Inhibitor Coordination Interactions in the Binuclear Manganese Cluster of Arginase

摘要

Arginase is a manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to form L-ornithine and urea.The structure and stability of the binuclear manganese cluster are critical for catalytic activity as it activates the catalytic nucleophile,metal-bridging hydroxide ion,and stabilizes the tetrahedral intermediate and its flanking states.Here,we report X-ray structures of a series of inhibitors bound to the active site of arginase,and each inhibitor exploits a different mode of coordination with the Mn~(2+)_2 cluster.Specifically,we have studied the binding of fluoride ion (F~-;an uncompetitive inhibitor) and L-arginine,L-valine,dinor-N~(omega)-hydroxy-L-arginine,descarboxy-nor-N~(omega)-hydroxy-L-arginine,and dehydro-2(S)-amino-6-boronohexanoic acid.Some inhibitors,such as fluoride ion,dinor-N~(omega)-hydroxy-L-arginine,and dehydro-2(S)-amino-6-boronohexanoic acid,cause the net addition of one ligand to the Mn~(2+)_2 cluster.Other inhibitors,such as descarboxy-nor-N~(omega)-hydroxy-L-arginine,simply displace the metal-bridging hydroxide ion of the native enzyme and do not cause any net change in the metal coordination polyhedra.The highest affinity inhibitors displace the metal-bridging hydroxide ion (and sometimes occupy a Mn~(2+)_A site found vacant in the native enzyme) and maintain a conserved array of hydrogen bonds with their a-amino and -carboxylate groups.