Characteristics of Proteins

α-Amino acids, peptides and proteins.


Next to water, proteins are the most abundant substances in most cells - from 10% to 20% of the cell’s mass. All proteins contain the elements carbon, hydrogen, oxygen, and nitrogen; most also contain sulfur. The presence of nitrogen in proteins sets them apart from carbohydrates and lipids, which generally do not contain nitrogen. The average nitrogen content of proteins is 15.4% by mass. Other elements, such as phosphorus and iron, are essential constituents of certain specialized proteins.

Casein, the main protein of milk, contains phosphorus, an element very important in the diet of infants and children. Hemoglobin, the oxygen-transporting protein of blood, contains iron.

Amino acids - the building blocks for proteins.  The word protein comes from the Greek proteios, which means "of first importance." This derivation alludes to the key role that proteins play in life processes.

А protein is in polymer in which the monomer units are amino acids. Thus the starting point for а discussion of proteins is an understanding of the structures and chemical properties of amino acids.

An amino acid is an organic compound that contains both an amino (–NН3) group and a carboxyl (-СООН) group. The amino acids found in proteins are always α-amino acids - that is, amino acids in which the amino group is attached to the α-carbon atom of the carboxylic acid carbon chain. The general structural formula for an α-amino acid is:

The R group present in an α-amino acid is called the amino acid side chain. The nature of this side chain distinguishes а-amino acids from each other. Side chains vary in size, shape, charge, acidity, functional groups present, hydrogen-bonding ability, and chemical reactivity.

Over 700 different naturally occurring amino acids are known, but only 20 of them, called standard amino acids, are normally present in proteins. А standard amino acid is one of the 20 α-amino acids normally found in proteins. Amino acids are grouped according to side-chain polarity. In this system there are four categories: (1) nonpolar amino acids, (2) polar neutral amino acids, (3) polar acidic amino acids, and (4) polar basic amino acids. This classification system gives insights into how various types of amino acid side chains help determine the properties of proteins.

Nonpolar amino acids contain one amino group, one carboxyl group, and a nonpolar side chain. When incorporated into а protein, such amino acids are hydrophobic (“water fearing”); that is, they are not attracted to water molecules. They are generally found in the interior of proteins, where there is limited contact with water. There are eight nonpolar amino acids.

The three types of polar amino acids have varying degrees of affinity for water. Within а protein, such amino acids are said to be hydrophilic ("water-loving"). Hydrophilic amino acids are often found on the surfaces of proteins.

Polar neutral amino acids contain one amino group, one carboxyl group, and а side chain that is polar but neutral. The side chain is neutral in that it is neither acidic nor basic in solution at physiological pH. There are seven polar neutral amino acids.

Polar acidic amino acids contain one amino group and two carboxyl groups, the second carboxyl group being part of the side chain. In solution at physiological рН, the side chain of а polar acidic amino acid bears а negative charge; the side-chain carboxyl group has lost its acidic hydrogen atom. There are two polar acidic amino acids: aspartic acid and glutamic acid.

Polar basic amino acids contain one amino groups and one carboxyl group, the second amino group being part of the side chain. In solution at physiological рН, the side chain of а polar basic amino acid bears а positive charge; the nitrogen atom of the amino group has accepted а proton. There are three polar basic amino acids: lysine, arginine, and histidine.

Classification and structure of amino acids

The names of the standard amino acids are often abbreviated using three-letter codes. Except in four cases, these abbreviations are the first three letters of the amino acid’s name. In addition, а new one-letter code for amino acid names is currently gaining popularity (particularly in computer applications). These abbreviations are used extensively when describing peptides and proteins, which contain tens and hundreds of amino acid units.

The essential amino acids. All of the 20 amino acids are necessary constituents of human protein. Adequate amounts of 11 of the 20 amino acids can be synthesized from carbohydrates and lipids in the body if а source of nitrogen is also available. Because the human body is incapable of producing 9 of these 20 acids fast enough or in sufficient quantities to sustain normal growth, these 9 amino acids, called essential amino acids, must be obtained from food. Essential amino acids are amino acids that must be obtained from food. An adequate human diet must include foods that contain these essential amino acids.

The human body can synthesize small amounts of some of the essential amino acids, but not enough to meet its needs, especially in the case of growing children.

The 9 essential amino acids for adults are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. (In addition, arginine is essential for children).

