creatine and creatinine (7,5 %), salts of ammonia and indicane (0,5 %), other compounds (about 13 %).
Urea is formed in liver during the degradation of amino acids, pyrimidine nucleotides and other nitrogen containing compounds. Amino acids are formed as result of protein decomposition or owing to the conversion of fatty acids or carbohydrates to amino acids. The pool of amino acids in blood is also supported by the process of their absorption in intestine. Creatine is produced in kidneys and liver from amino acids glycine and arginine, creatinine is formed in muscles as result of creatine phosphate splitting. In result of ammonia neutralization the ammonia salts can be formed. Indicane is the product of indol neutralization in the liver.
The content of residual nitrogen in blood is 0,2 – 0,4 g/l.
The pathways of convertion of amino acid nonnitrogen residues.
The removal of the amino group of an amino acid by transamination or oxidative deamination produces an α-keto acid that contains the carbon skeleton from the amino acid (nonnitrogen residues). These α-keto acids can be used for the biosynthesis of non-essential amino acids or undergoes a different degradation process. For alanine and serine, the degradation requires a single step. For most carbon arrangements, however, multistep reaction sequences are required. There are only seven degradation sequences for 20 amino acids. The seven degradation products are pyruvate, acetyl CoA, acetoacetyl CoA, α-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate. The last four products are intermediates in the citric acid cycle. Some amino acids have more than one pathway for degradation.
The major point of entry into the tricarboxylate cycle is via acetyl-CoA; 10 amino acids enter by this route. Of these, six (alanine, glycine, serine, threonine, tryptophan and cysteine) are degraded to acetyl-CoA via pyruvate, five (phenylalanine, tyrosine, leucine, lysine, and tryptophan) are degraded via acetoacetyl-CoA, and three (isoleucine, leucine and tryptophan) yield acetyl-CoA directly. Leucine and tryptophan yield both acetoacetyl-CoA and acetyl-CoA as end products.
The carbon skeletons of five amino acids (arginine, histidine, glutamate, glutamine and proline) enter the tricarboxylic acid cycle via a-ketoglutarate.
The carbon skeletons of methionine, isoleucine, and valine are ultimately degraded via propionyl-CoA and methyl-malonyl-CoA to succinyl-CoA; these amino acids are thus glycogenic.
Fumarate is formed in catabolism of phenylalanine, aspartate and tyrosine.
Oxaloacetate is formed in catabolism of aspartate and asparagine. Aspartate is converted to the oxaloacetate by transamination.
Amino acids that are degraded to citric acid cycle intermediates can serve as glucose precursors and are called glucogenic. A glucogenic amino acid is an amino acid whose carbon-containing degradation product(s) can be used to produce glucose via gluconeogenesis.
Amino acids that are degraded to acetyl CoA or acetoacetyl CoA can contribute to the formation of fatty acids or ketone bodies and are called ketogenic. Aketogenic amino acid is an amino acid whose carbon-containing degradation product(s) can be used to produce ketone bodies.
Amino acids that are degraded to pyruvate can be either glucogenic or ketogenic. Pyruvate can be metabolized to either oxaloacetate (glucogenic) or acetyl CoA (ketogenic).
Only two amino acids are purely ketogenic: leucine and lysine. Nine amino acids are both glucogenic and ketogenic: those degraded to pyruvate (alanine, glycine, cysteine, serine, threonine, tryptophan), as well as tyrosine, phenylalanine, and isoleucine (which have two degradation products). The remaining nine amino acids are purely glucogenic (arginine, asparagine, aspartate, glutamine, glutamate, valine, histidine, methionine, proline)
The regulation of protein metabolism. Protein metabolism is regulated by different hormones. All hormones according to their action on protein synthesis or splitting are divided on two groups: anabolic and catabolic. Anabolic hormones promote to the protein synthesis. Catabolic hormones enhance the decomposition of proteins.
Somatotropic hormone (STH, growth hormone):
- stimulates the passing of amino acids into the cells;
- activates the synthesis of proteins, DNA, RNA.
Thyroxine and triiodthyronine:
- in normal concentration stimulate the synthesis of proteins and nucleic acids;
- in excessive concentration activate the catabolic processes.
- increases the permeability of cell membranes for amino acids;
- activates synthesis of proteins and nucleic acids;
- inhibits the conversion of amino acids into carbohydrates.
- stimulates the conversion of amino acids into carbohydrates.
