Collagen

Collagen is the most abundant protein in the body, comprising approximately 25% of the body's protein. There are actually at least 19 distinct but related types. The molecules of each type consist of three long polypeptide chains tightly wound together to form a triple helix. Types I, II, III and V have long and fibrous molecules: the other types are non-fibrillar. In some cases there are two identical chains and one that is different, while in other types the three chains are identical. As far as is known the individual polypeptides in the various types are all encoded by separate genes. Most of the mature chains are about 1000 amino acids long and are characterised by the presence of a glycine residue at every third position through a good deal of their length. Its tiny side chain can be easily accommodated inside the tightly wound helix. They are also unusually rich in proline, and many prolyl residues (and also some lysyl ones) undergo post-translational hydroxylation. There are extensions at each end of the molecules (telo-peptides) that are removed after the chains are wound together to form the triple helix. In some types the regular repetitions of glycine followed by two other amino acids are interrupted at intervals so that the molecules may be kinked rather than straight. The exons of the genes of the fibrillar collagens tend to be rather short, and the gene for Type I contains no fewer than 54 exons that mostly contain 45 or 54 nt or multiples or sums of these numbers, beginning with complete glycine codons.