FIBROUS PROTEINS COLLAGEN Major fibrous protein of epithelial

FIBROUS PROTEINS COLLAGEN

Major fibrous protein of epithelial tissues (cytoskeleton) is: keratin Major fibrous proteins of connective tissue are: Collagen Elastin

Important Fibrous Proteins • Intermediate filaments of the cytoskeleton – Structural scaffold inside the cell • Keratin in hair, horns and nails • Extracellular matrix – Bind cells together to make tissues – Secreted from cells and assemble in long fibers • Collagen – fiber with a glycine every third amino acid in the protein • Elastin – unstructured fibers that gives tissue an elastic characteristic

OVERVIEW Collagen and Elastin are examples of common, well-characterized FIBROUS PROTEINS of the extracellular matrix that serve structural functions in the body. Collagen and Elastin are found as components of skin, connective tissue, tendons, blood vessel walls, sclera and cornea of the eye. Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which are obtained by combining specific amino acids into regular, secondary structural elements. This is in contrast to globular proteins, whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements.

Collagen It is the major component of most connective tissues, constitute approximately 25 -30% of protein of mammals and it is the most abundant protein in the animal world. It provides extracellular framework for all animals and exists in every animal tissues. At least 25 distinct types of collagen made up of over 30 distinct polypeptide chains have been identified in human tissues.

TYPES OF COLLAGEN Variations in the amino acid sequence of the α chains result in structural components that are about the same size (approximately 1, 000 amino acids long), but with slightly different properties. These α chains are combined to form the various types of collagen found in the tissues.

Although these molecules are found throughout the body, their types and organization are dictated by the structural role collagen plays in a particular organ. In some tissues, collagen may be dispersed as a gel that gives support to the structure, as in the extracellular matrix or the vitreous humor of the eye. In other tissues, collagen may be bundled in tight, parallel fibers that provide great strength, as in tendons. In the cornea of the eye, collagen is stacked so as to transmit light with a minimum of scattering. Collagen of bone occurs as fibers arranged at an angle to each other so as to resist mechanical shear from any direction.

Collagen Where: 25 -30% of total protein. Major protein of connective tissues: bones, tendons, ligaments, basement membranes, dentin, cementum, (not enamel). Cellular location: extracellular matrix. Structure: triple helix (tropocollagen). Subsequent to secretion, tropocollagen is assembled and crosslinked to make insoluble collagen fibers. Function: Provides tensile strength to soft connective tissues. Tissues that must be elastic but exhibit tensile strength (e. g. ligaments) have a mixture of collagen and elastin. Collagen fibers in bone reinforce against fracture.

Structure of Collagen • Each unit of tropocollagen is about 1. 5 nm wide and 300 nm long. Bundles of these 3 -stranded supercoils can be seen as collagen fibres in the electron microscope. • Mature collagen has extensive covalent crosslinkages (H bonds) between individual collagen molecules.

COLLAGEN NOMENCLATURE: An individual polypeptide: pro a 1 collagen Assembled triple helix with propeptides still on: procollagen Triple helix after propeptide removal: tropocollagen Assembled into 50 nm fiber: Collagen microfibril

A typical collagen MOLECULE is a long, rigid structure in which three polypeptides (referred to as “α chains”) are wound around one another in a rope-like triple helix

Structure of Collagen • The structural unit of collagen is a TROPOCOLLAGEN, a supercoil made up of 3 helices, with a molecular mass of ~285 kdal. • Each collagen helix consists of ~ 1000 amino acid residues. The helix is left-handed. It is not an a-helix. • The helix contains 3 amino acids per turn, with a pitch of 0. 94 nm. (continued)

Triple-helical structure: Elongated, triple-helical structure places many of its amino acid side chains on the surface of the triplehelical molecule. [Note: This allows bond formation between the exposed R-groups of neighboring collagen monomers, resulting in their aggregation into long fibers. ]

Hydroxyproline and hydroxylysine: Collagen contains hydroxy -proline (hyp) and hydroxylysine (hyl), which are not present in most other proteins. These residues result from the hydroxylation of some of the proline and lysine residues after their incorporation into polypeptide chains. The hydroxylation is, thus, an example of post translational modification. Hydroxy - proline is important in stabilizing the triple-helical structure of collagen because it maximizes interchain hydrogen bond formation.

Collagen 3 stranded helix

AMINO ACID SEQUENCE: Collagen is rich in proline and glycine, both of which are important in the formation of the triple-stranded helix. PROLINE facilitates the formation of the helical conformation of each α chain because its ring structure causes “kinks” in the peptide chain. Glycine, the smallest amino acid, is found in every third position of the polypeptide chain. It fits into the restricted spaces where three chains of the helix come together. The glycine residues are part of a repeating sequence, –Gly–X–Y–, where X is frequently proline and Y is often hydroxyproline Thus, most of the α chain can be regarded as a polytripeptide whose sequence can be represented as (–Gly–Pro–Hyp–).

Stabilizing Cross-Links • Cross linkages can be between 2 parts of a protein or between 2 subunits • Disulfide bonds (S-S) form between adjacent -SH groups on the amino acid cysteine

Structure of Collagen • There is no intra-helical Hbonding in collagen helices. • Rather, H-bonding occurs between the amide N of GLYCINE residues in the central axis and the carbonyls of other residues in the adjacent chains. Often PROLINE and HYDROXYPROLINE are involved. (continued)

Fibril-forming collagens: Types I, II, and III are the fibrillar collagens, and have the rope-like structure described above for a typical collagen molecule. In the electron microscope, these linear polymers of fibrils have characteristic banding patterns, reflecting the regular taggered packing of the individual collagen molecules in the fibril.

Network-forming collagens: Types IV and VII form a threedimensional mesh, rather than distinct fibrils. For example, type IV molecules assemble into a sheet or meshwork that constitutes a major part of basement membranes. Basement membranes are thin, sheet-like structures that provide mechanical support for adjacent cells, and function as a semipermeable filtration barrier to macromolecules in organs such as the kidney and the lung.

Fibril-associated collagens: Types IX and XII bind to the surface of collagen fibrils, linking these fibrils to one another and to other components in the extracellular matrix.

Examples of collagen nomenclature: An individual polypeptide: pro a 1 collagen Assembled triple helix with propeptides still on: procollagen Triple helix after propeptide removal: tropocollagen Assembled into 50 nm fiber: Collagen microfibril

Degradation of collagen Normal collagens are highly stable molecules, having half-lives as long as several years. However, connective tissue is dynamic and is constantly being remodeled, often in response to growth or injury of the tissue. Breakdown of collagen fibers is dependent on the proteolytic action of collagenases, which are part of a large family of matrix metalloproteinases. For type I collagen, the cleavage site is specific, generating three-quarter and one-quarter length fragments. These fragments are further degraded by other matrix proteinases to their constituent amino acids.