А complete dietary protein contains all the essential amino acids in the same relative amounts in which human being require them. А complete dietary protein may or may not contain all the nonessential amino acids. Most animal proteins, including casein from milk and proteins found in meat, fish, and eggs, are complete proteins, although gelatin is an exception (it lacks tryptophan). Proteins from plants (vegetables, grains, and legumes) have quite diverse amino acid patterns, and some tend to be limiting in one or more essential amino acids. Some plant proteins (for example, corn protein) are far from complete. Others (for example, soy protein) are complete. Thus vegetarians must eat а variety of plant foods to obtain all of the essential amino acids in appropriate quantities.

Chirality and amino acids. Four different groups are attached to the α-carbon atom in all of the standard amino acids except glycine, where the R group is а hydrogen atom.

This means that the structures of 19 of the 20 standard amino acids possess а chiral center at this location, so enantiomeric forms (left- and right-handed forms) exist for each of these amino acids.

With few exceptions (in some bacteria), the amino acids found in nature and in proteins are isomers. Thus, as is the case with monosaccharides, nature favors one mirror-image form over the other. Interestingly, for amino acids the L isomer is the preferred form, whereas for monosaccharides the n isomer is preferred.

The rules for drawing Fischer projections for amino acid structures follow.

1. The - СООН group is put at the top of the projection, the R group at the bottom. This positions the carbon chain vertically.

2. The – NН2 group is in а horizontal position. Positioning it on the left denotes the L isomer, and positioning it on the right denotes the D isomer.


Acid - base properties of amino acids. In pure form, amino acids are white crystalline solids with relatively high decomposition points. (Most amino acids decompose before they melt.) Also most amino acids are not very soluble in water because of strong intermolecular forces within their crystal structures. Such properties are those often exhibited by compounds in which charged species are present. Studies of amino acids confirm that they are charged species both in the solid state and in solution.

Both an acidic group (-СООН) and а basic group (-NН2) are present on the same carbon in an α-amino acid.

We learned that in neutral solution, carboxyl groups have а tendency to lose protons (Н+), producing а negatively charged species:

СООН = – СОО- + Н+

We learned that in neutral solution, amino groups have а tendency to accept protons (Н+), producing а positively charged species:

 –NH2 + H+ ==  –NH3+

As is consistent with the behavior of these groups, in neutral solution, the –СООН group of an amino acid donates а proton to the –NH2 of the same amino acid. We can characterize this behavior as an internal acid — base reaction. The net result is that in neutral solution, amino acid molecules have the structure

Such а molecule is known as а zwitterion, from the German term meaning “double ion”. А zwitterion is а molecule that has а positive charge on one atom and а negative charge on another atom. Note that the net charge on а zwitterion is zero even though parts of the molecule carry charges. In solution and also in the solid state, α-amino acids are zwitterions.

Zwitterion structure changes when the pH of а solution containing an amino acid is changed from neutral either to acidic (low pH) by adding an acid such as НС1 or to basic (high pH) by adding а base such as NaOH. In an acidic solution, the zwitterion accepts а proton (Н+) to form а positively charged ion.

In basic solution, the –NH3+ of the zwitterion loses а proton, and а negatively charged species is formed.

Thus, in solution, three different amino acid forms can exist (zwitterion, negative ion, and positive ion). The three species are actually in equilibrium with each other, and the equilibrium shifts with pH change. The overall equilibrium process can be represented as follows:

In acidic solution, the positively charged species on the left predominates; nearly neutral solutions have the middle species (the zwitterion) as the dominant species; in basic solution, the negatively charged species on the right predominates.

The previous discussion assumed that the side chain (R group) of an amino acid remains unchanged in solution as the pH is varied. This is the case for neutral amino acids but not for acidic or basic ones. For these latter compounds, the side chain can also acquire а charge, because it contains an amino or а carboxyl group that can, respectively, gain or lose а proton.

Because of the extra site that can be protonated or deprotonated, acidic and basic amino acids have four charged forms in solution.

The existence of two low-pH forms for aspartic acid results from the two carboxyl groups being deprotonated at different pH values. For basic amino acids, two high-pH forms exist because deprotonation of the amino groups does not occur simultaneously. The side-chain amino group deprotonates before the α-amino group.

The isoelectric point for an amino acid is the pH at which the total charge on the amino acid is zero. Every amino acid has а different isoelectric point. Fifteen of the 20 amino acids, those with nonpolar or polar neutral side chains, have isoelectric points in the range of 4.8 - 6.3. The three basic amino acids have higher isoelectric points (His = 7.59, Lys = 9.74, Arg = 10.76), and the two acidic amino acids have lower ones (Asp = 2.77, Glu = 3.22).

