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) 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 form 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. This subgroup includes both gram-positive and gram-negative bacteria.
Comment:The overall CS reaction involves both closed and open conformational forms of the enzyme.
Structure:6CSC_A, Gallus gallus type I CS dimer bound with citrate, contacts at 3.5 A. - View structure with Cn3D
Structure:6CTS_A, Gallus gallus heart type I CS monomer bound with citryl-thioester CoA, contacts at 3.5 A. - View structure with Cn3D
Comment:Citryl-thioester CoA is a potent inhibitor of chicken heart CS. It is extremely similar to the citryl-CoA intermediate; the only difference is that the O-3 of citrate has been substituted with two hydrogen atoms preventing hydrolysis of the compound.