Saccharomyces cerevisiae (Sc) citrate synthase (CS)-like. 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). This group includes three S. cerevisiae CS proteins, ScCit1,-2,-3. ScCit1 is a nuclear-encoded mitochondrial CS with highly specificity for AcCoA; in addition to having activity with AcCoA, it 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. ScCit3 is a mitochondrial CS and functions in the metabolism of PrCoA; it is a dual specificity CS and 2MCS, having similar catalytic efficiency with both AcCoA and PrCoA. The pattern of expression of the ScCIT3 gene follows that of the ScCIT1 gene and its expression is increased in the presence of a ScCIT1 deletion. Included in this group is the Tetrahymena 14 nm filament protein which functions as a CS in mitochondria and as a cytoskeletal component in cytoplasm and Geobacter sulfurreducens (GSu) CS. GSuCS is dimeric and eukaryotic-like; it lacks 2MCS activity and is inhibited by ATP. In contrast to eukaryotic and other prokaryotic CSs, GSuCIT is not stimulated by K+ ions. 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. - 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
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