- activates the protein decomposition.
- stimulate the catabolic processes (protein decomposition) in connective, lymphoid and muscle tissues and activate the processes of protein synthesis in liver;
- stimulate the activity of aminotransferases;
- activate the synthesis of urea.
- stimulate the processes of protein, DNA, RNA synthesis;
- cause the positive nitrogenous balance.
The role of liver in protein metabolism:
– synthesis of plasma proteins. Most of plasma proteins are synthesized in liver: all albumins, 75-90 % of α-globulins, 50 % of β-globulins, all proteins of blood clotting systems (prothrombin, fibrinogen, proconvertin, proaccelerine). Only γ-globulins are synthesized in the cells of reticuloendothelial system.
– synthesis of urea and uric acid;
– synthesis of choline and creatine;
– transamination and deamination of amino acids.
Clinical significance of residual nitrogen measurement in blood. The kinds of azotemia.
Azotemia - increase of the residual nitrogen content in blood. There are two kinds of azotemia: absolute and relative.
Absolute azotemia – accumulation of the components of residual nitrogen in blood. Relative azotemia occurs in dehydration of the organism (diarrhea, vomiting).
Absolute azotemia can be divided on the productive azotemia and retention azotemia. Retention azotemia is caused by the poor excretion of the nitrogen containing compounds via the kidneys; in this case the entry of nitrogen containing compounds into the blood is normal.
Retention azotemia can be divided on the renal and extrarenal. Renal retention azotemia occurs in kidney diseases (glomerulonephritis, pyelonephritis, kidney tuberculosis et c.). Extrarenal retention azotemia is caused by the violations of kidney hemodynamic and decrease of glomerulus filtration processes (heart failure, local disorders of kidney hemodynamic).
Productive azotemia is conditioned by the enhanced entry of nitrogen containing compounds into the blood. The function of kidneys in this case doesn’t suffer.Productive azotemia can be observed in cachexia, leukoses, malignant tumors, treatment by glucocorticoids.
Alternate Names : Azotemia - Prerenal, Renal Underperfusion, Uremia
All peripheral cells express the LDL-receptor (LDLR), and recycle it to the cell surface upon need for cholesterol. Cholesterol is delivered to these cells through binding of LDL to LDLR, which triggers endocytosis (internalization) of both species. When the need for cholesterol is satisfied, the recycling of LDLR is discontinued. Normally, an LDL particle stays in circulation for no more than a few days before being consumed by a cholesterol needing cell. However, under conditions of sustained cholesterol excess, the particle stays in circulation for longer periods of time, and becomes more vulnerable to undesired modifications (e.g. oxidation). As high levels of oxidized LDL are commonly found in atherosclerotic plaques, they are thought to be the major inducer of atherosclerotic lesions. Hence, LDL became known as bad cholesterol. However, today we know that not all LDL particles are bad, and that some LDL particles, especially very large ones (with diameter >21.3nm), may even provide protection against atherosclerosis (2). LDL and HDL particle sizes are largely determined by a LP-associated protein, called CETP (cholesteryl ester transfer protein). This protein enhances exchange of non-polar lipids, primarily CE and TG, and facilitates tight packaging of CE within the core of the particles. The end result of prolonged and/or efficient CETP action is smaller LDL and HDL particles. [The LP-anchored CETP can be envisioned as having a hand that rotates between the interior and exterior of the particle and capable of holding only one lipid molecule at a time. Grasping of one molecule releases another and vise versa.]
Genetic variation at the human CETP gene generates proteins with varying degrees of activity. For example, a single codon variation, from isoleucine to valine at position 405, generates a mutant protein, designated I405V, which manifests significantly reduced CETP activity (3, 4). In a new observational study, Barzilai, N. et al. (2) found that people with homozygosity for the I405V allele have larger HDL and LDL particles, and that this genotype is associated with exceptional longevity and a markedly reduced risk of coronary artery disease (CAD). Of the 213 centenarians enrolled in the study, 80% had a high proportion of large LDL particles, compared to just 8% of the subjects in the control group (256 people in their 60’s and 70’s) (2). Interestingly, HDL and LDL particle sizes are significantly larger in women than in men, which may account, at least in part, for the longer life expectancies of women.
