BIOCHEMISTRY OF
CONNECTIVE TISSUE
Connective tissue
Connective tissue (CT) is a
kind of biological tissue that supports, connects, or separates different types
of tissues and organs of the body. It is one of the four general classes of
biological tissues—the others of which are epithelial, muscular, and nervous
tissues.
All CT has three main
components: cells, fibers, and extracellular matrices, all immersed in the body
fluids.
Connective tissue can be
broadly subdivided into connective tissue proper, special connective tissue,
and series of other, less classifiable types of connective tissues. Connective
tissue proper consists of loose connective tissue and dense connective tissue
(which is further subdivided into dense regular and dense irregular connective
tissues.) Special connective tissue consists of reticular connective tissue,
adipose tissue, cartilage, bone, and blood. Other kinds of connective tissues
include fibrous, elastic, and lymphoid connective tissues.
Fibroblasts are the cells
responsible for the production of some CT.
Type-I collagen, is present
in many forms of connective tissue, and makes up about 25% of the total protein
content of the mammalian body.
BIOMEDICAL
IMPORTANCE
Most mammalian cells are located in tissues where
they are surrounded by a complex extracellular matrix (ECM) often
referred to as “connective tissue.” The ECM contains three major classes
of biomolecules:
(1)
the structural
proteins, collagen, elastin, and fibrillin;
(2) certain specialized proteins such as
fibrillin, fibronectin, and laminin; and
(3) proteoglycans, whose chemical natures
are described below.
The extracellular matrix
The ECM has been found to be involved in many
normal and pathologic processes—eg, it
plays important roles in development, in inflammatory states, and in the spread
of cancer cells. Involvement of certain components of the ECM has been
documented in both rheumatoid arthritis and osteoarthritis. Several
diseases (eg, osteogenesis imperfecta and a number of types of the
Ehlers-Danlos syndrome) are due to genetic disturbances of the synthesis of
collagen. Specific components of proteoglycans (the glycosaminoglycans; GAGs)
are affected in the group of genetic disorders known as the mucopolysaccharidoses.
Changes occur in the ECM during the aging
process. This chapter describes the basic biochemistry of the three major
classes of biomolecules found in the ECM and illustrates their biomedical
significance.
Major biochemical features of two specialized
forms of ECM—bone and cartilage—and of a number of diseases involving them are
also briefly considered.
Functions of connective tissue
- Storage of energy
- Protection of organs
- Provision of structural
framework for the body
- Connection of body tissues
- Connection of epithelial
tissues to muscle tissues
Characteristics
of connective tissue and fiber types
Cells are spread through an extracellular fluid.
Ground substance - A clear, colorless, and
viscous fluid containing glycosaminoglycans and proteoglycans to fix the
bodywater and the collagen fibers in the intercellular spaces. Ground substance
slows the spread of pathogens.
Fibers. Not all types of CT are fibrous. Examples include
adipose tissue and blood. Adipose tissue gives "mechanical
cushioning" to our body, among other functions. Although there is no dense
collagen network in adipose tissue, groups of adipose cells are kept together
by collagen fibers and collagen sheets in order to keep fat tissue under
compression in place (for example, the sole of the foot). The matrix of blood
is plasma.
Both the ground substance and proteins (fibers)
create the matrix for CT.
Types of fibers:Tissue → Purpose → Components → Location
Collagenous fibers → Alpha
polypeptide chains → tendon, ligament, skin, cornea, cartilage, bone,
blood vessels, gut, and intervertebral disc.
Elastic fibers → elastic
microfibril & elastin → extracellular
matrix
Reticular fibers → Type-III
collagen → liver, bone marrow,
lymphatic organs.
Structure and functions of collagen.
Collagen is a group of naturally occurring proteins
found in animals,
especially in the flesh and connective
tissues of vertebrates.
It is the main
component of connective tissue, and is the most abundant
protein in mammals, making up about 25% to 35% of the whole-body protein
content. Collagen, in the form of elongated fibrils, is
mostly found in fibrous tissues such as tendon, ligament
and skin, and is also abundant in cornea, cartilage, bone, blood vessels, the
gut, and intervertebral disc. The fibroblast
is the most common cell which creates collagen. In muscle tissue, it serves as
a major component of the endomysium.
Fig.Functions of collagen
Collagen constitutes
one to two percent of muscle tissue, and accounts for 6% of the weight of
strong, tendinous muscles. Gelatin, which is used in food and industry, is collagen that
has been irreversibly hydrolyzed. Collagen is composed of a triple helix, which
generally consists of two identical chains (α1) and an additional chain
that differs slightly in its chemical composition (α2). The amino acid
composition of collagen is atypical for proteins, particularly with respect to
its high hydroxyproline content. The most common motifs
in the amino acid sequence of collagen are glycine-proline-X
and glycine-X-hydroxyproline, where X is any amino acid other than glycine,
proline or hydroxyproline.
Figure 1. Collagen structures
Synthesis
First, a three dimensional stranded structure
is assembled, with the amino acids glycine and proline as its principal
components. This is not yet collagen but its precursor, procollagen. A recent
study shows that vitamin C must have an important role in its synthesis.
Prolonged exposure of cultures of human connective-tissue cells to ascorbate
induced an eight-fold increase in the synthesis of collagen with no increase in
the rate of synthesis of other proteins. Since the production of procollagen
must precede the production of collagen, vitamin C must have a role in this
step. The conversion involves a reaction that substitutes a hydroxyl group, OH,
for a hydrogen atom, H, in the proline residues at certain points in the
polypeptide chains, converting those residues to hydroxyproline. This hydroxylation
reaction organizes the chains in the conformation necessary for them to form a
triple helix. The hydroxylation, next, of the residues of the amino acid
lysine, transforming them to hydroxylysine, is then needed to permit the
cross-linking of the triple helices into the fibers and networks of the
tissues.
Figure Collagen
Synthesis
These hydroxylation reactions are
catalyzed by two different enzymes: prolyl-4-hydroxylase and lysyl-hydroxylase.
Vitamin C also serves with them in inducing these reactions. in this service,
one molecule of vitamin C is destroyed for each H replaced by OH. The synthesis
of collagen occurs inside and outside of the cell. The formation of collagen
which results in fibrillary collagen (most common form) is discussed here.
Meshwork collagen, which is often involved in the formation of filtration
systems is the other form of collagen. It should be noted that all types of
collagens are triple helixes, and the differences lie in the make-up of the
alpha peptides created in step 2.
1.
Transcription of
mRNA: There are approximately 34 genes
associated with collagen formation, each coding for a specific mRNA sequence,
and typically have the "
2.
Pre-pro-peptide
Formation: Once the final mRNA exits from the
cell nucleus and enters into the cytoplasm it links with the ribosomal subunits
and the process of translation occurs. The early/first part of the new peptide
is known as the signal sequence. The signal sequence on the N-terminal
of the peptide is recognized by a signal recognition particle on the
endoplasmic reticulum, which will be responsible for directing the
pre-pro-peptide into the endoplasmic reticulum. Therefore, once the synthesis
of new peptide is finished, it goes directly into the endoplasmic reticulum for
post-translational processing. Note that it is now known as pre-pro-collagen.
Fig. Biosynthesis of collagen from
Preprocollagen
3.
Alpha Peptide to
Procollagen: Three modifications of the
pre-pro-peptide occur leading to the formation of the alpha peptide. Secondly,
the triple helix known as procollagen is formed before being transported in a
transport vesicle to the golgi apparatus. 1) The signal peptide on the
N-terminal is dissolved, and the molecule is now known as propeptide
(not procollagen). 2) Hydroxylation of lysines and prolines on propeptide by
the enzymes prolyl hydroxylase and lysyl hydroxylase (to produce
hydroxyproline and hydroxylysine) occurs to aid crosslinking of the alpha
peptides. It is this enzymatic step that requires vitamin C as a cofactor. In
scurvy, the lack of hydroxylation of prolines and lysines causes a looser
triple helix (which is formed by 3 alpha peptides). 3) Glycosylation occurs by
adding either glucose or galactose monomers onto the hydroxy groups that were
placed onto lysines, but not on prolines. From here the hydroxylated and
glycosylated propeptide twists towards the left very tightly and then three
propeptides will form a triple helix. It is important to remember that this
molecule, now known as procollagen (not propeptide) is composed of a
twisted portion (center) and two loose ends on either end. At this point the
procollagen is packaged into a transfer vesicle destined for the golgi
apparatus.
4.
Golgi Apparatus
Modification: In the golgi apparatus, the procollagen
goes through one last post-translational modification before being secreted out
of the cell. In this step oligosaccharides (not monosaccharides like in step 3)
are added, and then the procollagen is packaged into a secretory vesicle
destined for the extracellular space.
5.
Formation of
Tropocollagen: Once outside the cell, membrane
bound enzymes known as collagen peptidases, remove the "loose
ends" of the procollagen molecule. What is left is known as tropocollagen.
Defect in this step produces one of the many collagenopathies known as Ehlers-Danlos syndrome. This step is
absent when synthesizing type III, a type of fibrilar collagen.
6.
Formation of the Collagen
Fibril: Lysyl oxidase an
extracellular enzyme produces the final step in the collagen synthesis pathway.
This enzyme acts on lysines and hydroxylysines producing aldehyde groups, which
will eventually undergo covalent bonding between tropocollagen molecules. This
polymer of tropocollogen is known as a collagen fibril.
Amino acids
Collagen has an unusual amino acid
composition and sequence:
·
Glycine
is found at almost every third residue
·
Proline
(Pro) makes up about 17% of collagen
·
Collagen contains
two uncommon derivative amino acids not directly inserted during translation. These amino acids are found
at specific locations relative to glycine and are modified post-translationally
by different enzymes, both of which require vitamin C
as a cofactor.
o
Hydroxyproline
(Hyp), derived from proline.
o
Hydroxylysine
(Hyl), derived from lysine
(
Cortisol
stimulates degradation of (skin) collagen into amino
acids.
