Fibrous proteinsCollagen, elastin and keratin

Functions:

- Structural functions in the body, collagen and elastin are found as component of skin,

connective tissues, sclera and cornea of the eye and blood vessel walls

- Keratin is found in skin and hair.

- Each fibrous protein has its special mechanical properties resulting from its unique but

relatively simple structure. -Fibrous proteins consist of specific amino acids arranged into regular secondary

structural elements. This is in contrast to globular proteins whose structure resulted from

a complex interaction of secondary, tertiary, and sometimes quaternary structural

elements.

A. Collagen

- Collagen is the most abundant protein in the human body-In some tissues, collagen found as gel that serves to strengthen the structures as in the

extracellular matrix or vitreous humor of the eye. In other tissues collagen may be

bundled in tight parallel fibers that provide great strength as tendons,-In other tissues collagen may be bundled in tight parallel fibers that provide great

strength as tendons,- in the cornea of the eye, collagen stacked so transmit light with minimum of scattering.

- Collagen in the bone occurs as fibers arranged at an angle to each other so as to

resist mechanical shear from any direction

Structure of collagen

Types of collagen

Collagen is formed from three polypeptides called α-chains which wrap around each

other in a triple helix forming a rope-like structure.

The three polypeptide chains are held together by H-bonds

Variations in amino acids sequence of the α-chains result in structural components

that are in the same size (around 1000 amino acids) but slightly with different

properties.

The α-chains are combined to form the various types of collagen found d in tissue.

Figure 3.23

Amino acid sequence:

the primary structure of collagen is unusual in that glycine is found in every third

position of the polypeptide chain, the glycine residue is a part of a repeating

sequence –Gly-X-Y where X is frequently is proline and Y often hydroxyproline or

hydroxylysine

Triple-helical structure-Collagen doesn't fold into a compact structure

- -It has an elongated triple-helical structure , the amino acid side chains are placed

outside of the molecule .

This allow the interaction between triple-helical molecules that lead to aggregation of

collagen monomers into long fiber

Hydroxyproline and hydroxylysine:

These amino acids are rarely found in other proteins and found extensively in collagen

They resulted from hydroxylation of Proline and Lysine after their incorporation into

polypeptide chains. (Posttranslation modification)

Hydroxproline is important in stabilizing the triple-helical structure

Glycosylation

The hydroxyl group of the hydroxylysine residues of collagen maybe glycosylated.

Most commonly glucose and galactose.

Elastin Elastin is a connective tissue protein with a rubber like properties. Elastin fibers like

are found at lungs, wall of blood vessels and elastic ligaments. Elastin can be

stretched to several times their normal length but recoil to their original shape when

the stretching force is relaxed

Structure of elastin

Amino acid composition Elastin is composed primarily of small, non-polar amino acid residues as glycine,

alanine and valine. Elastin also rich in lysine and proline but little hydroxyproline and no

hydroxylysine.

Interchain cross-linkElastin fiber are formed as three dimensional network of cross-linked polypeptide

that have an irregular conformation. The cross-link involve lysine. 4 lysine residue from 4 separate chains can be

covalently joined to produce a desmosine cross link results in inter-connected,

rubbery network that can stretch and bend in any direction when stressed giving

connective tissue its elasticity.

α-Keratins The α-Keratins are proteins that form tough fibers. They are found in

hair, nails and outer epidermal layer of mammals.α-Keratins are also constituents of intermediate filaments of eth

cytoskeleton in certain cells.α-Keratins are rich in cysteine covalent disulfide cross-links

between adjacent polypeptide chains thus producing fibers that insoluble

and resistant to stretching.The α-Keratins of hair is an example of a protein constructed almost of

α-helices. Hair is composed of dead cells. Each cell is packed with keratin

macrofibrilsMacrofibriles are formed of microfibril embedded into a protein matrix.

Each microfibril is formed from protofibrils. Protofibrils are formed from α-helix protein