Thiolase

Thiolases (also known as acetyl-CoA acyltransferases) are enzymes that catalyze the reversible condensation of two molecules of acetyl-CoA to acetoacetyl-CoA in the thiolytic cleavage of β-ketoacyl-CoA. They are members of the acyltransferase family. Thiolases are crucial enzymes in several metabolic pathways, including fatty acid oxidation, ketone body metabolism, and the biosynthesis of various molecules.

Function

The primary function of thiolase is to catalyze the following reversible reaction:

Acyl-CoA + Acetyl-CoA ⇌ CoA-SH + Acyl-Acetyl-CoA

Specifically, during beta-oxidation of fatty acids, thiolase cleaves the bond between the α- and β-carbon atoms of a β-ketoacyl-CoA molecule, releasing acetyl-CoA and shortening the fatty acid chain by two carbon atoms. The released acetyl-CoA can then enter the citric acid cycle (Krebs cycle) for further oxidation and energy production.

Conversely, thiolase also plays a role in the reverse reaction during the synthesis of compounds like ketone bodies. Two acetyl-CoA molecules are condensed to form acetoacetyl-CoA, which is a precursor to other ketone bodies such as acetoacetate and β-hydroxybutyrate.

Classification

Thiolases are classified into two main types:

• Acetoacetyl-CoA thiolase (EC 2.3.1.9): Primarily involved in ketone body metabolism and steroid biosynthesis.

• 3-ketoacyl-CoA thiolase (EC 2.3.1.16): Primarily involved in the degradation of fatty acids via β-oxidation.

Some organisms may also express different isoforms of thiolase with slightly different substrate specificities and tissue distributions.

Mechanism

The catalytic mechanism of thiolase involves the formation of a covalent intermediate between the enzyme and one of the substrates. A cysteine residue in the active site of the enzyme acts as a nucleophile to attack the carbonyl carbon of the acyl-CoA substrate, forming a thioester intermediate. This intermediate is then attacked by the second substrate, acetyl-CoA, leading to the formation of acetoacetyl-CoA and the regeneration of the free thiol group on the cysteine residue.

Clinical Significance

Deficiencies in thiolase can lead to metabolic disorders affecting fatty acid oxidation and ketone body metabolism. These deficiencies can manifest as various symptoms, including hypoglycemia, muscle weakness, and neurological problems. Diagnosis often involves measuring enzyme activity in tissues like fibroblasts or leukocytes. Genetic testing can also be used to identify mutations in the genes encoding thiolase.