Collagen is the main protein of connective tissue. It has great tensile strength, and is the main component of ligaments and tendons. It is responsible for skin elasticity, and its degradation leads to wrinkles that accompany aging. Collagen also fills out the cornea where it is present in crystalline form.
Collagen has very interesting amino acid composition. It contains a lot of glycine and proline, as well as two amino acids that are not inserted directly by ribosomes: hydroxyproline and hydroxylysine, the former as a rather large percentage of the total amino acids. They are changed from proline and lysine in enzymatic processes, for which vitamin C is required.
Another rare feature of collagen is its regular arrangement of amino acids. Its sequence generally follows the pattern Gly-X-Y, where X is proline, and Y is proline or hydroxyproline. There are very few other proteins with such regularity. The resulting structure is called a collagen helix.
Collagen is a major structural protein, forming molecular cables that strengthen the tendons and vast, resilient sheets that support the skin and internal organs. Bones and teeth are made by adding mineral crystals to collagen. Collagen provides structure to our bodies, protecting and supporting the softer tissues and connecting them with the skeleton. But, in spite of its critical function in the body, collagen is a relatively simple protein. Collagen is composed of three chains, wound together in a tight triple helix. The illustration included here shows only a small segment of the entire molecule--each chain is over 1400 amino acids long and only about 20 are shown here. A repeated sequence of three amino acids forms this sturdy structure. Every third amino acid is glycine, a small amino acid that fits perfectly inside the helix. Many of the remaining positions in the chain are filled by two unexpected amino acids: proline and a modified version of proline, hydroxyproline. We wouldn't expect proline to be this common, because it forms a kink in the polypeptide chain that is difficult to accommodate in typical globular proteins. But, as you can see on the next page, it seems to be just the right shape for this structural protein.
Like other proteins, collagen consists of polypeptide chains; the long chains of this fibrous molecule contain about one thousand amino-acid residues, about sixteen thousand atoms. It differs from almost all other proteins in being substantially composed of but two amino acids, glycine and hydroxyproline. Collagen is a kind of supermolecule, however, in its three-dimensional architecture. The polypeptide chains of the two amino acids, alternating with one another and punctuated by the presence of certain other amino acids, are coiled in a left-handed helix. Three of these helical strands are twisted around on another, like strands of a rope, in a right handed superhelix, to compose the complete molecule.
Collagen is an inert, rigid protein found predominantly in skin, ligaments, bones and teeth. Its most distinctive characteristic in its role as a transmitter of mechanical force is its inelasticity. The fundamental structural unit is a tropo-collagen, a molecular rod about 2600 Å in length and 15 Å in diameter with a molecular weight of 300,000 daltons. In tendons, these macromolecules, grouped as collagen fibrils, run parallel to the axis; in skin the fibrils are interlaced and branched. Collagen fibers with limited cross-linkages (i.e., unaged) will dissolve to some extent in dilute acid or concentrated neutral salt solutions.