Citrate synthase (CS) catalyzes the condensation of acetyl coenzyme A (AcCoA) and oxalacetate (OAA) to form citrate and coenzyme A (CoA), the first step in the oxidative citric acid cycle (TCA or Krebs cycle). Peroxisomal CS is involved in the glyoxylate cycle. This group also includes CS proteins which functions as a 2-methylcitrate synthase (2MCS). 2MCS catalyzes the condensation of propionyl-CoA (PrCoA) and OAA to form 2-methylcitrate and CoA during propionate metabolism. This group contains proteins which functions exclusively as either a CS or a 2MCS, as well as those with relaxed specificity which have dual functions as both a CS and a 2MCS. The overall CS reaction is thought to proceed through three partial reactions and involves both closed and open conformational forms of the enzyme: a) a carbanion or equivalent is generated from AcCoA by base abstraction of a proton, b) nucleophilic attack of this carbanion on OAA to generate citryl-CoA, and c) hydrolysis of citryl-CoA to produce citrate and CoA. CSs are found in two structural types: type I (homodimeric) and type II CSs (homohexameric). Type II CSs are unique to gram-negative bacteria. Type I CSs are found in eukarya, gram-positive bacteria, archaea, and in some gram-negative bacteria. Type I CS is active as a homodimer, both subunits participating in the active site. Type II CS is a hexamer of identical subunits (approximated as a trimer of dimers). Some type II CSs are strongly and specifically inhibited by NADH through an allosteric mechanism. This subgroup includes both gram-positive and gram-negative bacteria.
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