Functional residues at the active site of aminopeptidase N

Sequence analysis of aminopeptidase N has shown that this zinc exopeptidase contains a consensus sequence (Val-Xaa-Xaa-His-Glu-Xaa-Xaa-His), generally found at the active site of zinc endopeptidases [Jongeneel, C. V., Bouvier, J. and Bairoch, A. (1989) FEBS Lett. 242, 211-214]. This suggests that the active site of aminopeptidase N may be closer to that of a classical zinc endopeptidase, such as thermolysin, than to that of an exopeptidase, such as carboxypeptidase A, which does not contain the above sequence. However, the nature of the other amino acids involved in the enzymatic activity of the eukaryotic aminopeptidase N remains unknown. Chemical modifying agents have now been used to characterize the active site of aminopeptidase N further. The location of the modified residues was also determined by comparing the protection given by three competitive inhibitors which interact with different subsites of the active site. Aminopeptidase N was rapidly inactivated by 2,3-butanedione and diethylpyrocarbonate and partially inactivated by N-acetylimidazole, diazoacetamide and a soluble carbodiimide, suggesting the presence of functional arginyl, histidyl, tyrosyl and aspartyl/glutamyl residues. In each case the reaction kinetics showed that the inactivation could be correlated with modification of a single residue. The protection experiments indicated that the residues are at the active site of the enzyme and that the arginine and tyrosine are probably located in the S'1-S'2 subsites, histidine in the S1 subsite and the acidic residue near the zinc binding site and the S'1 subsite. Steady-state kinetics showed that the arginine, histidine and acidic residues are involved in substrate binding, while the tyrosine may play a role in the catalytic process. All these data support an endopeptidase-like structure for the active site of aminopeptidase N.