This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide? \nSelect and explain. \nA. Trypsin, B. Carboxypeptidase, C. Phenylisothiocyanate, D. Cyanogen bromide', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Phenylisothiocyanate is used in the Edman degradation for N-terminal analysis. Trypsin is an endopeptidase and cleaves internal peptide bonds and therefore is not generally useful in determining the N-terminal amino acid residue. Carboxypeptidase is an exopeptidase that sequentially removes one amino acid at a time from the carboxyl end of the polypeptide chain and is used to determine the C-terminal amino acid. Cyanogen bromide cleaves at internal methionine residues in a polypeptide chain.', 'role': 'assistant'}]