This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
The most predominantly occurring amino acid in collagen is 
Select and explain. 
A. Valine, B. Cysteine, C. Arginine, D. Glycine
D: Glycine
Answer: D: Glycine 
Explanation: 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