Citrate Synthase

The enzyme citrate synthase (E.C. 2.3.3.1 ) exists in nearly all living cells and stands as a pace-making enzyme in the first step of the Citric Acid Cycle (or Krebs Cycle). Citrate synthase is localized within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial. It is synthesized using cytoplasmic ribosomes, then transported into the mitochondrial matrix. Citrate synthase is commonly used as a quantitative enzyme marker for the presence of intact mitochondria.

Citrate synthase catalyzes the condensation reaction of the two-carbon acetate residue from acetyl coenzyme A and a molecule of four-carbon oxaloacetate to form the six-carbon citrate. Oxaloacetate will be regenerated after the completion of one round of the Krebs Cycle.

• acetyl-CoA

• Oxaloacetic acid

• Citric acid

acetyl-CoA + oxaloacetate + H₂O → citrate + CoA-SH

Oxaloacetate is the first substrate to bind to the enzyme. This induces the enzyme to change its conformation, and creates a binding site for the acetyl-CoA. Only when this citroyl-CoA has formed will another conformational change cause thioester hydrolysis and release coenzyme A. This ensures that the energy released from the thioester bond cleavage will drive the condensation.