The most predominantly occurring amino acid in collagen is

Tropocollagen, the repeating unit of a mature collagen fiber, consists of three collagen polypeptides, each containing about 1000 amino acids, bundled together in a unique conformation, the collagen triple helix. Every third amino acid residue in collagen is a glycine residue. Collagen is also rich in proline and hydroxyproline, yielding a repetitive Gly-X-Y pattern in which Y generally is proline or hydroxyproline.Reference: Explanation: Harper's Biochemistry; 30th edition; Chapter 5; proteins: higher Orders of Structure