А рН below the isoelectric point favors the positively charged form of the amino acid. Conversely, а рН above the isoelectric point favors the negatively charged form of the amino acid.

When two electrodes (one positively charged and one negatively charged) are immersed in а solution containing an amino acid, molecules with а net positive charge are attracted to the negatively charged electrode, and negatively charged amino acid molecules migrate toward the positively charged electrode. The zwitterion form exhibits no net migration toward either electrode. This behavior is the basis for the measurement of isoelectric points. The pH of the solution is adjusted until no net migration occurs.

Mixtures of amino acids in solution can be separated by using their different migration patterns at various pH values. This type of analytical separation is called electrophoresis. Electrophoresis is the process of separating charged molecules on the basis of their migration toward charged electrodes.

Reaction of amino acids.

Reaction with alcohols – esters formation:

Reaction with ammonia – amides formation. The amides of aspartic and glutamic acid acids, asparagine and glutamine, play important role in the transport of ammonia in the body.

Decarboxylation. Amino acids may be decarboxylated by heat, acids, bases or specific enzymes to the primary amines:

Some of the decarboxylation reaction are of great importance in the body, decarboxylation  of histidine to histamine:

In the presence of foreign protein introduced into the body, very large quantities of histamine are produced in the body and allergic reactions become evident. In extreme cases shock may result. The physiological effects of histamine may be neutralized or minimized by the use of chemical compounds known as antihistamines.

Salts are formed. All amino acids can react with some inorganic acids and bases and form two kind sold:


1.                              oxidation deamination – important pathway for the biodegradation of α-amino acids:

2.                              hydrolitic deamination – reaction with nitrous acid. Amino acids react with nitrous acid to give hydroxy acid along with the evolution of nitrogen.

The nitrogen can be collected and measured. Thus this reaction constitutes one of the methods for the estimation of amino acids.

3.                              intramolecular deamination - unsaturated acids are formed

4.                              redaction deamination – saturated carboxylic acid formation:

Peptide formation. Two amino acids can react in а similar way - the carboxyl group of one amino acid reacts with the amino group of the other amino acid. The products are а molecule of water and а molecule containing the two amino acids linked by an amide bond.

Removal of the elements of water from the reacting carboxyl and amino groups and the ensuing formation of the amide bond are better visualized when expanded structural formulas for the reacting groups are used.


In amino acid chemistry, amide bonds that link amino acids together are given the specific name of peptide bond. А peptide bond is а bond between the carboxyl group of one amino acid and the amino group of another amino acid.

Under proper conditions, many amino acids can bond together to give chains of amino acids containing numerous peptide bonds. For example, four peptide bonds are present in а chain of five amino acids.

Short to medium-sized chains of amino acids are known as peptides. А peptide is а sequence of amino acids, of up to 50 units, in which the amino acids are joined together through amide (peptide) bonds. А compound containing two amino acids joined by а peptide bond is specifically called а dipeptide; three amino acids in а chain constitute а tripeptide; and so on. The name oligopeptide is loosely used to refer to peptides with 10 to 20 amino acid residues and polypeptide to larger peptides.

In all peptides, the amino acid at one end of the amino acid sequence has а free H3N+ group, and the amino acid at the other end of the sequence has а free СОО- group. The end with the free H3N+ group is called the N-terminal end, and the end with the free СОО- group is called the С-terminal end. By convention, the sequence of amino acids in а peptide is written with the N-trminal end amino acid at the left. The individual amino acids within а peptide chain are called amino acid residues.

The structural formula for а polypeptide may be written out in full, or the sequence of amino acids present may be indicated by using the standard three-letter amino acid abbreviations. The abbreviated formula for the tripeptide:

which contains the amino acids glycine, alanine, and serine, is Gly –  Ala  Ser. When we use this abbreviated notation, by convention, the amino acid at the N-terminal end of the peptide is always written on the left.

The repeating chain of peptide bonds and α-carbon atoms in а peptide is referred to as the backbone of the peptide. The R group side chains are substituents on the backbone.

Peptides that contain the same amino acids but in different order are different molecules (structural isomers) with different properties. For example, two different dipeptides can be formed from one molecule of alanine and one molecule of glycine.

 In the first dipeptide, the alanine is the N-terminal residue, and in the second molecule, it is the С-terminal residue. These two compounds are isomers with different chemical and physical properties.