Unlike LDL, HDL is not recognized by LDLR, and cannot deliver cholesterol to tissue cells. Instead, it has the ability to remove excess peripheral cholesterol and return it to the liver for recycling and excretion. This process, called reverse cholesterol transport, is thought to protect against atherosclerosis. Observational studies over the last 2 decades have consistently shown strong correlation between elevated HDL levels and low incidents of coronary heart disease (CHD). Hence HDL has been dubbed “good” cholesterol.
HDL is synthesized in the liver and intestine as a nascent, discoid-shaped particle that contains predominantly apoA-I, and some phospholipids. Upon maturation, HDL assumes a spherical shape, and the composition of its core lipids becomes very similar to that of LDL. However, the relative higher protein content in HDL renders the particle denser and more resistant to undesired modifications. Unlike the case of LDL, the clearance of HDL from circulation is not negatively affected by excess cholesterol, which may be another reason why HDL, despite being much smaller particle than LDL (10nm versus 20nm), is not found in atherosclerotic plaques. It’s worth noting, that the potential of LPs to become harmful is also influenced by the character of their lipid constituents. For example, vitamin E and lipids containing omega-3 fatty acid moieties appear to protect the particles from harmful oxidation and from getting stuck on the walls of blood vessels.
The functional difference between LDL and HDL results primarily from the different character of their major apoproteins, apoB-100 and apoA-I, respectively. ApoB-100, which is found in VLDL, IDL, and LDL, but not in HDL, serves as a ligand for LDLR, and provides LDL with the means to deliver cholesterol to tissue cells. On the other hand, apoA-I, which is found exclusively in HDL, has a unique ability to capture and solubilze free cholesterol. This apoA-I ability enables HDL to act as a cholesterol scavenger.
A mutant apoA-I protein, called apoA-I Milano (apoA-Im), has been identified in a group of people that live in a small village in northern Italy (5). Carriers of this protein, all heterozygous for the mutation, had very low levels of HDL (7-14 mg/dl) but showed no clinical signs of atherosclerosis (5-7). HDL particles in these subjects were markedly larger than control (12nm versus 9.4nm), which may account for their immunity against premature atherosclerosis. ApoA-Im differs from natural apoA-I by having a cysteine residue at position 173 instead of arginine. This cysteine residue forms disulfide bridges with other apoA-I molecules or with apoA-II (6, 7), which apparently lead to larger HDL particles. It also renders apoA-I more susceptible to catabolism (8), accounting for the low HDL levels in apoA-Im carriers.
The therapeutic potential of apoA-I has been recently assessed in patients with acute coronary syndromes (9). Of the 47 patients that participated in a randomized controlled trial, 36 received 5 weekly infusions of recombinant apoA-Im/phospholipid complexes, and 11 received only saline infusions. The results showed significant regression in coronary atherosclerotic volume in the apoA-Im treated group, and virtually no change in the control group (9). These results, if reproduced in larger clinical trials, may constitute a revolutionary breakthrough in the non-invasive treatment of cardiovascular disease. They should also encourage further exploration into the therapeutic usefulness of apoA-Im and normal apoA-I in managing atherosclerotic vascular diseases.
The lilipoproteins - Any of the series of soluble lipid-protein complexes which are transported in the blood; each aggregate particle consists of a spherical hydrophobic core containing triglycerides and cholesterol esters surrounded by an amphipathic monolayer of phopholipids, cholesterol and apolipoproteins; classes of lipoproteins include chylomicrons, very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).
chylomicrons - The class of largest diameter soluble lipid-protein complexes which the lowest in density (mass to volume ratio); their composition is ~2% apolipoproteins, ~5% cholesterol, and ~93% triglycerides and phospholipids; their normal role is to be synthesized by the intestinal mucosal cells to transport dietary (exogenous) triglycerides and other lipids from the intestines via the lacteals and lymphatic system to the systemic circulation to the adipose tissue and liver for storage and use; they are only present in the blood in significant quantities after the digestion of a meal.
low-density lipoproteins (LDL) - The class of large diameter soluble lipid-protein complexes which the fourth lowest in density (mass to volume ratio); their composition is ~25% apolipoproteins, ~45% cholesterol, and ~30% triglycerides and phospholipids; their normal role is to transport cholesterol and other lipids from the liver and intestines to the tissues for use; elevated levels of LDL are associated with increased risk of cardiovascular disease. nickname - bad cholesterol
high-density lipoproteins (HDL) - The class of small diameter soluble lipid-protein complexes which the highest in density (mass to volume ratio); their composition is ~45% apolipoproteins, ~25% cholesterol, and ~30% triglycerides and phospholipids; their normal role is to transport cholesterol and other lipids from the tissues to the liver for disposal; elevated levels of HDL are associated with decreased risk of cardiovascular disease.
very low-density lipoproteins (VLDL) - The class of very large diameter soluble lipid-protein complexes which the second lowest in density (mass to volume ratio); their composition is ~10% apolipoproteins, ~40% cholesterol, and ~50% triglycerides and phospholipids; their normal role is to transport triglycerides and other lipids from the liver and intestines to the tissues for use; elevated levels of VLDL are associated with some increased risk of cardiovascular disease.
