Mechanism-based inactivation of bacterial kynureninase by beta-substituted amino acids.

Kynureninase from Pseudomonas marginalis has been shown to catalyze the elimination of beta-functionalities of beta-substituted amino acids such as beta-chloro-L-alanine, resulting in the formation of aminoacrylate-pyridoxal phosphate-enzyme complex. This intermediate can be processed further to produce either pyruvate, ammonia, and active enzyme or an inactive enzyme complex. Approximately 1 in 500 turnovers leads to inactivation of the enzyme. The mechanism of inactivation appears to involve nucleophilic addition of a carboxylate group at the active site to the beta-carbon of the aminoacrylate complex. Both subunits of kynureninase have been shown to be catalytically competent although the native enzyme contains only one pyridoxal phosphate per dimer. Since both aspartate beta-decarboxylase and kynureninase catalyze mechanistically similar reactions, these results further support the notion that the two active sites may have several common features.