Collagen I formation
Most collagen forms in a similar
manner, but the following process is typical for type I:
1.
Inside the cell
1.
Two types of peptide
chains are formed during translation on ribosomes along the rough endoplasmic reticulum (RER): alpha-1
and alpha-2 chains. These peptide chains (known as preprocollagen) have registration peptides on
each end and a signal peptide.
2.
Polypeptide chains
are released into the lumen of the RER.
3.
Signal peptides are cleaved
inside the RER and the chains are now known as pro-alpha chains.
4.
Hydroxylation
of lysine
and proline
amino acids occurs inside the lumen. This process is dependent on ascorbic acid
(Vitamin C) as a cofactor.
5.
Glycosylation
of specific hydroxylysine residues occurs.
6.
Triple ɣ helical structure is formed inside the endoplasmic
reticulum from each two alpha-1 chains and one alpha-2 chain.
7.
Procollagen
is shipped to the Golgi apparatus, where it is packaged and
secreted by exocytosis.
2.
Outside the cell
1.
Registration
peptides are cleaved and tropocollagen is formed by procollagen peptidase.
2.
Multiple
tropocollagen molecules form collagen fibrils, via covalent cross-linking (aldol
reaction) by lysyl oxidase which links hydroxylysine and
lysine residues. Multiple collagen fibrils form into collagen fibers.
3.
Collagen may be
attached to cell membranes via several types of protein, including fibronectin
and integrin.
Synthetic pathogenesis
Vitamin C deficiency
causes scurvy,
a serious and painful disease in which defective collagen prevents the formation of
strong connective tissue. Gums
deteriorate and bleed, with loss of teeth; skin discolors, and wounds do not heal. Prior
to the eighteenth century, this condition was notorious among long duration
military, particularly naval, expeditions during which participants were deprived
of foods containing Vitamin C.
An autoimmune disease such as lupus erythematosus or rheumatoid arthritis may attack healthy
collagen fibers.
Many bacteria and
viruses have virulence factors which destroy collagen (such as the enzyme collagenase)
or interfere with its production.
Molecular structure
The tropocollagen or collagen
molecule is a subunit of larger collagen aggregates such as fibrils. At
approximately 300 nm long and 1.5 nm in diameter, it is made up of
three polypeptide
strands (called alpha peptides, see step 2), each possessing the conformation
of a left-handed helix
(its name is not to be confused with the commonly occurring alpha helix, a
right-handed structure). These three left-handed helices are twisted together
into a right-handed coiled coil, a triple helix or "super
helix", a cooperative quaternary structure stabilized by
numerous hydrogen bonds. With type I collagen and
possibly all fibrillar collagens if not all collagens, each triple-helix
associates into a right-handed super-super-coil referred to as the collagen
microfibril. Each microfibril is interdigitated with its neighboring
microfibrils to a degree that might suggest they are individually unstable,
although within collagen fibrils, they are so well ordered as to be
crystalline.
A distinctive feature of collagen is
the regular arrangement of amino acids in each of the three chains of
these collagen subunits. The sequence often follows the pattern Gly-Pro-X
or Gly-X-Hyp, where X may be any of various other amino
acid residues. Proline or hydroxyproline constitute about 1/6 of the total
sequence. With glycine accounting for the 1/3 of the sequence, this means
approximately half of the collagen sequence is not glycine, proline or
hydroxyproline, a fact often missed due to the distraction of the unusual GX1X2
character of collagen alpha-peptides. The high glycine content of collagen is
important with respect to stabilization of the collagen helix as this allows
the very close association of the collagen fibers within the molecule,
facilitating hydrogen bonding and the formation of intermolecular cross-links.
This kind of regular repetition and high glycine content is found in only a few
other fibrous proteins, such as silk fibroin. About 75-80% of silk is (approximately) -Gly-
Because glycine is the smallest amino
acid with no side chain, it plays a unique role in fibrous structural proteins.
In collagen, Gly is required at every third position because the assembly of
the triple helix puts this residue at the interior (axis) of the helix, where
there is no space for a larger side group than glycine’s single hydrogen atom.
For the same reason, the rings of the Pro and Hyp must point outward. These two
amino acids help stabilize the triple helix—Hyp even more so than Pro; a lower
concentration of them is required in animals such as fish, whose body
temperatures are lower than most warm-blooded animals. Lower proline
and hydroxyproline contents are characteristic of cold-water, but not warm-water
fish; the latter tend to have similar proline and hydroxyproline contents to
mammals. The lower proline and hydroxproline contents of cold-water fish and
other poikilotherm animals leads to their collagen
having a lower thermal stability than mammalian collagen. This lower thermal
stability means that gelatin derived from fish collagen is not suitable for many
food and industrial applications.
The tropocollagen subunits
spontaneously self-assemble, with regularly staggered
ends, into even larger arrays in the extracellular
spaces of tissues. In the fibrillar collagens, the molecules are staggered from
each other by about 67 nm (a unit that is referred to as ‘D’ and changes depending
upon the hydration state of the aggregate). Each D-period contains four plus a
fraction collagen molecules, because 300 nm divided by 67 nm does not
give an integer (the length of the collagen molecule divided by the stagger
distance D). Therefore, in each D-period repeat of the microfibril, there is a
part containing five molecules in cross-section, called the “overlap”, and a
part containing only four molecules, called the "gap". The
triple-helices are also arranged in a hexagonal or quasihexagonal array in
cross-section, in both the gap and overlap regions.
There is some covalent
crosslinking within the triple helices, and a variable amount of covalent
crosslinking between tropocollagen helices forming well organized aggregates
(such as fibrils). Larger fibrillar bundles are formed with the aid of several
different classes of proteins (including different collagen types),
glycoproteins and proteoglycans to form the different types of mature tissues
from alternate combinations of the same key players. Collagen's insolubility
was a barrier to the study of monomeric collagen until it was found that
tropocollagen from young animals can be extracted because it is not yet fully crosslinked.
However, advances in microscopy techniques (i.e. electron microscopy (EM) and
atomic force microscopy (AFM)) and X-ray diffraction have enabled researchers
to obtain increasingly detailed images of collagen structure in situ. These later advances are
particularly important to better understanding the way in which collagen
structure affects cell-cell and cell-matrix communication, and how tissues are
constructed in growth and repair, and changed in development and disease. For
example using AFM –based nanoindentation it has been shown that a single
collagen fibril is a heterogeneous material along its axial direction with
significantly different mechanical properties in its gap and overlap regions,
correlating with its different molecular organizations in these two regions.
Collagen
fibrils are semicrystalline aggregates of collagen
molecules. Collagen fibers are bundles of fibrils.
Fig. Synthesis Of
Collagen
Collagen fibrils/aggregates are
arranged in different combinations and concentrations in various tissues to provide
varying tissue properties. In bone, entire collagen triple helices lie in a
parallel, staggered array. 40 nm gaps between the ends of the
tropocollagen subunits (approximately equal to the gap region) probably serve
as nucleation sites for the deposition of long, hard, fine crystals of the
mineral component, which is (approximately) Ca10(OH)2(PO4)6.
Type I collagen gives bone its tensile
strength.
Types and associated disorders
Collagen occurs in many places
throughout the body. Over 90% of the collagen in the body, however, is of type
I.
So far, 28 types of
collagen have been identified and described. The five most common types are:
·
Collagen I: skin,
tendon, vascular ligature, organs, bone (main component of the organic part of
bone)
·
Collagen II: cartilage
(main component of cartilage)
·
Collagen III:
reticulate (main component of reticular
fibers), commonly found alongside type I.
·
Collagen IV: forms
bases of cell basement membrane
·
Collagen V: cell
surfaces, hair and placenta
Collagen-related
diseases most commonly arise from genetic defects or nutritional deficiencies
that affect the biosynthesis, assembly, postranslational modification,
secretion, or other processes involved in normal collagen production.
Tabl. Types Of Collagen
A Number of Genetic Diseases Result From
Abnormalities in the Synthesis of Collagen
About 30 genes encode collagen, and its pathway
of biosynthesis is complex, involving at least eight enzyme-catalyzed
posttranslational steps. Thus, it is not
surprising that a number of diseases are due to mutations in collagen
genes or in genes encoding some of the enzymes involved in these
posttranslational
modifications. The diseases affecting bone (eg,
osteogenesis imperfecta) and cartilage (eg, the chondrodysplasias) will be discussed
later in this chapter.
Ehlers-Danlos syndrome comprises a group of inherited disorders whose
principal clinical features are hyperextensibility of the skin, abnormal tissue
fragility,
and increased joint
mobility. The clinical picture is variable, reflecting underlying extensive
genetic heterogeneity.
At least 10 types have been recognized, most but not
all of which reflect a variety of lesions in the synthesis of collagen. Type
IV is the most serious because of its tendency for spontaneous rupture of
arteries or the bowel, reflecting abnormalities in type III collagen.
Patients with type VI, due to a deficiency of
lysyl hydroxylase, exhibit marked joint hypermobility and a tendency to ocular
rupture. A deficiency of procollagen
N-proteinase,
causing formation of abnormal thin, irregular collagen fibrils, results in type
VIIC, manifested by marked joint hypermobility and soft skin.
Alport syndrome is the designation applied to a number of genetic
disorders (both X-linked and autosomal) affecting the structure of type IV
collagen fibers, the major collagen found in the basement membranes of the
renal glomeruli (see discussion of laminin, below). Mutations in several genes
encoding type IV collagen fibers have been demonstrated. The presenting sign is
hematuria, and patients may eventually develop end-stage renal disease.
Electron microscopy reveals characteristic abnormalities of the structure of
the basement membrane and lamina densa.
