Thiolase

Thiolases, also known as acetyl-coenzyme A acetyltransferases (ACAT), are enzymes which convert two units of acetyl-CoA to acetoacetyl CoA in the mevalonate pathway.

The thiolase superfamily enzymes catalyse the carbon–carbon-bond formation via a thioester-dependent Claisen condensation[2] reaction mechanism.

[7] These thioesters are made by conjugating the fatty acid with the free SH group of the pantetheine moiety of either coenzyme A (CoA) or acyl carrier protein (ACP).

[8] In the first step of both the degradative and biosynthetic reactions, the nucleophilic Cys89 (or its equivalent) attacks the acyl-CoA (or 3-ketoacyl-CoA) substrate, leading to the formation of a covalent acyl-enzyme intermediate.

[10] Each of the tetrahedral reaction intermediates that occur during transfer of an acetyl group to and from the nucleophilic cysteine, respectively, have been observed in X-ray crystal structures of biosynthetic thiolase from A.

The crystal structure of the tetrameric biosynthetic thiolase from Zoogloea ramigera has been determined at 2.0 Å resolution.

[16][17] Thiolase is of central importance in key enzymatic pathways such as fatty-acid, steroid and polyketide synthesis.

[18] Harnessing the complicated catalytic versatility of the polyketide synthases for the synthesis of biologically and medically relevant natural products is also an important future perspective of the studies of the enzymes of this superfamily.

[23] Developmental delay may occur, even before the first acute episode, and bilateral striatal necrosis of the basal ganglia has been seen on brain MRI.

Mevalonate pathway
Reaction catalyzed by thiolase
Thiolase Mechanism. The two-step, ping-pong mechanism for the thiolase reaction. Red arrows indicate the biosynthetic reaction; Black arrows trace the degradative reaction. In both directions, the reaction is initiated by the nucleophilic attack of Cys89 on the substrate to form a covalent acetyl–enzyme intermediate. Cys89 is activated for nucleophilic attack by His348, which abstracts the sulfide proton of Cys89. In the second step of both the biosynthetic and degradative reactions, the substrate nucleophilically attacks the acetyl–enzyme intermediate to yield the final product and free enzyme. This nucleophilic attack is activated by Cys378, which abstracts a proton from the substrate.