This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Most commonly occurring amino acid in the collagen structure is 
Select and explain. 
A. Serine, B. Glycine, C. Hydroxy proline, D. Proline
B: Glycine
Answer: B: Glycine 
Explanation: About 33%of aminoacids of collagen is glycine.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 720