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
Trypsin cleaves 
Select and explain. 
A. Arginine, B. Glutamate, C. Lysine, D. Proline
A: Arginine
Answer: A: Arginine 
Explanation: Activation of zymogens: Enteropeptidase (formerly called enterokinase), an enzyme synthesized by and present on the luminal surface of intestinal mucosal cells of the brush border membrane, conves the pancreatic zymogen trypsinogen to trypsin by removal of a hexapeptide from the N-terminus of trypsinogen. Trypsin subsequently conves other trypsinogen molecules to trypsin by cleaving a limited number of specific peptide bonds in the zymogen. Enteropeptidase, thus, unleashesa cascade of proteolytic activity because trypsin is the common activator of all thepancreatic zymogens .