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Fibrous proteins
ELASTIN
Major fibrous protein of epithelial tissues is
a keratin
Major fibrous proteins of connective tissue are:
Collagen and Elastin
•Much or most of the polypeptide chain is parallel to
a single axis
•Fibrous proteins are often mechanically strong &
highly cross-linked
•Fibrous proteins are usually insoluble
•Usually play a structural role
ELASTIN
It is a connective tissue protein that is responsible for
properties of extensibiity and elastic recoil in tissues.
Although not as widespread as collagen, elastin is
present in large amounts, particularly in tissues that
require these physical properties e.g. lung, blood vessels
and elastic ligaments and small amounts in ear and skin.
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 are composed of elastin and
glycoprotein microfibrils and 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.
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).
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.
Elastin is also rich in proline and lysine, but contains only a
little hydroxyproline and hydroxylysine.
Some of the lysyl side chains of the tropoelastin polypeptides
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 .THIS PRODUCES ELASTIN—
an extensively interconnected, rubbery network that can
stretch and bend in any direction when stressed, giving
connective tissue elasticity
Elastin fibers in relaxed and stretched conformations.
DYNAMIC STRUCTURE OF ELASTIN:
The structure of elastin is
called a b spiral, and is
loosely held together by
the hydrophobic force.
It is easily deformed to an
extended configuration,
but will relax back to a
compact conformation.
Repeating unit: PGVGV
ELASTIC FIBERS. These
scanning electron micrographs show (A) a lowpower view of a segment of a dog's aorta and (B) a high-power view of
the dense network of longitudinally oriented elastic fibers in the outer
layer of the same blood vessel. All the other components have been
digested away with enzymes and formic acid.
FIBRILLIN
. A large glycoprotein (about 350 KDa) that is a structural
component of microfibrils 10-12 nm fibers found in many
tissues. It is secreted into ECM by fibroblast and becomes
incorporated into the insoluble microfibrils which appear
to provide a scaffold for deposition of elastin.
.
MARFAN SYNDROME.
It is a relatively prevalent inherited disease affecting
connective tissues. It is inherited as an autosomal
dominant trait. It affects the eyes, the skeletal system
and hyper-extensibility of the joints and cardiovascular
system. This syndrome is due to mutations in the gene
for fibrilin.
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.]
Deletion of elastin gene have been found in 90% of the
subjects with Williams syndrome, a developmental
disorder affecting connective tissue and the central
nervous system. A number of skin diseases are
associated with accumulation of elastin. Alternatively, a
decrease of elastin is found in – conditions such as
pulmonary emphysema and aging of the skin
Genetic defects in elastin underlie Williams Syndrome
Facial features associated with Williams Syndrome
Dental features include small widely
spaced teeth and malocclusion.