In epidermolysis bullosa, the skin breaks and
blisters as a result of minor trauma. The dystrophic form is due to mutations
in COL7A1, affecting the structure of type VII collagen. This collagen
forms delicate fibrils that anchor the basal lamina to collagen fibrils in the
dermis. These anchoring fibrils have been shown to be markedly reduced in this
form of the disease, probably resulting in the blistering. Epidermolysis
bullosa simplex, another variant, is due to mutations in keratin 5.
Scurvy affects
the structure of collagen. However, it is due to a deficiency of ascorbic acid
and is not a genetic disease. Its major signs are bleeding gums, subcutaneous hemorrhages, and poor wound
healing. These signs reflect impaired synthesis of collagen due to deficiencies
of prolyl and lysyl hydroxylases, both of which require ascorbic acid as a
cofactor.
Osteoporosis - Not inherited genetically, brought on with age,
associated with reduced levels of collagen in the skin and bones, growth hormone
injections are being researched as a possible treatment to counteract any loss
of collagen.
Knobloch syndrome - Caused by a mutation in the collagen XVIII gene,
patients present with protrusion of the brain tissue and degeneration of the
retina, an individual who has family members suffering from the disorder are at
an increased risk of developing it themselves as there is a hereditary link.
Characteristics
Collagen is one of
the long, fibrous structural proteins whose functions are
quite different from those of globular
proteins such as enzymes. Tough bundles of collagen called collagen fibers are a major component
of the extracellular matrix that supports most
tissues and gives cells structure from the outside, but collagen is also found
inside certain cells. Collagen has great tensile
strength, and is the main component of fascia, cartilage,
ligaments,
tendons,
bone and skin. Along with
soft keratin,
it is responsible for skin strength and elasticity, and its
degradation leads to wrinkles that accompany aging. It strengthens blood vessels
and plays a role in tissue development. It is present in the cornea and lens
of the eye
in crystalline
form.
Uses
Collagen has a wide variety of
applications, from food to medical. For instance, it is used in cosmetic
surgery and burns surgery. It is widely used in the form of
collagen casings for sausages.
If collagen is sufficiently
denatured, e.g. by heating, the three tropocollagen strands separate partially
or completely into globular domains, containing a different secondary structure
to the normal collagen polyproline II (PPII), e.g. random coils.
This process describes the formation of gelatin,
which is used in many foods,
including flavored gelatin desserts. Besides food, gelatin has
been used in pharmaceutical, cosmetic, and photography industries. From a
nutritional point of view, collagen and gelatin are a poor-quality sole source
of protein since they do not contain all the essential amino acids in the proportions
that the human body requires—they are not 'complete
proteins' (as defined by food science, not that they are partially
structured). Manufacturers of collagen-based dietary supplements claim that their products
can improve skin and fingernail quality as well as joint health. However,
mainstream scientific research has not shown strong evidence to support these
claims. Individuals with problems in these areas are more likely to be
suffering from some other underlying condition (such as normal aging, dry skin,
arthritis etc.) rather than just a protein deficiency.
From the Greek for glue, kolla, the word collagen means "glue
producer" and refers to the early process of boiling the skin and sinews of horses and other animals to
obtain glue. Collagen adhesive was used by Egyptians about 4,000
years ago, and Native Americans used it
in bows
about 1,500 years ago. The oldest glue in the world, carbon-dated as more than 8,000 years old, was
found to be collagen—used as a protective lining on rope baskets and embroidered
fabrics,
and to hold utensils together; also in crisscross
decorations on human
skulls.
Collagen normally converts to gelatin, but survived due to the dry conditions.
Animal glues are thermoplastic, softening again upon reheating,
and so they are still used in making musical instruments such as fine violins and guitars, which
may have to be reopened for repairs—an application incompatible with tough, synthetic plastic
adhesives, which are permanent. Animal sinews and skins, including leather,
have been used to make useful articles for millennia.
Elastin – main protein of elastic fibrils, structure and biological role.
Elastin is a protein in connective
tissue that is elastic and allows many tissues in the
body to resume their shape after stretching or contracting. Elastin helps skin
to return to its original position when it is poked or pinched. Elastin is also
an important load-bearing tissue in the bodies of vertebrates and used in
places where mechanical energy is required to be stored. In humans, elastin is
encoded by the ELN gene.
Function
This gene encodes a protein that is
one of the two components of elastic fibers. The encoded protein is rich in hydrophobic
amino acids such as glycine and proline, which form mobile hydrophobic regions bounded by
crosslinks between lysine
residues. Multiple transcript variants encoding different isoforms have been
found for this gene. The other name for elastin is tropoelastin. The
characterization of disorder is consistent with an entropy-driven mechanism of
elastic recoil. It is concluded that conformational disorder is a constitutive
feature of elastin structure and function.
Clinical significance
Deletions and
mutations in this gene are associated with supravalvular aortic stenosis (SVAS) and autosomal dominant cutis laxa.
Other associated defects in elastin include Marfan's Syndrome and emphysema
caused by α1-antitrypsin deficiency.
Composition
Elastic fiber is
composed of the protein fibrillin and elastin made of simple amino acids
such as glycine,
valine,
alanine,
and proline.
The total elastin ranges from 58 to 75% of the weight of the dry defatted
artery in normal canine arteries.[6]
Comparison between fresh and digested tissues shows that, at 35% strain, a
minimum of 48% of the arterial load is carried by elastin, and a minimum of 43%
of the change in stiffness of arterial tissue is due to the change in elastin
stiffness. Elastin is made by linking many soluble tropoelastin
protein molecules, in a reaction catalyzed
by lysyl oxidase,
to make a massive insoluble, durable cross-linked
array. The amino acid responsible for these cross-links is lysine.
Tropoelastin is a specialized protein with a molecular weight of 64 to 66 kDa,
and an irregular or random coil conformation made up of 830 amino acids.
Desmosine
and isodesmosine
are types of links for the tropoelastin molecules.
Elastic Fibers synthesis
Tissue distribution
Elastin serves an important function
in arteries
as a medium for pressure wave propagation to help blood flow
and is particularly abundant in large elastic blood vessels such as the aorta. Elastin is also
very important in the lungs,
elastic
ligaments, the skin, and the bladder,
elastic cartilage. It is present in all
vertebrates above the jawless fish.
Table
1 summarizes the main differences between collagen
and elastin.
Table 1. Major differences between collagen and elastin.
In contrast
to collagen, which forms fibers that are tough and have high tensile strength,
elastin is a connective tissue protein with rubber-like properties. Elastic
fibers composed of elastin and glycoprotein microfibrils are found in the
lungs, the walls of large arteries, and elastic ligaments.
They can be
stretched to several times their normal length, but recoil to their original
shape when the stretching force is relaxed.
A. Structure
of elastin
Elastin
is an insoluble protein polymer synthesized from a precursor, tropoelastin,
which is a linear polypeptide composed of about 700 amino acids that are
primarily small and nonpolar (for example, glycine, alanine, and valine).
Elastin is also rich in proline and lysine, but contains only a little
hydroxyproline and hydroxy lysine.
Tropoelastin
is secreted by the cell into the extracellular space. There it interacts with
specific glycoprotein microfibrils, such as fibrillin, which function as a
scaffold onto which tropoelastin is deposited. Some of the lysyl side chains of
the tropoelastin poly peptides are oxidatively deaminated by lysyl oxidase,
forming allysine residues. Three of the allysyl side chains plus one unaltered
lysyl side chain from the same or neighboring polypeptides form a desmosine
cross-link (Figure 4.12). This produces elastin—an extensively interconnected,
rubbery network that can stretch and bend in any direction when stressed,
giving connective tissue elasticity (Figure 4.13). Mutations in the fibrillin-1
protein are responsible for Marfan syndrome—a connective tissue disorder
characterized by impaired structural integrity in the skeleton, the eye, and
the cardiovascular system. With this disease, abnormal fibrillin protein is
incorporated into microfibrils along with normal fibrillin, inhibiting the
formation of functional microfibrils. [Note: Patients with OI, EDS, or Marfan
syndrome may have blue sclera due to tissue thinning that allows underlying
pigment to show through.]
B. Role of 1-antitrypsin
in elastin degradation
1.
1-Antitrypsin: Blood and other body fluids contain a protein, α1-antitrypsin
(α1-AT,
A1AT, currently also called α1-antiproteinase), that inhibits a number
of proteolytic enzymes (also called proteases or proteinases) that hydrolyze
and destroy proteins. [Note: The inhibitor was originally named α1-antitrypsin
because it inhibits the activity of trypsin (a proteolytic enzyme synthesized
as trypsinogen by the pancreas] α1-AT comprises more than 90% of the α1-globulin
fraction of normal plasma. α1-AT has the important physiologic
role of inhibiting neutrophil elastase––a powerful protease that is released
into the extracellular space, and degrades elastin of alveolar walls, as well
as other structural proteins in a variety of tissues (Figure 4.14). Most of the
α1-AT
found in plasma is synthesized and secreted by the liver. The remainder is
synthesized by several tissues, including monocytes and alveolar macrophages,
which may be important in the prevention of local tissue injury by elastase.
2. Role of
1-AT in the lungs: In the normal lung, the alveoli are
chronically exposed to low levels of neutrophil elastase released from
activated and degenerating neutrophils. This proteolytic activity can destroy
the elastin in alveolar walls if unopposed by the action of α1-AT,
the most important inhibitor of neutrophil elastase (see Figure 4.14). Because
lung tissue cannot regenerate, emphysema results from the destruction of the
connective tissue of alveolar walls.
3. Emphysema
resulting from 1-AT deficiency:
In the
The
polymerization of the mutated protein in the endoplasmic reticulum of
hepatocytes causes decreased secretion of
α1-AT by the liver. The accumulated polymer may result
in cirrhosis (scarring of the liver). In the
An individual
must inherit two abnormal α1-AT alleles to be at risk for the
development of emphysema. In a heterozygote, with one normal and one defective
gene, the levels of α1-AT are sufficient to protect the alveoli from
damage. [Note: A specific α1-AT methionine is required for the
binding of the inhibitor to its target proteases.
