Design of Amino Acid Aldehydes as Transition-State Analogue Inhibitors of Arginase

Abstract

Arginase is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea. Chiral l-amino acids bearing aldehyde side chains have been synthesized in which the electrophilic aldehyde CO bond is isosteric with the CN bond of l-arginine. This substitution is intended to facilitate nucleophilic attack by the metal-bridging hydroxide ion upon binding to the arginase active site. Syntheses of the amino acid aldehydes have been accomplished by reduction, oxidation, and Wittig-type reaction with a commercially available derivative of l-glutamic acid. Amino acid aldehydes exhibit inhibition in the micromolar range, and the X-ray crystal structure of arginase I complexed with one of these inhibitors, (S)-2-amino-7-oxoheptanoic acid, has been determined at 2.2 Å resolution. In the enzyme−inhibitor complex, the inhibitor aldehyde moiety is hydrated to form the gem-diol: one hydroxyl group bridges the Mn2+2 cluster and donates a hydrogen bond to D128, and the second hydroxyl group donates a hydrogen bond to E277. The binding mode of the neutral gem-diol may mimic the binding of the neutral tetrahedral intermediate and its flanking transition states in arginase catalysis.