The number of isomeric peptides possible increases rapidly as the length of the peptide chain increases. Let us consider the tripeptide Ala – Ser – Cys as another example. In addition to this sequence, five other arrangements of these three components are possible, each representing another isomeric tripeptide: Ala – Cys – Ser, Ser –Ala – Cys, Ser – Cys – Ala, Cys – Ala – Ser, and Cys – Ser – Ala. For а pentapeptide containing 5 different amino acids, 120 isomers are possible.

More than two hundred peptides have been isolated and identified as essential to the proper functioning of the human body. In general, these substances serve as hormones or neurotransmitters. Their functions range from controlling pain to controlling muscle contraction or kidney fluid excretion.

Two important hormones produced by the pituitary gland are oxytocin and vasopressin, Each hormone is а nonapeptide (nine amino acid residues) with six of the residues hells in the form of а loop by а disulfide bond formed from the interaction of two cysteine residues.

Oxytocin regulates uterine contractions and lactation. Vasopressin regulates the excretion of water by the kidneys; it also affects blood pressure. The structure of vasopressin differs from that of oxytocin at only two amino acid positions: the third and eighth amino acid residues. The result of these variations is а significant difference in physiological action.

Endorphins are peptides that bind at receptor sites in the brain to reduce pain. These compounds are synthesized by the brain itself. А subclass of such molecules, the enkephalins, are simple pentapeptides. Two important enkephalins are methionitlen  enkephalin (Tyr – Gly – Gly – Phe – Met ) and leucine enkephalin (Tyr – Gly – Gly – Phe – Leu).

The action of the prescription painkillers morphine and codeine is based on their binding at the same receptor sites in the brain as naturally occurring enkephalins.

Biuret  test. The protein is warmed gently with 10 % solution of sodium hydroxide and then а drop of very dilute copper sulphate solution is added, the formation of reddish - violet colour indicates the presence of peptide link, СО NH – . The test is given by all proteins, peptones and peptides. Its name is derived from the fact that the test is also positive for the compound biuret, Н2Х –CONH – CONH2 obtained from urea by heating.

It should be noted that dipeptides do not give the biuret test, while all other polypeptides do so. Hence biuret test is important to know whether hydrolysis of proteins is complete or not. If the biuret test is negative, hydrolysis is complete, at least to the dipeptide stage.

Xanthoproteic test. On treatment with concentrated nitric acid, certain proteins give yellow colour. This yellow colour is the same that is formed on the skin when the latter comes in contact with the concentrated nitric acid. The test is given only by the proteins having at least one mole of aromatic amino acid, such as tryptophan, phenylalanine, and tyrosine which are actually nitrated during treatment with concentrated nitric acid.

Millon's test. Protein on adding Millon's reagent (а solution of mercuric and mercurous nitrates in nitric acid containing а little nitrous acid) followed by heating the solution give а red precipitate or colour. The test is responded by the proteins having tyrosine. The hydroxyphenyl group of tyrosine is the structure responsible for this test. Moreover, the non-proteinous material having phenolic group also responds the test.

Foll reaction. This reaction reveals the sulfur containing amino acids (cysteine, cystine). Treatment of the sulfur containing amino acids with salt of lead and alkali yields a black sediment. 

Adamkevich reaction. This reaction detects the amino acid tryptophan containing indol ring. The addition of the concentrated acetic and sulfuric acids to the solution of tryptophan results in the formation of red-violet ring appearing on the boundary of different liquids.

Ninhydrin test.  The ninhydrin colour reaction is the most commonly test used for the detection of amino acids. This is an extremely delicate test, to which proteins, their hydrolytic products, and α-amino acids react. Although the test is positive for all free amino groups in amino acids, peptides, or proteins, the test is much weaker for peptides or proteins because not as many free groups are available as in amino acids. For certain amino acids the test is positive in dilutions as high as 1 part in 100,000 parts of water.

When ninhydrin is added to а protein solution and the mixture is heated to boil, blue to violet colour appears on cooling. The colour is due to the formation of а complex compound.

The test is also given by ammonia, ammonium salts, and certain amines. Ninhydrin is also used as а reagent for the quantitative determination of free carboxyl groups in solutions of amino acids.

Nitroprusside test. Proteins containing free -SH groups (of cysteine) give а reddish colour with sodium nitroprusside in ammonical solution.

Proteins are polypeptides that contain more than 50 amino acid units. The dividing line between а polypeptide and а protein is arbitrary. The important point is that proteins are polymers containing а large number of amino acid units linked by peptide bonds. Polypeptides are shorter chains of amino acids. Some proteins have molecular masses in the millions. Some proteins also contain more than one polypeptide chain.

