Question

why does collagen always occur as a trimer of three subunits?

Answer 1

Collagen is a fibrous protein. It is found in skin, tendons, cartilage, bones and the walls of blood vessels. It occurs as trimer of three sub units beacuse it provide it the great tensile strength. Which is due to that every third residue of each polypeptide passes through the centre of the triple helix, which is so crowded that only the small R group of glycine (Gly) can fit in. This allows the three helical a-chains to pack tightly together. Furthermore, the triple helix is held together by an extensive network of hydrogen bonds. Covalent crosslinks are also present within tropocollegen molecules to impart the collegen fibre with strength and rigidity. The repetitive regular tripeptide sequence of amino acids with an unusually high amount of glycine and praline residues make it insoluble in water.