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Study Title:

Collagen: The Fibrous Proteins of the Matrix

Study Abstract

Collagen is the major insoluble fibrous protein in the extracellular matrix and in connective tissue. In fact, it is the single most abundant protein in the animal kingdom. There are at least 16 types of collagen, but 80 – 90 percent of the collagen in the body consists of types I, II, and III (Table 22-3). These collagen molecules pack together to form long thin fibrils of similar structure (see Figure 5-20). Type IV, in contrast, forms a two-dimensional reticulum; several other types associate with fibril-type collagens, linking them to each other or to other matrix components. At one time it was thought that all collagens were secreted by fibroblasts in connective tissue, but we now know that numerous epithelial cells make certain types of collagens. The various collagens and the structures they form all serve the same purpose, to help tissues withstand stretching.

The Basic Structural Unit of Collagen Is a Triple Helix

Because its abundance in tendon-rich tissue such as rat tail makes the fibrous type I collagen easy to isolate, it was the first to be characterized. Its fundamental structural unit is a long (300-nm), thin (1.5-nm-diameter) protein that consists of three coiled subunits: two α1(I) chains and one α2(I).* Each chain contains precisely 1050 amino acids wound around one another in a characteristic right-handed triple helix (Figure 22-11a). All collagens were eventually shown to contain three-stranded helical segments of similar structure; the unique properties of each type of collagen are due mainly to segments that interrupt the triple helix and that fold into other kinds of three-dimensional structures.
Figure 22-11. The structure of collagen.
Figure 22-11

The structure of collagen. (a) The basic structural unit is a triple-stranded helical molecule. Each triple-stranded collagen molecule is 300 nm long. (b) In fibrous collagen, collagen molecules pack together side by side. Adjacent molecules are displaced (more...)

The triple-helical structure of collagen arises from an unusual abundance of three amino acids: glycine, proline, and hydroxyproline. These amino acids make up the characteristic repeating motif Gly-Pro-X, where X can be any amino acid. Each amino acid has a precise function. The side chain of glycine, an H atom, is the only one that can fit into the crowded center of a three-stranded helix. Hydrogen bonds linking the peptide bond NH of a glycine residue with a peptide carbonyl (C═O) group in an adjacent polypeptide help hold the three chains together. The fixed angle of the C – N peptidyl-proline or peptidyl-hydroxyproline bond enables each polypeptide chain to fold into a helix with a geometry such that three polypeptide chains can twist together to form a three-stranded helix. Interestingly, although the rigid peptidyl- proline linkages disrupt the packing of amino acids in an α helix, they stabilize the rigid three-stranded collagen helix.