To aid us in describing protein structure, we will consider four levels of substructure: primary, secondary, tertiary, and quaternary. Even though we consider these structure levels one by one, remember that it is the combination of all four levels of structure that controls protein function.

The primary structure of а protein is the sequence of amino acids present in its peptide chain or chains. Knowledge of primary structure tells us which amino acids are present, the number of each, their sequence, and the length and number of polypeptide chains.


The first protein whose primary structure was determined was insulin, the hormone that regulates blood-glucose level; а deficiency of insulin leads to diabetes.


The primary structure of а specific protein is always the same, regardless of where the protein is found within an organism. The structures of certain proteins are even similar among different species of animals. For example, the primary structures of insulin in cows, pigs, sheep, and horses are very similar both to each other and to human insulin. Until recently, this similarity was particularly important for diabetics who required supplemental injections of insulin.

The secondary structure of а protein is the arrangement in space of the atoms in the backbone of the protein. Three major types of protein secondary structure are known; the alpha helix, the beta pleated sheet, and the triple helix. The major force responsible for all three types of secondary structure is hydrogen bonding between а carbonyl oxygen atom of а peptide linkage and the hydrogen atom of an amino group (-NH) of another peptide linkage farther along the backbone. This hydrogen-bonding interaction may be diagrammed as follows:


Interactions responsible for tertiary structure. Four types of attractive interactions contribute to the tertiary structure of а protein:

(1)  covalent disulfide bonds,

(2)  electrostatic attractions (salt bridges),


(3)  hydrogen bonds,


(4)  hydrophobic attractions.

All four of these interactions are interactions between amino acid R groups. This is а major distinction between tertiary-structure interactions and secondary-structure interactions. Tertiary-structure interactions involve the R groups of amino acids; secondary-structure interactions involve the peptide linkages between amino acid units.

Disulfide bonds, the strongest of the tertiary-structure interactions, result from the –SH groups of two cysteine molecules reacting with each other to form а covalent disulfide. This type of interaction is the only one of the four tertiary-structure interactions that involves а covalent bond. That – SH groups are readily oxidized to give а disulfide bond, –  S –  S –. Disulfide bonds may involve two cysteine units in the same chain or in different chains.

Electrostatic interactions, also called salt bridges, always involve amino acids with charged side chains. These amino acids are the acidic and basic amino acids. The two R groups, one acidic and one basic, interact through ion — ion attractions. Figure.b shows an electrostatic interaction.

Hydrogen bonds can occur between amino acids with polar R groups. А variety of polar side chains can be involved, especially those that possess the following functional groups:

Hydrogen bonds are relatively weak and are easily disrupted by changes in pH and temperature. Hydrophobic interactions result when two nonpolar side chains are close to each other, In aqueous solution, many proteins have their polar R groups outward, toward the aqueous solvent (which is also polar), and their nonpolar R groups inward (away from the polar water molecules). The nonpolar R groups then interact with each other. Hydrophobic interactions are common between phenyl rings and alkyl side chains. Although hydrophobic interactions are weaker than hydrogen bonds or electrostatic interactions, they are a significant force in some proteins because there are so many of them; their cumulative effect can be greater in magnitude than the effects of hydrogen bonding.

Collagen, the most abundant of all proteins in humans (30% of total body protein), is а major structural material in tendons, ligaments, blood vessels, and skin; it is also the organic component of bones and teeth. The predominant structural feature within collagen molecules, three chains of amino acids wrapped (wound) into а triple helix, has already been considered.

The rich content of the amino acid proline (up to 20%) in collagen is one reason why it has а triple-helix conformation rather than the simpler а helix structure. Proline amino acid residues do not fit into regular а helices because of the cyclic' nature of the side chain present and its accompanying different "geometry."

An additional structural feature of collagen is the presence of the nonstandard amino acids 4-hydroxyproline (5%) and 5-hydroxylysine (1%) – derivatives of the standard amino acids proline and lysine.

 The presence of carbohydrate units (mostly glucose, galactose, and their disaccharides) attached by glycosidic linkages to collagen at its 5-hydroxylysine residues causes collagen to be classified as а glycoprotein. The function of the carbohydrate groups in collagen is related to cross-linking; they direct the assembly of collagen triple helices into more complex aggregations called collagen fibrils.

