1. Collagen, elastin and keratin
Fibrous proteins
Collagen, 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.

2. - 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

3. 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.

4. 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-link
Elastin 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.

5. 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 macrofibrils
Macrofibriles are formed of microfibril embedded into a protein matrix. Each microfibril is formed from protofibrils.
Protofibrils are formed from α-helix protein