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
[{'content': 'Cystine is formed by \nSelect and explain. \nA. Hydroxylation of cysteine molecule, B. Carboxylation of cysteine molecule, C. Peptide bond between two cysteine molecule, D. Disulfide bond between cysteine molecule', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The difference in structure of cysteine and cystine. Two molecules of cysteine are joined together by S--S bond to form one molecule of cystine. Ref: M.N. Chatterjea - Textbook of Biochemistry, 8th edition, page no: 504', 'role': 'assistant'}]