Collagen molecules (triple helices) are very long, thin, and rigid. Many such molecules, lined up alongside each other, combine to make collagen fibrils. Cross-linking between helices gives the fibrils extra strength. The greater the number of cross links, the more rigid the fibril is. The stiffening of skin and other tissues associated with aging is thought to result, at least in part, from an increasing amount of cross-linking between collagen molecules. The process of tanning, which converts animal hides to leather, involves increasing the degree of cross-linking.

When collagen is boiled in water, under basic conditions, it is converted to the water-soluble protein gelatin. This process involves both denaturation and hydrolysis. Heat acts as а denaturant, causing rupture of the hydrogen bonds supporting collagen's triple-helix structure. Regions in the amino acid chains where prolinе and hydroxyproline concentrations are high are particularly susceptible to hydrolysis, which breaks up the polypeptide chains. Meats become more tender when cooked because of the conversion of some collagen to gelatin. Tougher cuts of meat (more cross-linking), such as stew meat, need longer cooking times.

Immunoglobulins are among the most important and interesting of the soluble proteins in the human body. Immunoglobulins are glycoprotein molecules produced by an organism as а protective response to the invasion of microorganisms or foreign molecules. Different classes of immunoglobulins, identified by differing carbohydrate content and molecular mass, exist.

Immunoglobulins serve as antibodies to combat invasion of the body by antigens. Antigens are foreign substances, such as bacteria and viruses, that invade the body. Antibodies are molecules that counteract specific antigens. The immune system of the human body has the capability to produce immunoglobulins that respond to several thousand different antigens.

All types of immunoglobulin molecules have а similar basic structure, which includes the following features:

1. Four polypeptide chains are present: two identical heavy (Н) chains and two identical light (L) chains.

2. The Н chains, which usually contain 400 — 500 amino acid residues, are approximately twice as long as the Ь chains.

3. Both the Н and Ь chains have constant and variable regions. The constant regions have the same amino acid sequence from immunoglobulin to immunoglobulin, and the variable regions have а different amino acid sequence in each immunoglobulin.

4. The carbohydrate content of various immunoglobulins varies from 1% to 12% by mass.

5. The secondary and tertiary structures are similar for all immunoglobulins. They involve а Y-shaped conformation, with disulfide linkages between Н and L chains stabilizing the structure.

The interaction of an immunoglobulin molecule with an antigen occurs at the "tips" (upper-most part) of the Y structure. These tips are the variable-composition region of the immunoglobulin structure. It is here that the antigen binds specifically, and it is here that the amino acid sequence differs from one immunoglobulin to another.

Each immunoglobulin has two identical active sites and can thus bind to two molecules of the antigen it is "designed for." The action of many such immunoglobulins of given type in concert with each other creates an antigen – antibody complex that precipitates from solution. Eventually, an invading antigen can be eliminated from the body through such precipitation. The bonding of an antigen to the variable region of an immunoglobulin occurs through dipole — dipole interactions and hydrogen bonds rather than covalent bonds.

The importance of immunoglobulins is amply and tragically demonstrated by the effects of AIDS (acquired immune deficiency syndrome). The AIDS virus upsets the body's normal production of immunoglobulins and leaves the body susceptible to what would otherwise not be debilitating and deadly infections.

Individuals who receive organ transplants must be given drugs to suppress the production of immunoglobulins against foreign proteins in the new organ, thus preventing rejection of the organ. The major reason for the increasing importance of organ transplants is the successful development of drugs that can properly manipulate the body's immune system.

Many reasons exist for а mother to breast- еес1 а newborn infant. One of the most important is immunoglobulins. During the first two or three days of lactation, the breasts produce colostrum, а premilk substance containing immunoglobulins from the mother’s blood. Colostrum helps protect the newborn infant from those infections to which the mother has developed immunity. These diseases are the ones in her environment - precisely those the infant needs protection from. Breast milk, once it is produced, is а source of immunoglobulins for the infant for а short time. (After the first week of nursing, immunoglobulin concentrations in the milk decrease rapidly.) Infant formula used as a substitute for breast milk is almost always nutritionally equivalent, but it does not contain immunoglobulins.

Lipoproteins are conjugated proteins composed of both lipids and amino acids. The major function of such proteins is to help suspend lipids and transport them through the blood-stream. Lipids, in general; are insoluble in blood (an aqueous medium) because of their nonpolar nature.

The presence or absence of various types of lipoproteins in the blood appears to have implications for the health of the heart and blood vessels. Lipoprotein levels in the blood are now used as an indicator of heart attack risk.



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