Cholesterol is a waxy fat found in the body and, despite what you may have been told, is a necessary nutrient for the body. Cholesterol is used in the formation of cell membranes and plays an important role in hormone, bile and vitamin D production. Cholesterol comes from two sources: the foods that we eat, such as meat, dairy products and eggs, and our own liver, which produces about eighty percent of all the cholesterol in the body. That means that only about twenty percent of our total cholesterol is obtained from food. Since cholesterol is not water-soluble, the liver packages the cholesterol into tiny spheres called lipoproteins so that the cholesterol can be transported through the blood. The lipoproteins can be divided into two different categories: low density and high density lipoproteins.
Low density lipoprotein (LDL): LDL, often dubbed the "bad" cholesterol, carries most of the cholesterol in the blood and seems to play a role in the deposition of fat in arteries. These deposits result in blockages called plaque. In addition to narrowing the arteries and increasing blood pressure, plaque contributes to the hardening of artery walls, a condition known as atherosclerosis.
High density lipoprotein (HDL): HDL is known as the "good" cholesterol. HDL carries cholesterol from the blood back to the liver for elimination. It is also responsible for removing the plaque buildup along the artery walls. Elevated levels of HDL are very desirable because it helps to clear blockages in the arteries, reduces LDL and decreases blood pressure.
Triglycerides are lipids normally found in increased levels in the blood following the digestion of fats in the intestine. Consumed calories that are not immediately used are stored in fat cells in the form of triglycerides and are later released from fatty tissues when the body needs energy between meals. The major transporter of triglycerides is a forerunner of LDL, a simpler molecule known as VLDL (very low density lipoprotein). As the VLDL loses triglycerides, the VLDL particle is converted into intermediate and then low density lipoprotein. Over time, elevated triglyceride levels may result in pancreatitis—a condition that can cause malabsorption of nutrients and lead to diabetes. As pancreatitis progresses, damage can spread to other organs, including the heart, lungs and kidneys. High triglyceride levels also promote the deposition of cholesterol in the arteries and are associated with known risk factors for heart disease. The exact role that triglycerides play as an independent risk factor is not yet clear because people with high LDL and low HDL levels also have high triglyceride levels.
One study by Koren-Morag, Graff and Goldbourt, published in the American Heart Association journal Circulation, found that individuals with elevated triglyceride levels have a nearly thirty percent increased probability of suffering a stroke, even after taking into account other risk factors such as cholesterol levels. One of the most important aspects of the study is that it clarifies the independent link of triglyceride levels to stroke, meaning that a causal relationship is likely.
Excess LDL cholesterol clings to arterial walls as it is transported through the system. Macrophages eat the LDL and become "foam cells." The cells eventually rupture and begin to form a lipid layer called plaque. Connective fibers form in and around the fatty layer, causing it to harden. Over time, the fibrous layer thickens, narrowing the arterial pathway. When calcium deposits form a crust, the plaque becomes brittle and is more likely to rupture.
High blood cholesterol levels increase the likelihood that the fat will be deposited as plaque on the inner surface of arterial walls. As these deposits increase, the channel of the artery narrows, contributing to an increase in blood pressure. To compensate, the heart must work harder to pump the same volume of blood through the narrower arteries. When the coronary arteries themselves are affected by plaque, the harder working heart receives less oxygen, thus increasing the risk of heart attack. Plaque also contributes to hardening of the arteries, or atherosclerosis. This loss of flexibility in arterial walls elevates blood pressure, putting the heart at additional risk. When the plaque deposits become unstable, they burst, releasing their cholesterol into the bloodstream all at once. This can trigger clotting in small coronary arteries. When the artery is completely obstructed, blood flow stops and a heart attack occurs.
