Probing the role of the hyper-reactive histidine residue of arginase

Rat liver arginase (arginase 1) is potently inactivated by diethyl pyrocarbonate, with a second-order rate constant of 113 M(-1)s(-1) for the inactivation process at pit 7.0, 25 degrees C. Partial protection front inactivation is provided by the product of the reaction, L-ornithine, while nearly complete protection is afforded by the inhibitor pair, t-ornithine and borate. The role of H141 has been probed by mutagenesis, chemical modulation, and X-ray diffraction. The hyper-reactivity of H141 towards diethyl pyrocarbonate can be explained by its proximity to E277. A proton shuttling role for H141 is Supported by its conformational mobility observed among the known arginase structures. H141 is proposed to serve as an acid/base catalyst, deprotonating the metal-bridging water molecule to generate the metal-bridging hydroxide nucleophile, and by protonating the amino group of the product to facilitate its departure. (c) 2005 Elsevier Inc. All rights reserved.