Saccharomyces cerevisiae (Sc) citrate synthases Cit1-2_like. Citrate synthases (CS) catalyzes the condensation of acetyl coenzyme A (AcCoA) with oxaloacetate (OAA) to form citrate and coenzyme A (CoA), the first step in the citric acid cycle (TCA or Krebs cycle). Some CS proteins function as 2-methylcitrate synthase (2MCS). 2MCS catalyzes the condensation of propionyl-coenzyme A (PrCoA) and OAA to form 2-methylcitrate and CoA during propionate metabolism. 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). ScCit1 is a nuclear-encoded mitochondrial CS with highly specificity for AcCoA. In addition to its CS function, ScCit1 plays a part in the construction of the TCA cycle metabolon. Yeast cells deleted for Cit1 are hyper-susceptible to apoptosis induced by heat and aging stress. ScCit2 is a peroxisomal CS involved in the glyoxylate cycle; in addition to having activity with AcCoA, it may have activity with PrCoA. Chicken and pig heart CS, two Arabidopsis thaliana (Ath) CSs, CSY4 and -5, and Aspergillus niger (An) CS also belong to this group. Ath CSY4 has a mitochondrial targeting sequence; AthCSY5 has no identifiable targeting sequence. AnCS encoded by the citA gene has both an N-terminal mitochondrial import signal and a C-terminal peroxisiomal target sequence; it is not known if both these signals are functional in vivo. 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.
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.
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.
Structure:6CTS_A, Gallus gallus heart type I CS monomer bound with citryl-thioester CoA, contacts at 3.5 A.
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