Clostridium kluyveri Re-citrate synthase and related proteins, catalytic TIM barrel domain
Re-citrate synthase (Re-CS) is a Clostridium kluyveri enzyme that converts acetyl-CoA and oxaloacetate to citrate. In most organisms, this reaction is catalyzed by Si-citrate synthase which is Si-face stereospecific with respect to C-2 of oxaloacetate, and phylogenetically unrelated to Re-citrate synthase. Re-citrate synthase is also found in a few other strictly anaerobic organisms. This family belongs to the DRE-TIM metallolyase superfamily. DRE-TIM metallolyases include 2-isopropylmalate synthase (IPMS), alpha-isopropylmalate synthase (LeuA), 3-hydroxy-3-methylglutaryl-CoA lyase, homocitrate synthase, citramalate synthase, 4-hydroxy-2-oxovalerate aldolase, re-citrate synthase, transcarboxylase 5S, pyruvate carboxylase, AksA, and FrbC. These members all share a conserved triose-phosphate isomerase (TIM) barrel domain consisting of a core beta(8)-alpha(8) motif with the eight parallel beta strands forming an enclosed barrel surrounded by eight alpha helices. The domain has a catalytic center containing a divalent cation-binding site formed by a cluster of invariant residues that cap the core of the barrel. In addition, the catalytic site includes three invariant residues - an aspartate (D), an arginine (R), and a glutamate (E) - which is the basis for the domain name "DRE-TIM".
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