Inhibitor coordination interactions in the binuclear manganese cluster of arginase

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-NO-hydroxy-L-arginine, descarboxy-nor-N-omega-hydroxy-L-arginine, and dehydro-2(S)-amino-6-boronchexanoic 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-A(2+) site found vacant in the native enzyme) and maintain a conserved array of hydrogen bonds with their alpha-amino and -carboxylate groups.