♦ See referenced article, J. Biol. Chem. 2011, 286, 27399–27405
In the DUF849 family of prokaryotic proteins, a novel cleavage enzyme that metabolizes 3-keto-5-aminohexanoate (KAH), an intermediate of lysine fermentation, was discovered and reported a few years ago in JBC. In their current JBC Paper of the Week, Bellinzoni et al. bring dramatic mechanistic insights to the study of the KAH cleavage enzyme (Kce). The authors reveal a Claisen-type enzyme mechanism in which acetyl-CoA and the six-carbon KAH substrate are reversibly converted into two four-carbon species: acetoacetate and 3-aminobutyryl-CoA. Crystallographic data, with Kce bound to KAH or acetoacetate, confirm a triose phosphate isomerase (TIM) barrel core and resolve the coordination of Zn2+ by both active site residues and the β-keto acid functions of substrates. A well-ordered water molecule and an Asp-Arg dyad are readily modeled into a charge relay system that supports enolate transitioning of both KAH and acetoacetate. Site-directed mutagenesis supports this mechanism, which excludes an acyl-enzyme intermediate and rather posits the intramolecular shift of the CoA moiety within the eight-carbon bound intermediate. This work exemplifies the value to be excavated from genomic data through biochemical expertise.
Familiar folds, novel chemistry.