Fibrous proteins collagen and elastin

The collagens constitute what is now known to be a highly specialized family of glycoproteins. At present, 20 genetically distinct types encoded by at least 30 genes have been described. All collagens contain a domain with a triple helical conformation and are integral components of the extracellular matrix.

In triple helical domains three polypeptide (a) chains, each with a left-handed conformation, wind round each other to form a right-handed superhelix. Every third residue is glycine and of the repeating triplet [Gly-X-Y] that makes up the chain, approximately 20% of X and Y are imino acids, proline and hydroxyproline. This triple helical domain varies in length for the different collagen types and can either be continuous or interrupted with nonhelical domains.

The molecular configuration and tissue distribution of some of the collagens are shown in Table 1. Attempts have been made to classify collagens, both with regard to their gene organization and supramolecular structure, and while some of the more recently described collagens cannot be categorized in this way, it does provide a useful guide as to their role in the ECM.

The bulk of the collagen exists as fibrillar collagens, types I, II, III, V and XI. In these assemblies, molecules pack in quarter-staggered arrays to produce banded fibrils and fibers which provide mechanical support to the tissue (Figure 1). In contrast the FACIT (fibril associated collagens with interrupted triple helices), types IX, XII and XIV, are associated with the fibrillar collagens and may serve as a molecular link between fibrillar collagens and other matrix molecules. A different form of assembly is found in basement membranes, where type IV collagen forms a three-dimensional network (Figure 1). In addition beaded microfilaments are produced from type VI collagen (Figure 1), antiparallel dimers

Table 1 The collagen family

Collagen types Number Supramolecular form Distribution of a-

Collagen types Number Supramolecular form Distribution of a-

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