Mechanistic Studies of 1-Aminocyclopropane-1-carboxylate Deaminase (ACCD): Characterization of an Unusual PLP-dependent Reaction

摘要

1-Aminocyclopropane-1-carboxylic acid (ACC) deaminase (ACCD) is a pyridoxal 5′-phosphate (PLP) dependent enzyme that cleaves the cyclopropane ring of ACC, to give α-ketobutyric acid and ammonia as products. The cleavage of the Cα-Cβ bond of an amino acid substrate is a rare event in PLP-dependent enzyme catalysis. Potential chemical mechanisms involving nucleophile- or acid-catalyzed cyclopropane ring opening have been proposed for the unusual transformation catalyzed by ACCD, but the actual mode of cyclopropane ring cleavage remains obscure. In this report, we aim to elucidate the mechanistic features of ACCD catalysis by investigating the kinetic properties of ACCD from Pseudomonas sp. ACP and several of its mutant enzymes. Our studies suggest that the pKa of the conserved active site residue, Tyr294, is lowered by a hydrogen bonding interaction with a second conserved residue, Tyr268. This allows Tyr294 to deprotonate the incoming amino group of ACC in order to initiate the aldimine exchange reaction between ACC and the PLP coenzyme, and also likely helps to activate Tyr294 for a role as nucleophile to attack and cleave the cyclopropane ring of the substrate. In addition, solvent kinetic isotope effect, proton inventory, and ¹³C-KIE studies of the wild type enzyme suggest that the Cα-Cβ bond cleavage step in the chemical mechanism is at least partially rate limiting under kcat/Km conditions, and is likely preceded in the mechanism by a partially rate limiting step involving the conversion of a stable gem-diamine intermediate into a reactive external aldimine intermediate that is poised for cyclopropane ring cleavage. When viewed within the context of previous mechanistic and structural studies of ACCD enzymes, our studies are most consistent with a mode of cyclopropane ring cleavage involving nucleophilic catalysis by Tyr294.