Thiolases

Thiolases are enzymes of the thiolase superfamily that catalyze reversible thiolysis and condensation reactions of acyl-CoA substrates. The best-known reaction is the condensation of two acetyl-CoA to form acetoacetyl-CoA, and the reverse thiolytic cleavage of acetoacetyl-CoA into two acetyl-CoA. These activities are central to fatty acid metabolism and to the production and utilization of ketone bodies.

Two major activities are distinguished: thiolase I (acetyl-CoA acetyltransferase), which condenses acetyl-CoA to acetoacetyl-CoA; and thiolase II (acetoacetyl-CoA thiolase), which cleaves acetoacetyl-CoA into acetyl-CoA. In eukaryotes these enzymes are found in mitochondria and/or peroxisomes, depending on tissue and species; bacteria possess thiolases as well. In mammals, mitochondrial thiolases participate in beta-oxidation and energy production, while acetoacetyl-CoA generated by thiolase I serves as a precursor for ketone body and cholesterol biosynthesis.

Mechanistically, thiolases operate via a catalytic cysteine residue that forms a covalent acetyl-enzyme intermediate, aided by histidine and asparagine residues in the active site. The reaction proceeds through a ping-pong mechanism with multiple CoA substrates. The enzymes are typically homodimers or homotetramers and adopt the conserved thiolase fold.

Defects in mitochondrial acetoacetyl-CoA thiolase cause beta-ketothiolase deficiency, a rare metabolic disorder characterized by ketoacidosis, hypoglycemia, and metabolic crises, especially in infancy.

Thiolases are around 390 amino acids in length and function in central carbon metabolism, linking fatty acid oxidation to energy production, ketogenesis, and isoprenoid/cholesterol biosynthesis.