2-isopropylmalate synthase (IPMS), N-terminal catalytic TIM barrel domain
2-isopropylmalate synthase (IPMS) catalyzes an aldol-type condensation of acetyl-CoA and 2-oxoisovalerate yielding 2-isopropylmalate and CoA, the first committed step in leucine biosynthesis. This family includes the Arabidopsis thaliana IPMS1 and IPMS2 proteins, the Glycine max GmN56 protein, and the Brassica insularis BatIMS protein. This family also includes a group of archeal IPMS-like proteins represented by the Methanocaldococcus jannaschii AksA protein. AksA catalyzes the condensation of alpha-ketoglutarate and acetyl-CoA to form trans-homoaconitate, one of 13 steps in the conversion of alpha-ketoglutarate and acetylCoA to alpha-ketosuberate, a precursor to coenzyme B and biotin. AksA also catalyzes the condensation of alpha-ketoadipate or alpha-ketopimelate with acetylCoA to form, respectively, the (R)-homocitrate homologs (R)-2-hydroxy-1,2,5-pentanetricarboxylic acid and (R)-2-hydroxy-1,2,6- hexanetricarboxylic acid. 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".