Citrate synthase (E.C. 2.3.3.1 (previously 4.1.3.7)) is an enzyme that exists in nearly all living cells. It functions as a pace-making enzyme in the first step of the citric acid cycle (or Krebs cycle). Citrate synthase is located 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. Maximal activity of citrate synthase indicates the mitochondrial content of skeletal muscle. The maximal activity can be increased by endurance training or high-intensity interval training, with high-intensity interval training having the greater effect.
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 is regenerated after the completion of one round of the Krebs cycle.
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 citryl-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.
