Collagens are found in tendons and other connective ligaments. Collagens have a triple helix as the major structure.The keratin structure is described more fully in the next section. The main differences in various keratins arises from their sulfur content. If there are many cysteine disulfide cross-links, then there is very little flexibility as in horns, claws, hooves, or nails. In wool, skin, and muscle proteins, there are fewer disulfide cross-links which allows some stretching but returns to normal upon relaxation of tension.
The quaternary structure of collagen consists of three left-handed helices twisted into a right-handed coil. This structure is shown in the graphic on the left.The basic properties of collagen are rigidity and resistance to stretching. The helices are grouped in a variety of more complex fiber type structures. A rigid pattern of interchain amide-carbonyl hydrogen bonding is best accommodated by a primary amino acid sequence of:
-gly-X-pro- or -gly-X-hypro
where X is any amino acid and hypro is hydroxyproline. In the graphic on the left X = arginine. Molecular model studies show that this sequence works the best for the triple helix structure. Glycine is needed because it is small and is the only amino acid which can fit in the interior of the triple helix.
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