This complex LIPOPROTEIN is a globular micelle-like particle that consists of a nonpolar core of triacylglycerols and cholesterol esters surrounded by an amphiphilic coating of protein, phospholipid, and cholesterol.
Theoretically, a 17g egg yolk that contains 2.8g of protein would contain 1.4g of apoB (49% total yolk protein, MW = 5.5 x 105). Because VLDLy contains only one apoB protein per particle, this single egg yolk would contain 1.5 x 1018 VLDLy particles. The hen would be producing VLDLy particles at a rate of 1.5 x 1014 particles per minute for seven days!
Viruses, bacteria, fungi, and parasites that enter the body of vertebrates of are recognized and attacked by the immune system. Endogenous cells that have undergone alterations— e. g., tumor cells—are also usually recognized as foreign and destroyed. The immune system is supported by physiological changes in infected tissue, known asinflammation. This reaction makes it easier for the immune cells to reach the site of infection. Two different systems are involved in the immune response. The innate immune system is based on receptors that can distinguish between bacterial and viral surface structures or foreign proteins (known as antigens) and those that are endogenous. With the help of these receptors, phagocytes bind to the pathogens, absorb them by endocytosis, and break them down. The complement system (see p. 298) is also part of the innate system. The acquired (adaptive) immune system is based on the ability of the lymphocytes to form highly specific antigen receptors “on suspicion,” without ever having met the corresponding antigen. In humans, there are several billion different lymphocytes, each of which carries a different antigen receptor. If this type of receptor recognizes “its” cognate antigen, the lymphocyte carrying it is activated and then plays its special role in the immune response. In addition, a distinction is made between cellular and humoral immune responses.
The T lymphocytes (T cells) are responsible for cellular immunity. They are named after the thymus, in which the decisive steps in their differentiation take place. Depending on their function, another distinction is made between cytotoxic T cells (green) and helper T cells (blue).
Humoral immunity is based on the activity of the B lymphocytes (B cells, light brown), which mature in the bone marrow. After activation by T cells, B cells are able to release soluble forms of their specific antigen receptors, known as antibodies (see p. 300), into the blood plasma. The immune system’s “memory” is represented by memory cells. These are particularly long–lived cells that can arise from any of the lymphocyte types described. Simplified diagram of the immune response.
Pathogens that have entered the body—e. g., viruses (top)—are taken up by antigen-presenting cells (APCs) and proteolytically degraded (1). The viral fragments produced in this way are then presented on the surfaces of these cells with the help of special membrane proteins (MHC proteins; see p. 296) (2). The APCs include B lymphocytes, macrophages, and dendritic cells such as the skin’s Langerhans cells. The complexes of MHC proteins and viral fragments displayed on the APCs are recognized by T cells that carry a receptor that matches the antigen (“T-cell receptors”) (3). Binding leads to activation of the T cell concerned and selective replication of it (4, “clonal selection”).
The proliferation of immune cells is stimulated by interleukins (IL). These are a group of more than 20 signaling substances belonging to the cytokine family (see p. 392), with the help of which immune cells communicate with each other. For example, activated macrophages release IL-1 (5), while T cells stimulate their own replication and that of other immune cells by releasing IL-2 (6). Depending on their type, activated T cells have different functions. Cytotoxic T cells (green) are able to recognize and bind virusinfected body cells or tumor cells (7). They then drive the infected cells into apoptosis (see p. 396) or kill them with perforin, a protein that perforates the target cell’s plasma membrane (8). B lymphocytes, which as APCs present viral fragments on their surfaces, are recognized by helper T cells (blue) or their T cell receptors (9). Stimulated by interleukins, selective clonal replication then takes place of B cells that carry antigen receptors matching those of the pathogen (10). Thesemature into plasma cells (11) and finally secrete large amounts of soluble antibodies (12).
