Kynureninase (KYNU)

Kynureninase is a PLP dependent enzyme that catalyses the cleavage of Kyn into Ant. It can also act on 3hKyn (to produce 3hAnt) and some other (3-arylcarbonyl)-alanines.KYNU is part of the pathways for the catabolism of Trp and the biosynthesis of NAD cofactors from Trp. In KYNU reaction, PLP facilitates C_β-C_γ bond cleavage. The reaction follows the same steps as the transamination reaction but does not hydrolyze the tautomerized Schiff base. The proposed reaction mechanism involves an attack of an enzyme nucleophile on the carbonyl carbon (C_γ) of the tautomerized 3hKyn-PLP Schiff base. This is followed by C_β-C_γ bond cleavage to generate an acyl-enzyme intermediate together with a tautomerized Ala-PLP adduct. Hydrolysis of the acyl-enzyme then yields 3hAnt.