Smoking
causes the oxidation and subsequent inactivation of that methionine residue,
thereby rendering the inhibitor powerless to neutralize elastase. Smokers with α1-AT
deficiency, therefore, have a considerably elevated rate of lung destruction
and a poorer survival rate than nonsmokers with the deficiency.] The deficiency
of elastase inhibitor can be reversed by augmentation therapy—weekly
intravenous administration of α1-AT. The α1-AT diffuses
from the blood into the lung, where it reaches therapeutic levels in the fluid
surrounding the lung epithelial cells.
Laminin Structure
§
cross-shaped glycoprotein
§
3 polypeptides a, b1, b2
§
carbohydrate (13% by weight)
§
Mr 900K
§
separate binding domains
§
collagen IV
§
heparin
§
heparin sulphate
§
cell binding
§
cell specific binding -
liver, nerve
§
cell surface receptor
Laminin Function
§
cell adhesion
§
migration pathways
§
stimulates growth of axons
§
development and regeneration
§
differentiation
§
basal laminae
§
most abundant linking glycoprotein
PROTEOGLYCANS & GLYCOSAMINOGLYCANS
The
Glycosaminoglycans Found in Proteoglycans Are Built Up of Repeating
Disaccharides
Proteoglycans are proteins that contain covalently linked
glycosaminoglycans. A number of them have been characterized and given names
such as syndecan, betaglycan, serglycin, perlecan, aggrecan, versican, decorin,
biglycan, and fibromodulin. They vary in tissue distribution, nature of the
core protein, attached glycosaminoglycans, and function. The proteins bound
covalently to glycosaminoglycans are called “core proteins”; they have
proved difficult to isolate and characterize, but the use of recombinant DNA
technology is beginning to yield important information about their structures.
The amount of carbohydrate in a proteoglycan is usually much greater than is
found in a glycoprotein and may comprise up to 95% of its weight. Figures 2 and
3 show the general structure of one particular proteoglycan, aggrecan, the
major type found in cartilage.
Figure 2. Dark field electron micrograph of a proteoglycan aggregate in
which the proteoglycan subunits
and filamentous backbone are particularly well extended.
It is very large (about 2 × 103 kDa), with its
overall structure resembling that of a bottle brush. It contains a long strand
of hyaluronic acid (one type of GAG) to which link proteins are attached
noncovalently.
Figure 3. Schematic
representation of the proteoglycan aggrecan.
In turn, these latter interact noncovalently with core
protein molecules from which chains of other GAGs (keratan sulfate and
chondroitin sulfate in this case) project. More details on this macromolecule
are given when cartilage is discussed below.
There are at least seven glycosaminoglycans (GAGs):
hyaluronic
acid, chondroitin sulfate, keratan sulfates
I and II, heparin, heparan sulfate, and dermatan sulfate. A GAG is an unbranched polysaccharide made
up of repeating disaccharides, one component of which is always an amino
sugar (hence the name GAG), either D-glucosamine
or D-galactosamine. The other component of the repeating
disaccharide (except in the case of keratan sulfate) is a uronic acid,
either L-glucuronic acid (GlcUA) or its 5′-epimer, L-iduronic acid
(IdUA). With the exception of hyaluronic acid, all the GAGs contain
sulfate groups, either as O-esters or as N-sulfate (in heparin and
heparan sulfate).
Hyaluronic acid affords another exception because
there is no clear evidence that it is attached covalently to protein, as the
definition of a proteoglycan given above specifies. Both GAGs and proteoglycans
have proved difficult to work with, partly because of their complexity.
However, they are major components of the ground substance; they have a number
of important biologic roles; and they are involved in a number of disease
processes—so that interest in them is increasing rapidly.
Proteoglycans
(mucoproteins) are formed of glycosaminoglycans (GAGs) covalently attached to
the core proteins.
They are found in all connective tissues, extracellular matrix (ECM) and on the surfaces of many cell types.
Proteoglycans are remarkable for their diversity (different cores, different numbers of GAGs with
various lenghts and compositions).
Glycosaminoglycans forming the proteoglycans are the most abundant heteropolisaccharides in
the body. They are long unbranched molecules containing a repeating disaccharide unit.
Usually one sugar is an uronic acid
(either D-glucuronic or L-iduronic) and the other is either GlcNAc
or GalNAc. One or both sugars contain sulfate groups (the only exception is hyaluronic acid).
GAGs are highly
negatively charged what is essential for their function.
THE
SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE ARE :
Hyaluronic
acid (D-glucuronate + GlcNAc)
Occurence : synovial fluid,
ECM of loose connective tissue
Hyaluronic acid is unique
among the GAGs because it does not contain any sulfate and is not found
covalently attached to proteins. It forms non-covalently linked complexes
with proteoglycans in the ECM. Hyaluronic acid
polymers are very large (100 - 10,000 kD) and can displace a large volume of
water.
Dermatan sulfate (L-iduronate + GlcNAc
sulfate)
Occurence : skin,
blood vessels, heart valves
Chondroitin sulfate (D-glucuronate
+ GalNAc sulfate)
Occurence : cartilage, bone,
heart valves ; It is the most abundant GAG.
Heparin and heparan sulfate (D-glucuronate
sulfate + N-sulfo-D-glucosamine)
Heparans have less sulfate groups than heparins
Occurence :
·
Heparin :component of intracellular granules of mast cells lining the arteries of
the lungs, liver and skin.
Figure. Structure of heparin. The
polymer section illustrates structural features typical of heparin; however, the sequence of variously
substituted repeating disaccharide units has been arbitrarily selected. In addition, non-O-sulfated or
3-O-sulfated glucosamine residues may also occur.
·
Heparan sulfate: basement membranes, component of cell surfaces.
Keratan sulfate ( Gal + GlcNAc sulfate)
Occurence : cornea,
bone, cartilage;
Keratan sulfates are often aggregated with
chondroitin sulfates.
Structure of proteoglycans
The GAGs extend perpendicular from the core protein
in a bottlebrush- like structure.
The linkage of GAGs such as (heparan sulfates and
chondroitin sulfates) to the protein core involves a specific trisaccharide
linker :
Some forms of keratan sulfates are linked to
the protein core through an N-asparaginyl bond.
The protein cores of proteoglycans are rich in Ser
and Thr residues which allows multiple GAG attachment.
Proteoglycans Have Numerous Functions
As indicated above, proteoglycans are remarkably complex molecules and
are found in every tissue of the body, mainly in the ECM or “ground substance.”
There they are associated with each other and also with the other
major structural components of the matrix, collagen and elastin, in quite specific manners.
Some proteoglycans
bind to collagen and others to elastin.
These interactions are important in determining the structural
organization of the matrix. Some proteoglycans (eg, decorin) can also bind growth factors such as TGF-â, modulating their effects on cells. In addition, some of them
interact with certain adhesive proteins such as fibronectin and laminin (see above), also
found in the
matrix. The GAGs present in the proteoglycans are polyanions and hence bind polycations and
cations such as Na+
and K+. This latter ability attracts water by osmotic pressure into the extracellular matrix and
contributes to its turgor. GAGs also gel at relatively low concentrations.
Because of the long extended nature of the polysaccharide chains of GAGs and their ability
to gel, the
proteoglycans can act as sieves, restricting the passage of large macromolecules into the ECM but
allowing relatively
free diffusion of small molecules.
Again, because of their extended structures and the huge
macromolecular aggregates they often form, they occupy a large volume of the matrix relative to
proteins.
SOME FUNCTIONS OF SPECIFIC GAGS & PROTEOGLYCANS
Hyaluronic acid is especially
high in concentration in embryonic tissues and is thought to play an
important role in permitting cell migration during
morphogenesis and wound repair. Its ability to attract water into
the extracellular
matrix and thereby “loosen it up” may be important in this regard. The high concentrations
of hyaluronic
acid and chondroitin sulfates present in cartilage contribute to
its compressibility (see below).
Chondroitin sulfates are located
at sites of calcification in endochondral bone and are also found in
cartilage.
They are also located inside certain neurons and may provide
an endoskeletal structure, helping to maintain their shape.
Both keratan
sulfate I and dermatan sulfate are present in
the cornea. They lie between collagen fibrils and play a critical role in corneal transparency.
Changes in
proteoglycan composition found in corneal scars disappear when the cornea
heals. The presence of dermatan
sulfate in the sclera may also play a role in
maintaining the overall shape of the eye. Keratan sulfate I is also present
in cartilage.
Heparin is an
important anticoagulant. It binds with factors IX and XI, but its most important
interaction is with plasma
antithrombin III. Heparin can
also bind specifically to lipoprotein lipase present in capillary walls,
causing a release of this enzyme into the circulation.
Certain proteoglycans (eg, heparan sulfate) are
associated with the plasma membrane of cells, with their core proteins
actually spanning that membrane. In it they may act as receptors and may also participate
in the mediation
of cell growth and cell-cell communication.
The attachment of cells to their substratum in
culture is mediated
at least in part by heparan sulfate. This proteoglycan is also found in the basement membrane of the kidney
along with type IV collagen and laminin (see above), where it plays a major role in determining the charge
selectiveness of glomerular filtration.
Proteoglycans are also found in intracellular
locations such as the nucleus; their function in this
organelle has not been elucidated. They are present in some storage
or secretory granules, such as the chromaffin granules of the adrenal medulla. It has been
postulated that they play a role in release of the contents of such
granules. The various functions of GAGs are summarized in Table 2.
Table 2. Some functions of glycosaminoglycans
and proteoglycans.
