Stereochemistry and mechanism of aldol reactions catalyzed by Kynureninase

Article abstract of DOI:10.1021/ja00019a039

Kynureninase from Pseudomonas has been reported to catalyze aldol and retro-aldol reactions, in addition to the physiological hydrolytic cleavage of L-kynurenine to anthranilic acid and L-alanine. However, the stereochemistry of these novel aldol reactions has not been previously determined. We have determined that the reaction of L-kynurenine and benzaldehyde catalyzed by kynureninase results in (2S,4A)-2-amino-4-hydroxy-4-phenylbutanoic acid. Similarly, the 4R isomer of dihydro-L-kynurenine readily undergoes retro-aldol cleavage, while the 4S isomer is unreactive as a substrate. Both isomers of dihydro-L-kynurenine are competitive inhibitors of kynureninase from Pseudomonas. However, the 4S isomer of dihydro-L-kynurenine is the most potent inhibitor, with a K_i of 0.3 μM. These results provide additional support for a general base mechanism for kynureninase, and suggest that the hydration occurs on the re face of the carbonyl group of kynurenine to give an (S)-gem-diolate intermediate.