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Chloroflexus aurantiacus (Ca) citrate synthase (CS)_like. CS catalyzes the condensation of acetyl coenzyme A (AcCoA) and oxalacetate (OAA) to form citrate and coenzyme A (CoA), the first step in the citric acid cycle (TCA or Krebs cycle). This group is similar to gram-negative Escherichia coli (Ec) CS (type II, gltA) and Arabidopsis thaliana (Ath) peroxisomal (Per) CS. However EcCS and AthPerCS are not found in this group. The overall CS reaction is thought to proceed through three partial reactions and involves both closed and open conformational forms of the enzyme: a) the carbanion or equivalent is generated from AcCoA by base abstraction of a proton, b) the nucleophilic attack of this carbanion on OAA to generate citryl-CoA, and c) the hydrolysis of citryl-CoA to produce citrate and CoA. There are two types of CSs: type I CS and type II CSs. Type I CSs are found in eukarya, gram-positive bacteria, archaea, and in some gram-negative bacteria and are homodimers with both subunits participating in the active site. Type II CSs are unique to gram-negative bacteria and are homohexamers of identical subunits (approximated as a trimer of dimers). Some type II CSs are strongly and specifically inhibited by NADH through an allosteric mechanism. C. aurantiacus is a gram-negative thermophilic green gliding bacterium, its CS belonging to this group may be a type I CS; it is not inhibited by NADH or 2-oxoglutarate and is inhibited by ATP. Both gram-positive and gram-negative bacteria are found in this group.
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