Streptomyces rubellomurinus FrbC and related proteins, catalytic TIM barrel domain
FrbC (NifV) of Streptomyces rubellomurinus catalyzes the condensation of acetyl-CoA and alpha-ketoglutarate to form homocitrate and CoA, a reaction similar to one catalyzed by homocitrate synthase. The gene encoding FrbC is one of several genes required for the biosynthesis of FR900098, a potent antimalarial antibiotic. This protein is also required for assembly of the nitrogenase MoFe complex but its exact role is unknown. This family also includes the NifV proteins of Heliobacterium chlorum and Gluconacetobacter diazotrophicus, which appear to be orthologous to FrbC. 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".