Many antigen receptors belong to the immunoglobulin superfamily. The common characteristic of these proteins is that they aremade up from “immunoglobulin domains.” These are characteristically folded substructures consisting of 70–110 amino acids, which are also found in soluble immunoglobulins (Ig; see p. 300). The illustration shows schematically a few of the important proteins in the Ig superfamily. They consist of constant regions (brown or green) and variable regions (orange). Homologous domains are shown in the same colors in each case. All of the receptors have transmembrane helices at the C terminus, which anchor them to the membranes. Intramolecular and intermolecular disulfide bonds are also usually found in proteins belonging to the Ig family. Immunoglobulin M (IgM), a membrane protein on the surface of B lymphocytes, serves to bind free antigens to the B cells. By contrast, T cell receptors only bind antigens when they are presented by another cell as a complex with an MHC protein (see below). Interaction between MHC-bound antigens and T cell receptors is supported by co-receptors. This group includes CD8, a membrane protein that is typical in cytotoxic T cells. T helper cells use CD4 as a co-receptor instead (not shown). The abbreviation “CD” stands for “cluster of differentiation.” It is the term for a large group of proteins that are all located on the cell surface and can therefore be identified by antibodies. In addition to CD4 and CD8, there are many other co-receptors on immune cells
The MHC proteins are named after the “major histocompatibility complex”—the DNA segment that codes for them. Human MHC proteins are also known as HLA antigens (“human leukocyte-associated” antigens). Their polymorphism is so large that it is unlikely that any two individuals carry the same set of MHC proteins—except formonozygotic twins. Class I MHC proteins occur in almost all nucleated cells. They mainly interact with cytotoxic T cells and are the reason for the rejection of transplanted organs. Class I MHC proteins are heterodimers (áâ). The â subunit is also known as â2-microglobulin. Class II MHC proteins also consist of two peptide chains, which are related to each other. MHC II molecules are found on all antigen- presenting cells in the immune system. They serve for interaction
T-cell activation The illustration shows an interaction between a virus-infected body cell (bottom) and a CD8- carrying cytotoxic T lymphocyte (top). The infected cell breaks down viral proteins in its cytoplasm and transports the peptide fragments into the endoplasmic reticulum with the help of a special transporter (TAP) . Newly synthesized class I MHC proteins on the endoplasmic reticulum are loaded with one of the peptides and then transferred to the cell surface by vesicular transport . The viral peptides are bound on the surface of the á2 domain of the MHC protein in a depression formed by an insertion as a “floor” and two helices as “walls” (see smaller illustration). Supported by CD8 and other co-receptors, a T cell with a matching T cell receptor binds to the MHC peptide complex (5). This binding activates protein kinases in the interior of the T cell, which trigger a chain of additional reactions (signal transduction). Finally, destruction of the virus-infected cell by the cytotoxic T lymphocytes takes place.
The complement system is part of the innate immune system. It supports nonspecific defense against microorganisms. The system consists of some 30 different proteins, the “complement factors,” which are found in the blood and represent about 4% of all plasma proteins there. When inflammatory reactions occur, the complement factors enter the infected tissue and take effect there. The complement system works in three different ways: Chemotaxis. Various complement factors attract immune cells that can attack and phagocytose pathogens. Opsonization. Certain complement factors (“opsonins”) bind to the pathogens and thereby mark them as targets for phagocytosing cells (e. g., macrophages). Membrane attack. Other complement factors are deposited in the bacterial membrane, where they create pores that lyse the pathogen (see below).
• The reactions that take place in the complement system can be initiated in several ways. During the early phase of infection, lipopolysaccharides and other structures on the surface of the pathogens trigger the alternative pathway (right). If antibodies against the pathogens become available later, the antigen– antibody complexes formed activate theclassic pathway (left). Acute-phase proteins are also able to start the complement cascade (lectin pathway). Factors C1 to C4 (for “complement”) belong to the classic pathway, while factors B and D form the reactive components of the alternative pathway. Factors C5 to C9 are responsible for membrane attack. Other components not shown here regulate the system. As in blood coagulation (see p. 290), the early components in the complement system are serine proteinases, which mutually activate each other through limited proteolysis. They create a self-reinforcing enzyme cascade.
Factor C3, the products of which are involved in several functions, is central to the complement system. The classic pathway is triggered by the formation of factor C1 at IgG or IgM on the surface of microorganisms (left). C1 is an 18-part molecular complex with three different components (C1q, C1r, and C1s). C1q is shaped like a bunch of tulips, the “flowers” of which bind to the Fc region of antibodies (left). This activates C1r, a serine proteinase that initiates the cascade of the classic pathway. First, C4 is proteolytically activated into C4b, which in turn cleaves C2 into C2a and C2b. C4B and C2a together form C3 convertase , which finally catalyzes the cleavage of C3 into C3a and C3b. Small amounts of C3b also arise from non-enzymatic hydrolysis of C3.
