thiolase

Thiolases are enzymes that catalyze thiolase reactions on acyl-CoA thioesters and are classified under EC 2.3.1.9. They participate in both the condensation of acetyl-CoA into longer carbon chains and the thiolytic cleavage of Beta-ketoacyl-CoA, enabling key steps in fatty acid metabolism and ketone body formation. In many organisms, thiolases occur as mitochondrial or peroxisomal isoforms and can be organized as homodimers or multimers.

The primary activities are acetoacetyl-CoA thiolase and beta-ketoacyl-CoA thiolase. Acetoacetyl-CoA thiolase catalyzes the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA, with CoA released. Beta-ketoacyl-CoA thiolase performs thiolysis of 3-ketoacyl-CoA to yield acetyl-CoA and a shortened acyl-CoA. Both directions are reversible, allowing the enzyme to adapt to cellular energy status and substrate availability.

In metabolism, thiolases play essential roles in fatty acid beta-oxidation, where they cleave 3-ketoacyl-CoA to release acetyl-CoA and a shorter acyl-CoA, and in the formation of ketone bodies in liver mitochondria through condensation steps that build acetoacetyl-CoA. They also contribute to the mevalonate pathway in some organisms by supplying acetoacetyl-CoA substrates. The enzymes are widespread in bacteria, archaea, plants, and animals, reflecting their fundamental role in carbon metabolism.

Catalysis involves a catalytic cysteine residue that forms a thioester intermediate, assisted by adjacent histidine and asparagine residues that act as acid-base catalysts. The mechanism proceeds via a ping-pong bi-bi process, with CoA acting as the nucleophile and leaving group. Genetic deficiencies, such as beta-ketothiolase deficiency, can disrupt energy metabolism and cause metabolic crises.