ASSOCIATIONS WITH MAJOR DISEASES & WITH AGING
Hyaluronic acid may be important in permitting tumor cells to migrate
through the ECM. Tumor cells can induce fibroblasts to synthesize greatly
increased amounts of this GAG, thereby perhaps facilitating
their own spread. Some tumor cells have less heparan
sulfate at their surfaces, and this may play a role in the
lack of adhesiveness that these cells display.
The intima of the arterial wall contains hyaluronic acid and chondroitin sulfate, dermatan sulfate, and heparan sulfate proteoglycans. Of these proteoglycans, dermatan sulfate binds plasma low-density lipoproteins.
In addition, dermatan sulfate appears to be the major GAG synthesized by arterial smooth muscle cells. Because it is these cells that proliferate in atherosclerotic lesions in arteries, dermatan sulfate may play an important role in development of the atherosclerotic plaque.
In various types of arthritis, proteoglycans may act as autoantigens, thus contributing to the pathologic features of these conditions. The amount of chondroitin sulfate in cartilage diminishes with age, whereas the amounts of keratan sulfate and hyaluronic acid increase.
These changes may contribute to the development of osteoarthritis. Changes in the amounts of cer tain GAGs in the
skin are also observed with aging and help to account for the
characteristic changes noted in this organ in the elderly.
An exciting new phase in proteoglycan research is opening up with the findings that mutations that affect individual proteoglycans or the enzymes needed for their synthesis alter the regulation of specific signaling pathways in drosophila and Caenorhabditis elegans, thus affecting development; it already seems likely that similar effects exist in mice and humans.
EXAMPLES OF GAG BINDING PROTEINS:
Secreted proteases and antiproteases
For example antithrombin III (AT III) binds
tightly to heparin and certain heparan sulfates (so do its
substrates). Thus they control the blood coagulation. In the absence of GAGs
AT III inactivates proteases (such as thrombin, factors IXa and
XIa) very slowly. In the presence of appropriate GAGs these reactions
are accelerated 2000-fold.
GAGs are sufficiently long that both protease and
protease inhibitor can bind to the same chain (thus the likelyhood of the two
proteins binding to each other is increased enormously). GAGs also affect the
protein conformation that contributes to improving AT III binding kinetics.
Polypeptide growth factors
Members of the FGF family, as well as several
other growth factors, bind to heparin or heparan sulfate. Binding
to endogenous GAGs entraps these molecules in ECM from which they may be later
released. GAGs can alter the conformation, proteolytic susceptibility and
biological activity of some of these proteins. The bound growth factor is
resistant to degradation by extracellular proteases. Active hormone is released
by proteolysis of the heparan sulfate chains. It occurs during the tissue
growth and remodeling after infection.
ECM proteins
Most of the large, multidomain ECM proteins contain
at least one GAG binding site.
For example fibrous collagens (type I, III,
V) and fibronectin bind to heparan sulfate chains which are
attached to the integral membrane core proteins of cell surface proteoglycans
such as syndecan and fibroglycan. Cell surface proteoglycans
are thought to anchor cells to matrix fibers.
Cell-cell adhesion molecules
·
For example NCAM (see cadherins) interacts with cell surface
heparan sulfate proteoglycans. This interaction is required for its
function. NCAM has a distinct heparan binding domain.
·
Hyaluronan is bound to the surface receptors (e.g. CD44) of many migrating
cells. It is very important during differentiation (for example myoblasts
which are undifferentiated muscle cell precursors bear hyaluronan- rich coat that
prevents premature cell fusion). Because its loose, hydrated porous structure,
the hyaluronan coat keeps cells apart from each other. They are free to move
around and proliferate.
When the level of
hyaluronan is lower (e.g. because of digesting by hyaluronidase), there is
ceesation of cell movement and initiation of cell- cell attachment.
Mucopolysaccharidoses and collagenoses, their
biochemical diagnostics
Mucopolysaccharidoses are a group of metabolic disorders caused by the absence or
malfunctioning of lysosomal enzymes needed to break down molecules called glycosaminoglycans - long chains of sugar
carbohydrates in each of our cells
that help build bone,
cartilage,
tendons,
corneas,
skin and connective
tissue. Glycosaminoglycans (formerly called mucopolysaccharides) are
also found in the fluid that lubricates our joints.
People with a mucopolysaccharidosis disease
either do not produce enough of one of the 11 enzymes required to break down
these sugar chains into simpler molecules, or they produce enzymes that do not
work properly. Over time, these glycosaminoglycans collect in the cells, blood
and connective tissues. The result is permanent, progressive cellular damage
which affects appearance, physical abilities, organ and system functioning,
and, in most cases, mental development.
The mucopolysaccharidoses are part of
the lysosomal storage disease family, a group
of more than 40 genetic disorders that result when a specific organelle in our
body's cells – the lysosome – malfunctions. The lysosome is commonly referred
to as the cell’s recycling center because it processes unwanted material into
substances that the cell can utilize. Lysosomes break down this unwanted matter
via enzymes, highly specialized proteins essential for survival. Lysosomal
disorders like mucopolysaccharidosis are triggered when a particular enzyme
exists in too small an amount or is missing altogether.
Features
The mucopolysaccharidoses share many
clinical features but have varying degrees of severity. These features may not
be apparent at birth but progress as storage of glycosaminoglycans affects
bone, skeletal structure, connective tissues, and organs. Neurological
complications may include damage to neurons (which
send and receive signals throughout the body) as well as pain and impaired motor
function. This results from compression of nerves or nerve
roots in the spinal cord or in the peripheral nervous system, the part of the
nervous
system that connects the brain and spinal cord to sensory organs such as the eyes
and to other organs, muscles, and tissues throughout the body.
Depending on the
mucopolysaccharidosis subtype, affected individuals may have normal intellect
or have cognitive impairments, may experience developmental delay, or may have
severe behavioral problems. Many individuals have hearing loss, either
conductive (in which pressure behind the ear drum causes fluid from the lining
of the middle ear to build up and eventually congeal), neurosensitive (in which
tiny hair cells in the inner ear are damaged), or both. Communicating
hydrocephalus — in which the normal reabsorption of cerebrospinal fluid is
blocked and causes increased pressure inside the head — is common in some of
the mucopolysaccharidoses. Surgically inserting a shunt
into the brain can drain fluid. The eye's cornea often
becomes cloudy from intracellular storage, and glaucoma
and degeneration of the retina also may affect the patient's vision.
Physical symptoms generally include
coarse or rough facial features (including a flat nasal bridge, thick lips, and
enlarged mouth and tongue), short stature with disproportionately short trunk (dwarfism),
dysplasia
(abnormal bone size and/or shape) and other skeletal irregularities, thickened
skin, enlarged organs such as liver (hepatomegaly)
or spleen (splenomegaly), hernias,
and excessive body hair growth. Short and often claw-like hands, progressive
joint stiffness, and carpal tunnel syndrome can restrict hand
mobility and function. Recurring respiratory infections are common, as are
obstructive airway disease and obstructive sleep apnea.
Many affected individuals also have heart disease, often involving enlarged or
diseased heart valves.
Another lysosomal storage disease
often confused with the mucopolysaccharidoses is mucolipidosis.
In this disorder, excessive amounts of fatty materials known as lipids (another
principal component of living cells) are stored, in addition to sugars. Persons
with mucolipidosis may share some of the clinical features associated with the
mucopolysaccharidoses (certain facial features, bony structure abnormalities,
and damage to the brain), and increased amounts of the enzymes needed to break
down the lipids are found in the blood.
Types
Seven distinct clinical types and
numerous subtypes of the mucopolysaccharidoses have been identified. Although
each mucopolysaccharidosis (MPS) differs clinically, most patients generally
experience a period of normal development followed by a decline in physical
and/or mental function.
Diagnosis
Diagnosis often can
be made through clinical examination and urine tests (excess
mucopolysaccharides are excreted in the urine). Enzyme assays (testing a
variety of cells or body fluids in culture for enzyme deficiency) are also used
to provide definitive diagnosis of one of the mucopolysaccharidoses. Prenatal
diagnosis using amniocentesis and chorionic villus sampling can verify if a
fetus either carries a copy of the defective gene or is affected with the
disorder. Genetic counseling can help parents who have a family history of the
mucopolysaccharidoses determine if they are carrying the mutated gene that
causes the disorders.
Any of various diseases or abnormal states (as
rheumatoid arthritis, systemic lupus erythematosus, polyarteritis nodosa,
rheumatic fever, and dermatomyositis) characterized by inflammatory or degenerative
changes in connective tissue—called also collagen disease, collagenolysis, collagen vascular disease
SUMMARY
• The major components of the ECM are the
structural proteins collagen, elastin, and fibrillin; a number of specialized
proteins (eg, fibronectin and laminin); and various proteoglycans.
• Collagen is the most abundant protein in the
animal kingdom; approximately 19 types have been isolated.
• Collagen and elastin are fibrous proteins.
Collagen molecules contain an abundance of proline, lysine, and glycine, the
latter occurring at every third position in the primary structure. Collagen
also contains hydroxyproline, hydroxylysine, and glycosylated hydroxylysine,
each formed by posttranslational modification.
• Collagen molecules typically form fibrils
containing a long, stiff, triple-stranded helical structure, in which three
collagen polypeptide chains are wound around one another in a rope-like
superhelix (triple helix). Other types of collagen form mesh-like networks.
All collagens contain greater or lesser stretches
of triple helix and the repeating structure (Gly-X-Y)n.
• The biosynthesis of collagen is complex,
featuring many posttranslational events, including hydroxylation of proline and
lysine.
• Diseases associated with impaired synthesis of
collagen include scurvy, osteogenesis imperfecta, Ehlers-Danlos syndrome (many
types), and Menkes disease.
• Elastin confers
extensibility and elastic recoil on tissues.