The classic pathway is triggered by the formation of factor C1 at IgG or IgM on the surface of microorganisms (left). C1 is an 18-part molecular complex with three different components (C1q, C1r, and C1s). C1q is shaped like a bunch of tulips, the “flowers” of which bind to the Fc region of antibodies (left). This activates C1r, a serine proteinase that initiates the cascade of the classic pathway. First, C4 is proteolytically activated into C4b, which in turn cleaves C2 into C2a and C2b. C4B and C2a together formC3 convertase , which finally catalyzes the cleavage of C3 into C3a and C3b. Small amounts of C3b also arise from non-enzymatic hydrolysis of C3. The alternative pathwaystarts with the binding of factors C3b and B to bacterial lipopolysaccharides (endotoxins). The formation of this complex allows cleavage of B by factor D, giving rise to a second form of C3 convertase (C3bBb). Proteolytic cleavage of factor C3 provides two components with different effects. The reaction exposes a highly reactive thioester group in C3b, which reacts with hydroxyl or amino groups. This allows C3b to bind covalently to molecules on the bacterial surface (opsonization, right). In addition, C3b initiates a chain of reactions leading to the formation of the membrane attack complex Together with C4a and C5a (see below), the smaller product C3a promotes the inflammatory reaction and has chemotactic effects. The “late” factors C5 to C9 are responsible for the development of the membrane attack complex (bottom). They create an ion-permeable pore in the bacterial membrane, which leads to lysis of the pathogen. This reaction is triggered by C5 convertase . Depending on the type of complement activation, this enzyme has the structure C4b2a3b or C3bBb3b, and it cleaves C5 into C5a and C5b. The complex of C5b and C6 allows deposition of C7 in the bacterial membrane. C8 and numerous C9 molecules—which form the actual pore—then bind to this core. Antibodies
• Soluble antigen receptors, which are formed by activated B cells (plasma cells; see p. 294) and released into the blood, are known as antibodies. They are also members of the immunoglobulin family (Ig; see p. 296). Antibodies are an important part of the humoral immune defense system. They have no antimicrobial properties themselves, but support the cellular immune system in various ways: 1. They bind to antigens on the surface of pathogens and thereby prevent them from interacting with body cells (neutralization; see p. 404, for example). 2. They link single-celled pathogens into aggregates (immune complexes), which are more easily taken up by phagocytes (agglutination). 3. They activate the complement system (see p. 298) and thereby promote the innate immune defense system (opsonization). In addition, antibodies have become indispensable aids in medical and biological diagnosis. Domain structure of immunoglobulin G _
Type G immunoglobulins (IgG) are quantitatively the most important antibodies in the blood,where they form the fraction of ã-globulins (see p. 276). IgGs (mass 150 kDa) are tetramers with two heavy chains (H chains; red or orange) and two light chains (L chains; yellow). Both H chains are glycosylated (violet; see also p. 43). The proteinasepapain cleaves IgG into two Fab fragments and one Fc fragment. The Fab (“antigen-binding”) fragments, which each consist of one L chain and the N-terminal part of an H chain, are able to bind antigens. The Fc (“crystallizable”) fragment is made up of the C-terminal halves of the two H chains. This segment serves to bind IgG to cell surfaces, for interaction with the complement system and antibody transport. Immunoglobulins are constructed in a modular fashion from several immunoglobulin domains (shown in the diagram on the right in Ω form). Classes of immunoglobulins _
Human immunoglobulins are divided into five classes. IgA (with two subgroups), IgD, IgE, IgG (with four subgroups), and IgM are defined by their H chains, which are designated by the Greek letters á, ä, å, ã, and µ. By contrast, there are only two types of L chain (ê and ë). IgD and IgE (like IgG) are tetramers with the structure H2L2. By contrast, soluble IgA and IgM are multimers that are held together by disulfide bonds and additional J peptides (joining peptides). The antibodies have different tasks. IgMs are the first immunoglobulins formed after contact with a foreign antigen. Their early forms are located on the surface of B cells (see p. 296), while the later forms are secreted from plasma cells as pentamers. Their action targets microorganisms in particular. Quantitatively, IgGs are the most important immunoglobulins (see the table showing serum concentrations). They occur in the blood and interstitial fluid. As they can pass the placenta with the help of receptors, they can be transferred from mother to fetus. IgAs mainly occur in the intestinal tract and in body secretions. IgEs are found in low concentrations in the blood. As they can trigger degranulation of mast cells (see p. 380), they play an important role in allergic reactions. The function of IgDs is still unexplained. Their plasma concentration is also very low.
