An insoluble fibrous protein found extensively in the connective tissue of skin, tendons, and bone, in the walls of the large arteries, and in the extracellular matrix. The polypeptide chains of collagen (containing the amino acids glycine and proline predominantly) form triple-stranded helical coils that are bound together to form fibrils, which have great strength and limited elasticity. Collagen accounts for over 25% of the total body protein of mammals.