• Elastin lacks
hydroxylysine, Gly-X-Y sequences, triple helical structure, and sugars but
contains desmosine and isodesmosine cross-links not found in collagen.
• Elastin is a
connective tissue protein with rubber-like properties in tissues such as the
lung. 1-Antitrypsin (α1-AT), produced primarily by the liver but also by tissues
such as monocytes and alveolar macrophages, prevents elastin degradation in the
alveolar walls. A deficiency of α1-AT can cause emphysema and, in some
cases, cirrhosis of the liver.
• Fibrillin is located in microfibrils. Mutations
in the gene for fibrillin cause Marfan syndrome.
• The glycosaminoglycans (GAGs) are made up of
repeating disaccharides containing a uronic acid (glucuronic or iduronic) or
hexose (galactose) and a hexosamine (galactosamine or glucosamine). Sulfate is
also frequently present.
• The major GAGs are hyaluronic acid, chondroitin
4- and 6-sulfates, keratan sulfates I and II, heparin, heparan sulfate, and
dermatan sulfate.
• The GAGs are synthesized by the sequential
actions of a battery of specific enzymes (glycosyltransferases, epimerases,
sulfotransferases, etc) and are degraded by the sequential action of lysosomal
hydrolases. Genetic deficiencies of the latter result in mucopolysaccharidoses
(eg, Hurler syndrome).
• GAGs occur in tissues bound to various proteins
(linker proteins and core proteins), constituting proteoglycans.
These structures are often of very high molecular
weight and serve many functions in tissues.
• Many components of the ECM bind to proteins of
the cell surface named integrins; this constitutes one pathway by which the
exteriors of cells can communicate with their interiors.
• Bone and cartilage are specialized forms of the
ECM.
Collagen I and hydroxyapatite are the major
constituents of bone. Collagen II and certain proteoglycans are major
constituents of cartilage.
Current understanding of the
molecular events underlying muscle contraction is embodied in the sliding
filament model of muscle contraction. The model is applicable to smooth,
skeletal, cardiac, and other contractile activity, including mechanochemical
events such as single cell locomotion and receptor endocytosis. Since the
biochemistry of these activities are best understood for skeletal muscle, this
discussion focus on skeletal muscle (noting, where appropriate, differences in
the other muscle types). The biochemical characteristics that differentiate
fast-reacting and slow-reacting cells in muscle tissue and the biochemical
basis of some common pathophysiological states of muscle, including tetany,
fatigue, and rigor mortis are reviewed as well.
There are three types of muscle:
·
Skeletal muscle or "voluntary muscle" is anchored by tendons
to bone and is used to
affect skeletal
movement such as locomotion
and in maintaining posture. Though this postural control is generally
maintained as a subconscious reflex, the muscles responsible react to conscious
control like non-postural muscles. An average adult male is made up of 40-50%
of skeletal muscle and an average adult female is made up of 30-40%.
·
Smooth muscle or "involuntary muscle" is found within the walls of organs
and structures such as the esophagus,
stomach,
intestines,
bronchi,
uterus, urethra,
bladder,
and blood vessels,
and unlike skeletal muscle, smooth muscle is not under conscious control.
·
Cardiac muscle is also an "involuntary muscle" but is a specialized kind of
muscle found only within the heart.
Cardiac
muscle
Cardiac and skeletal muscle are
"striated" in that they contain sarcomeres
and are packed into highly-regular arrangements of bundles; smooth muscle has
neither. While skeletal muscles are arranged in regular, parallel bundles,
cardiac muscle connects at branching, irregular angles. Striated muscle
contracts and relaxes in short, intense bursts, whereas smooth muscle sustains
longer or even near-permanent contractions.
http://www.youtube.com/watch?v=InIha7bCTjM&feature=related
http://www.youtube.com/watch?v=-pg09F5V63U&feature=related
Skeletal muscles comprise about 40%
of the mass of the average human body and are formed of long multinucleate,
cylindrical cells called muscle fibers. Skeletal muscle fibers are grossly
divided into two type; slow twitch (type I) and fast twitch (type II). Type II
fibers are further divided into type IIa and type IIb fibers. Type IIa fibers
are intermediate fast twitch fibers and can utilize both aerobic and anaerobic
metabolism for ATP production. Type IIb fibers are the classic fast twitch
fibers. Slow twitch muscle fibers primarily utilize fatty acid oxidation and
contain a high concentration of mitochondria and myoglobin. These two facts are
the reason that slow twitch fibers are red in color. Fast twitch fibers
primarily utilize glucose oxidation to pyruvate for ATP production, contain
less mitochondria and myoglobin than slow twitch fibers and thus, are white
muscle fibers. Because slow twitch fibers prefer to oxidize fatty acids they
are also referred to as oxidative fibers whereas, fast twitch fibers that
utilize glucose are referred to as glycolytic fibers. Slow twitch fibers are
capable of continuous extended contractions and therefore, do not fatigue
quickly. Fast twitch fibers are used for short rapid bursts of energy and as
such fatigue more quickly than slow twitch fibers.
The plasma
membrane of muscle fibers is known as the sarcolemma. Each muscle is made up of
bundles of these fibers, or cells, embedded in a matrix of connective tissue
known as the endomysium. The bundle of fibers with its endomysium is surrounded
by a more fibrous connective tissue sheath known as the perimysium. The
composite of the perimysium and its contents is known as a fasciculus. A
complete muscle consists of numerous fasciculi surrounded by a thick outer
layer of connective tissue known as the perimysial septa. The translation of
contractile activity of individual muscle fibers to anatomical motion take
place through this continuous system of connective tissues and sheaths, which
ultimately meld into the tendons.
Within the sarcolemma is the
sarcoplasm, containing all the usual subcellular elements plus long prominent
myofibrils. Each myofibril is composed of bundles of filamentous contractile
proteins, some extending from end to end in the cell. Myofibrils are the most
conspicuous elements in skeletal myofibers making up about 60% of myofiber
protein. A single myofibril is composed of many short structural units, known
as sarcomeres, which are arranged end to end. The proteins at the junctions
between sarcomeres form the Z line, and thus a sarcomere extends along a
myofibril from one Z line to the next Z line. Sarcomeres are composed mostly of
actin thin filaments and myosin thick filaments. Sarcomeres represent the
minimal contractile unit of a muscle. It is the coordinated contraction and
elongation of millions of sarcomeres in a muscle that gives rise to mechanical
skeletal activity. The relationship between muscle proteins and muscles is
summarized in the table below:
|
Organization
of Contractile Proteins in Muscle |
|
|
Thick
Filament |
Composed of hundreds of long, contractile myosin molecules
arranged in a staggered side by side complex. |
|
Thin
Filament |
Composed of a linear array of hundreds of globular,
actin monomers in a double helical. arrangement. |
|
Sarcomere |
The unit of contractile activity composed mainly of actin
and myosin and extending from Z line to Z line in a myofibril. |
|
Myofiber |
A single multinucleate muscle cell containing all
the usual cell organelles plus many myofibrils. |
|
Myofibril |
End to end arrays of identical sarcomeres. |
|
Muscle |
Organized arrays of muscle fibers. |
Organization
of the Sarcomere
The organization of individual
contractile proteins making up a sarcomere is a key feature of the sliding filament
model. Each sarcomere is composed of hundreds of filamentous protein
aggregates, each known as a myofilament. Two kinds of myofilaments are
identifiable on the basis of their diameter and protein composition (see image
above). Thick myofilaments are composed of several hundred molecules of a
fibrous protein known as myosin. Thin myofilaments are composed of two
helically interwound, linear polymers of a globular protein known as actin.
Thin and thick filaments also contain accessory proteins, described below.
Proteins of the Z line, including α-actinin, serve as an embedding
matrix or anchor for one end of the thin filaments, which extend toward the
center of sarcomeres on either side of the Z line. The Z line proteins often
appear continuous across the width of a muscle fiber and seem to act to keep
the myofibrils within a myofiber in register. The distal end of each thin
filament is free in the sarcoplasm and is capped with a protein known as β-actinin.
Also depicted in the image above is a
second disk-like protein aggregate: the M-line, which is centrally located in
sarcomeres. Like Z line protein, the M line protein aggregate acts as an
embedding matrix, in this case for the myosin thick filaments. Thick filaments
extend from their point of attachment on both sides of the M line toward the
two Z lines that define a sarcomere.
Within a sarcomere the thick and thin
filaments interdigitate so that in cross section they are seen to form a
hexagonal lattice, in which 6 thin filaments are arrayed around each thick
filament. The thick filaments are also arranged hexagonally to each other.
During contraction and relaxation the distance between the Z lines varies,
decreasing with contraction and increasing with relaxation. The M line, with
its attached thick filaments, remains centrally located in the sarcomere. The
thin and thick filaments retain their extended linear structure except in
extreme situations. Changes in sarcomere length are caused by the thin
filaments being pulled along the thick filaments in the direction of the M
line.
Structure
and functions of sarcoplasma proteins (Myogene, Myoglobine, Myoalbumine)
The sarcoplasm of a muscle fiber is
comparable to the cytoplasm of other cells, but it houses unusually large
amounts of glycosomes (granules of stored glycogen) and significant amounts of
myoglobin, an oxygen binding protein. The calcium concentration in sarcoplasma
is also a special element of the muscular fiber by means of which the
contractions takes place and regulates.
It contains mostly myofibrils (which
are composed of sarcomeres), but its contents are otherwise comparable to those
of the cytoplasm of other cells. It has a Golgi apparatus, near the nucleus,
mitochondria just on the inside of the cytoplasmic membrane or sarcolemma, as well
as a smooth endoplasmic reticulum organized in an extensive network.
Myoglobin is an iron- and oxygen-binding protein found in
the muscle tissue of vertebrates in general and in almost all mammals. It is
related to hemoglobin, which is the iron- and oxygen-binding protein in blood,
specifically in the red blood cells. The only time myoglobin is found in the
bloodstream is when it is released following muscle injury. It is an abnormal
finding, and can be diagnostically relevant when found in blood.