There are three reasons for the extremely wide variability of antibodies: 1. Multiple genes. Various genes are available to code for the variable protein domains. Only one gene from among these is selected and expressed. 2. Somatic recombination. The genes are divided into several segments, of which there are various versions. Various (“untidy”) combinations of the segments during lymphocyte maturation give rise to randomly combined new genes (“mosaic genes”). 3. Somaticmutation. During differentiation of B cells into plasma cells, the coding genes mutate. In this way, the “primordial” germline genes can become different somatic genes in the individual B cell clones.
We can look at the basic features of the genetic organization and synthesis of immunoglobulins using the biosynthesis of a mouse ê chain as an example. The gene segments for this light chain are designated L, V, J, and C. They are located on chromosome 6 in the germ-line DNA (on chromosome 2 in humans) and are separated from one another by introns (see p. 242) of different lengths. Some 150 identical L segments code for the signal peptide (“leader sequence,” 17–20 amino acids) for secretion of the product
The V segments, of which there are 150 different variants, code formost of the variable domains (95 of the 108 amino acids). L and V segments always occur in pairs—in tandem, so to speak. By contrast, there are only five variants of the J segments (joining segments) at most. These code for a peptide with 13 amino acids that links the variable part of the ê chains to the constant part. A single C segment codes for the constant part of the light chain (84 amino acids). During the differentiation of B lymphocytes, individualV/J combinations arise in each B cell. One of the 150 L/V tandem segments is selected and linked to one of the five J segments.
The immune system is a complex, dynamic, and beautifully orchestrated mechanism with enormous responsibility. It defends against foreign invasion by microorganisms, screens out cancer cells, adapts as we grow, and modifies how we interact with our environment. When it malfunctions, disease, cancer or death can occur. Although it is not necessary to understand all the intimate details of the immune system, it is wise to have a basic grasp of its functions. More precisely, we should understand how to stay healthy.
It appears that the immune system has a training effect, similar to other areas of physiology (e.g., cardiovascular, muscular). In other words, a balanced training program of exercise and rest leads to better performance. Studies in the laboratory and epidemiological observations have shown improved immune function and fewer URIs in athletes as compared to their couch-potato counterparts. This is especially true in older athletes and it appears that regular exercise can help attenuate the age related decline in immune function.
On the other hand, too much exercise can lead to a dramatically increased risk of URIs. The stress of strenuous exercise transiently suppresses immune function. This interruption of otherwise vigorous surveillance can provide an "open window" for a variety of infectious diseases -- notably viral illnesses -- to take hold. This is especially true following single bouts of excessive exercise. For example, it has been observed that two-thirds of participants developed URIs shortly after completing an ultramarathon. Similarly, cumulative overtraining weakens the athlete's immune system, leading to frequent illness and injury.
The best model that accommodates clinical observations and laboratory experiments is described by the "J"-curve ( Fig. 1). It is important to note that this curve is individualized. What is moderate training for some is overtraining for others.
In addition to strenuous exercise, other forms of stress may also transiently suppress immune function. Since exercise is not the only stress factor, an athlete must consider a host of other variables. There are job responsibilities, family obligations, social interactions, financial concerns and other components that shape our lives. The sum of all of these affects a central axis in the body which ultimately influences immune function. Some of these (e.g., exercise) are under our direct control, and others only partially or not at all. Recognizing when excess stress occurs is easier if it just comes from one source. However, all too often it is the sum of many small, difficult to recognize changes that tips the scales and sends the athlete into the whirlpool of overtraining and immunosuppression. Alone and in isolation these small changes would be manageable, but combined they can overwhelm. (Fig. 2.)
Currently, the best way to stay healthy is to listen to your body. Recognizing the early warning signs and adapting the training schedule accordingly can help keep you healthy. In that light, here are some points to ponder and a few recommendations,
· Keep a training log. In addition to recording workouts, keep a fatigue score (scale 0-5). It is expected that a hard workout will make you tired, so it is more important to note the cumulative "feel" during the day. Granted, the scale is individualized and subjective, but this simple tool is very useful. If you notice that your fatigue is progressively increasing over days or weeks, then it is time to add more rest.
· To make sure your anti-oxidant defense system is tuned up, eat five servings of fruit or vegetables per day. Note: vitamin supplements do not appear to have the same benefits as fruits and vegetables.