Myoglobin (abbreviated Mb) is a
single-chain globular protein of 153 or 154 amino acids, containing a heme
(iron-containing porphyrin) prosthetic group in the center around which the
remaining apoprotein folds. It has eight alpha helices and a hydrophobic core.
It has a molecular weight of 17,699 daltons (with heme), and is the primary
oxygen-carrying pigment of muscle tissues. Unlike the blood-borne hemoglobin,
to which it is structurally related, this protein does not exhibit cooperative
binding of oxygen, since positive cooperativity is a property of
multimeric/oligomeric proteins only. High concentrations of myoglobin in muscle
cells allow organisms to hold their breaths longer. Diving mammals such as
whales and seals have muscles with particularly high myoglobin abundance.
Myoglobin was the first protein to
have its three-dimensional structure revealed. In 1958, John Kendrew and
associates successfully determined the structure of myoglobin by
high-resolution X-ray crystallography. For this discovery, John Kendrew shared
the 1962 Nobel Prize in chemistry with Max Perutz. Despite being one of the
most studied proteins in biology, its true physiological function is not yet
conclusively established: mice genetically engineered to lack myoglobin are
viable, but showed a 30% reduction in cardiac systolic output. They adapted to
this deficiency through hypoxic genetic mechanisms and increased vasodilation.
In humans myoglobin is encoded by the MB gene.
Meat
color
Myoglobin forms pigments responsible
for making meat red. The color that meat takes is partly determined by the
oxidation states of the iron atom in myoglobin and the oxygen species attached
to it. When meat is in its raw state, the iron atom is in the +2 oxidation
state, and is bound to a dioxygen molecule (O2). Meat cooked well done is brown
because the iron atom is now in the +3 oxidation state, having lost an
electron, and is now coordinated by a water molecule. Under some conditions,
meat can also remain pink all through cooking, despite being heated to high
temperatures. If meat has been exposed to nitrites, it will remain pink because
the iron atom is bound to NO, nitric oxide (true of, e.g., corned beef or cured
hams). Grilled meats can also take on a pink "smoke ring" that comes
from the iron binding to a molecule of carbon monoxide. Raw meat packed in a
carbon monoxide atmosphere also shows this same pink "smoke ring" due
to the same coordination chemistry. Notably, the surface of this raw meat also
displays the pink color, which is usually associated in consumers' minds with
fresh meat. This artificially induced pink color can persist in the meat for a
very long time, reportedly up to one year. Hormel and Cargill are both reported
to use this meat-packing process, and meat treated this way has been in the
consumer market since 2003. Myoglobin is found in Type I muscle, Type II A and
Type II B, but most texts consider myoglobin not to be found in smooth muscle.
Role in disease
Myoglobin is
released from damaged muscle tissue (rhabdomyolysis), which has very high
concentrations of myoglobin. The released myoglobin is filtered by the kidneys
but is toxic to the renal tubular epithelium and so may cause acute renal
failure. It is not the myoglobin itself that is toxic (it is a protoxin) but the
ferrihemate portion that is dissociated from myoglobin in acidic environments
(e.g., acidic urine, lysosomes).
Myoglobin is
a sensitive marker for muscle injury, making it a potential marker for heart
attack in patients with chest pain. However, elevated myoglobin has low
specificity for acute myocardial infarction (AMI) and thus CK-MB, cTnT, ECG,
and clinical signs should be taken into account to make the diagnosis.
Structure
and bonding
Myoglobin contains a porphyrin ring
with an iron center. There is a proximal histidine group attached directly to
the iron center, and a distal histidine group on the opposite face, not bonded
to the iron.
Many functional models of myoglobin
have been studied. One of the most important is that of picket fence porphyrin by
James P. Collman. This model was used to show the importance of the distal
prosthetic group. It serves three functions:
To form hydrogen bonds with the
dioxygen moiety, increasing the O2 binding constant
To prevent the
binding of carbon monoxide, whether from within or without the body.
Carbon monoxide binds to iron in an end-on fashion, and is hindered by the
presence of the distal histidine, which forces it into a bent conformation. CO
binds to heme 23,000 times better than O2, but only 200 times better in
hemoglobin and myoglobin. Oxygen binds in a bent fashion, which can fit with
the distal histidine.
To prevent irreversible dimerization
of the oxymyoglobin with another deoxymyoglobin species
myogen - proteins extracted
from skeletal muscle with cold water, largely the enzymes promoting glycolysis;
from the residue, alkaline 0.6 mol L-1 KCl extracts actin and myosin as
actomyosin, with myosin further separable into two meromyosins by proteinase
treatment.
Synonym(s): myosinogen
Proteins
of the Myofilaments
The
biochemical basis of muscle activity is related to the enzymatic and physical
properties of actin, myosin, and the accessory proteins that constitute the
thin and thick filaments. The following discussion summarizes the key protein
components of the myofilaments and their ATP-dependent interactions, which
produce contractile activity.
The proteins
of the thin and thick filaments can be separated into actin, myosin, and 6
accessory proteins. The accessory proteins are α-actinin, β-actinin,
tropomyosin, troponin, C protein, and M line protein. Solubilized myosin
molecules are long thin (fibrous) proteins with a molecular weight of about
500,000 daltons.
Each molecule is made up of 6
subunits, 2 very large, heavy chains (HC), and 4 smaller, light chains (LC). In
a given muscle fiber the 2 large subunits are identical, although there are
different HC isoforms in different types of muscle fibers. Heavy chains contain
a long linear C-terminal α-helical domain (1,300 amino acids)
and a prominent globular N-terminal domain of about 800 amino acids. The two
HC, α-helical
domains are helically interwound, giving the molecules a long, rigid
superhelical structure with 2 globular headpieces. A complete myosin molecule
also contains 4 relatively small proteins which are associated with the
globular headpieces. These small proteins, of molecular weight 16,000–24,000
daltons, are known as alkali light chains (LC1 or LC3) and DTNB light chains
(LC2). Each myosin molecule contains 2 subunits of LC2, 1 associated with each
HC globular domain. Each of the globular domains also contains a subunit of
either LC1 or LC3, with the proportions of LC1 and LC3 in the myosin molecules
varying in myosins from cardiac, skeletal, embryonic, and smooth muscle. All
light chains bind Ca2+ with high affinity, are phosphorylated by myosin light
chain kinase (MLCK), and generally serve in the regulation of myosin's ATPase
activity and its assembly into thick filaments.
Organization
of myofilaments
Several functionally important
landmarks exist on the myosin molecule. Near the midpoint of the long linear
superhelical region is a site defined by its ready susceptibility to
proteolytic trypsin digestion. Trypsin cleaves myosin into 2 portions: 1
containing both globular headpieces and some superhelical region, and the other
consisting of the remaining superhelical portion of the carboxy terminus. The
portion containing the headpiece is known as heavy meromyosin (HMM; molecular
weight 350,000). The C-terminal fragment is known as light meromyosin (LMM;
molecular weight 125,000).
The significance of the trypsin site
is that its susceptibility to protease action is thought to reflect an
interruption in the otherwise rigid superhelix, allowing this site to act as
one of a hinge point involved in converting the chemical energy of ATP into the
mechanical events of contraction and relaxation. A second proteolytic landmark
susceptible to papain has also been considered a hinge point. Papain cleaves a
site very close to the globular headpieces; these then separate to form 2
subfragments, each known as an SF-1 (for subfragment 1). The remaining
superhelical portion of the molecule is known as SF-2. The ATPase activity of
the myosin is associated with the SF-1 units.
A thick filament is composed of
approximately 400 myosin molecules, 200 arrayed on either side of the M line.
These molecules are maintained in bundles by C protein (clamp protein), M line
protein and the hydrophobic interactions of the myosin molecules themselves.
The myosin molecules are most tightly packed in the regions represented by the
LMM portion of the molecules.
At the trypsin hinge point the heavy
meromyosin angles sharply outward from the main axis of the thick filament.
This extension of the heavy meromyosin away from the main axis of the thick
filament helps bring the headpiece into close proximity to the actin thin
filaments lying between the thick filaments. The molecular event underlying
muscle contraction is the regulated binding of the myosin headpieces to actin
thin filaments, followed by rapid myosin conformational changes about its hinge
points with the bound actin being translocated toward the M line.
Organization
of Actin Thin Filaments
Thin filaments are composed of many
subunits of the globular protein G-actin (42 kD) and several accessory
proteins. In thin filaments, G-Actin is polymerized into long fibrous arrays
known as F-actin. A pair of linear F-actin arrays is helically wound to form
the backbone structure of 1 complete thin filament.
Each G-actin subunit has 1 ADP/ATP
binding site, presumed to be involved in polymerizing the thin filament. Once
polymerized, the actin is capped and the thin filament stabilized by a protein
known as β-actinin.
In addition to its nucleotide binding site, each G-actin molecule contains a
high-affinity myosin head-binding site. In skeletal and cardiac muscle,
accessory proteins of the thin filament (described below) physically regulate
the availability of this site for binding myosin. Thus, the accessory proteins
control contractile events.
The main thin
filament accessory proteins are tropomyosin and troponin. Tropomyosin is a
long, rod-like, αβ
helically-interwound heterodimer that spans a length of 7 G-actin residues. A
pair of tropomyosin molecules is associated with every 7 pairs of G-actin
residues along a thin filament, 1 tropomyosin molecule in each of the grooves
of the F-actin helix. In relaxed muscle, each tropomyosin molecule covers the
myosin binding sites of 7 G-actin residues, preventing interaction between
actin and myosin and thus maintaining the relaxed state. The onset of
contractile activity involves activating troponin, the second accessory protein
of thin filaments. Troponin is a heterotrimer attached to one end of each
tropomyosin molecule and to actin, physically linking tropomyosin to actin.
Conformational changes in the
bridging molecule, troponin, are responsible for moving tropomyosin on and off
myosin binding sites of actin and thus regulating muscle contraction. One of
the troponin subunits, troponin-C (Tn-C), is a calmodulin-like calcium-binding
protein. When Tn-C binds calcium, the whole troponin molecule undergoes the
conformational change that moves the attached tropomyosin away from the myosin
binding sites on actin. This event permits nearby myosin heads to interact with
myosin binding sites, and contractile activity ensues.
Events on the thin filament can be
summarized as follows: Prior to the appearance of free calcium in the
sarcoplasm, tropomyosin covers the myosin binding sites on actin. The
appearance of calcium in the sarcoplasm leads to calcium binding on Tn-C. The
resulting conformational changes in troponin move the attached tropomyosin
molecule more deeply into the helix groove of F-actin, uncovering the myosin
binding sites on G-actin subunits. The exposed sites are then available to
interact with myosin headpieces. Removing calcium from the sarcoplasm restores
the original conformational states of troponin and tropomyosin, preventing
interaction between actin and myosin and leading to the relaxed state.
Myosin
and the Power Stroke of Contraction
In a rested, non-contracting muscle,
myosin binding sites on actin are obscured and myosin exists a in high-energy
conformational state (M*), poised to carry out a contractile cycle. The energy
of ATP hydrolysis is used to drive myosin from a low-energy conformational
state (M) to the high-energy state, as depicted in the following equation:
(M-ATP)
<——> (M*-ADP-Pi)
When cytosolic calcium increases and
myosin binding sites on actin become available, an actomyosin complex is
formed, followed by the sequential dissociation of Pi and ADP with conversion
of myosin to its low-energy conformational state. These events are accompanied
by simultaneous translocation of the attached thin filament toward the M line
of the sarcomere. The latter events, summarized in the following 2 equations,
comprise the power stroke of the contractile cycle. Note that the energy of the
power stroke is derived from ATP, via ATP-driven conversion of a low-energy
myosin conformational state to a high-energy conformational state. A useful
analogy is that ATP cocks the myosin trigger and the formation of an actomyosin
complex pulls the trigger, releasing the energy stored in cocking the trigger.
(M*-ADP-Pi) + A
<——> (M*-ADP-A) + Pi
(M*-ADP-A)
<——> (M-A) + ADP
At the end of the power stroke the
actomyosin complex is remains intact until ATP becomes available. ATP binding
to myosin is a very exergonic reaction, with the result that ATP displaces
actin from the myosin head as indicated by the equation below. Thus, it is
often said that ATP is required for muscle relaxation. It is important to note
that in relaxed muscle, myosin is in its high-energy conformational state. Note
that the final product (M-ATP) is also the first reactant shown in the first
equation above, completing the reactions of the contractile cycle.
(MA) + ATP
<——> (M-ATP) + A
Troponin
Troponin is a
complex of three regulatory proteins (troponin C, troponin I and troponin T)
that is integral to muscle contraction in skeletal and cardiac muscle, but not
smooth muscle.
Discussions of troponin often pertain
to its functional characteristics and/or to its usefulness as a diagnostic
marker for various heart disorders.
Function
Troponin is attached to the protein tropomyosin
and lies within the groove between actin filaments in muscle tissue. In a
relaxed muscle, tropomyosin blocks the attachment site for the myosin
crossbridge, thus preventing contraction. When the muscle cell is stimulated to
contract by an action potential, calcium channels open in the sarcoplasmic
membrane and release calcium into the sarcoplasm. Some of this calcium attaches
to troponin which causes it to change shape, exposing binding sites for myosin
(active sites) on the actin filaments. Myosin binding to actin forms cross
bridges and contraction (cross bridge cycling) of the muscle begins. Troponin activation. Troponin C (red) binds Ca2+, which
stabilizes the activated state, where troponin I (yellow) is no longer bound to
actin. Troponin T (blue) anchors the complex on tropomyosin.Troponin is found
in both skeletal muscle and cardiac muscle, but the specific versions of
troponin differ between types of muscle. The main difference is that the TnC
subunit of troponin in skeletal muscle has four calcium ion binding sites,
whereas in cardiac muscle there are only three. Views on the actual amount of
calcium that binds to troponin vary from expert to expert and source to source.
Both cardiac
and skeletal muscles are controlled by changes in the intracellular calcium
concentration. When calcium rises, the muscles
contract, and when calcium falls, the muscles relax.
Troponin is a component of thin
filaments (along with actin and tropomyosin), and is the protein to which
calcium binds to accomplish this regulation. Troponin has three subunits, TnC,
TnI, and TnT. When calcium is bound to specific sites
on TnC, tropomyosin rolls out of the way of the actin filament active sites, so
that myosin (a molecular motor organized in muscle thick filaments) can attach
to the thin filament and produce force and/or movement. In the absence of
calcium, tropomyosin interferes with this action of myosin, and therefore
muscles remain relaxed.
Individual
subunits serve different functions:
Troponin C binds
to calcium ions to produce a conformational change in TnI
Troponin T
binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
Troponin I binds to actin in thin myofilaments to hold the
troponin-tropomyosin complex in place
Smooth muscle
does not have troponin.
Diagnostic use
The troponin
test can be used as a test of several different heart disorders, including
myocardial infarction.
Cardiac conditions
Certain subtypes
of troponin (cardiac troponin I and T) are very sensitive and specific
indicators of damage to the heart muscle (myocardium). They are measured in the
blood to differentiate between unstable angina and myocardial infarction (heart
attack) in patients with chest pain or acute coronary syndrome. A patient who
had suffered from a myocardial infarction would have an area of damaged heart
muscle and so would have elevated cardiac troponin levels in the blood. This
can also occur in patients with coronary vasospasm.
It is important to note that cardiac
troponins are a marker of all heart muscle damage, not just myocardial
infarction. Other conditions that directly or indirectly lead to heart muscle
damage can also increase troponin levels. Severe tachycardia (for example due
to supraventricular tachycardia) in an individual with normal coronary arteries
can also lead to increased troponins for example, presumably due to increased
oxygen demand and inadequate supply to the heart muscle.
Troponins are also increased in
patients with heart failure, where they also predict mortality and ventricular
rhythm abnormalities. They can rise in inflammatory conditions such as
myocarditis and pericarditis with heart muscle involvement (which is then
termed myopericarditis). Troponins can also indicate several forms of
cardiomyopathy, such as dilated cardiomyopathy, hypertrophic cardiomyopathy or
(left) ventricular hypertrophy, peripartum cardiomyopathy, Takotsubo
cardiomyopathy or infiltrative disorders such as cardiac amyloidosis.
Heart injury with increased troponins
also occurs in cardiac contusion, defibrillation and internal or external
cardioversion. Increased troponins are commonly increased in several procedures
such as cardiac surgery and heart transplantation, closure of atrial septal
defects, percutaneous coronary intervention or radiofrequency ablation.
Non-cardiac conditions
The distinction between cardiac and
non-cardiac conditions is somewhat artificial; the conditions listed below are
not primary heart diseases, but they exert indirect effects on the heart
muscle.
Troponins are increased in around 40%
of patients with critical illnesses such as sepsis. There is an increased risk
of mortality and length of stay in the intensive care unit in these patients.
In severe gastrointestinal bleeding there can also be a mismatch between oxygen
demand and supply of the myocardium.
Central nervous system disorders can
lead to increased sympathetic tone and/or catecholamine release which lead to
cardiac overstimulation. This is seen in subarachnoid hemorrhage, stroke, intracranial hemorrhage and (generalized) seizures (in
patients with epilepsy or eclampsia, for example).
Patients with end-stage renal disease
can have chronically elevated troponin T levels, which are linked to a poorer
prognosis. Troponin I is less likely to be falsely
elevated.
Strenuous endurance exercise such as
marathons or triathlons can lead to increased troponin levels in up to one
third of subjects, but it is not linked to adverse health effects in these
competitors. High troponin T levels have also been reported in patients with
inflammatory muscle diseases such as polymyositis or dermatomyositis. Troponins
are also increased in rhabdomyolysis.
Cardiac troponin T and I can be used
to monitor drug and toxin induced cardiomyocyte toxicity.
Tropomyosin
Tropomyosin is a two-stranded
alpha-helical coiled coil protein found in muscle.
All organisms contain structures
which provide physical integrity to their cells. These structures are collectively
known as the cytoskeleton and one of the most ancient systems is based on
filamentous polymers of the protein actin. During evolution a second polymer of
the protein, tropomyosin, arose and became an integral part of most actin
filaments in animals.
Tropomyosins are a large family of
integral components of actin filaments which play a critical role in regulating
the function of actin filaments in both muscle and nonmuscle cells. These
proteins consist of rod-shaped coiled-coil hetero- or homo-dimers that lie
along the α-helical groove of most actin filaments. Interaction occurs
along the length of the actin filament with dimers aligning in a head-to-tail
fashion.
Tropomyosins are often categorised into two groups,
muscle tropomyosin isoforms and nonmuscle tropomyosin isoforms. Muscle
tropomyosin isoforms are involved in regulating interactions between actin and
myosin in the muscle sarcomere and play a pivotal role in regulated muscle
contraction. Nonmuscle tropomyosin isoforms function in all cells, both muscle
and nonmuscle cells, and are involved in a range of
cellular pathways that control and regulate the cell’s cytoskeleton and other
key cellular functions.
The actin filament system that is involved in
regulating these cellular pathways is more complex than the actin filament systems that regulates muscle contraction. The contractile
system relies upon 4 actin filament isoforms and 5 tropomyosin isoforms , whereas the actin filament system of the
cytoskeleton uses 2 actin filament isoforms and over 40 tropomyosin isoforms.