This is one of the enzymes that catalyzes the last step of the mitochondrial beta-oxidation pathway, an aerobic process breaking down fatty acids into acetyl-CoA (PubMed:1715688, PubMed:7728148, PubMed:9744475). Using free coenzyme A/CoA, catalyzes the thiolytic cleavage of medium- to long-chain 3-oxoacyl-CoAs into acetyl-CoA and a fatty acyl-CoA shortened by two carbon atoms (PubMed:1715688, PubMed:7728148, PubMed:9744475). The activity of the enzyme is reversible and it can also catalyze the condensation of two acetyl-CoA molecules into acetoacetyl-CoA (PubMed:17371050). Thereby, it plays a major role in ketone body metabolism (PubMed:17371050, PubMed:1715688, PubMed:7728148, PubMed:9744475). 2 acetyl-CoA = acetoacetyl-CoA + CoA acetyl-CoA + propanoyl-CoA = 2-methyl-3-oxobutanoyl-CoA + CoA Activated by potassium ions, but not sodium ions. kcat is 21 sec(-1) for the degradation of acetoacetyl-CoA (at 25 degrees Celsius in the presence of 40 mM KCl) (PubMed:17371050). kcat is 8 sec(-1) for the degradation of acetoacetyl-CoA (at 25 degrees Celsius in the presence of 40 mM NaCl) (PubMed:17371050). kcat is 61 sec(-1) for the degradation of 2-methylacetoacetyl-CoA (at 25 degrees Celsius in the presence of 40 mM KCl) (PubMed:17371050). kcat is 14 sec(-1) for the degradation of 2-methylacetoacetyl-CoA (at 25 degrees Celsius in the presence of 40 mM NaCl) (PubMed:17371050). kcat is 3.5 sec(-1) for the synthesis of acetoacetyl-CoA (at 25 degrees Celsius in the presence of 40 mM KCl) (PubMed:17371050). Lipid metabolism; fatty acid beta-oxidation. Homotetramer. Succinylation at Lys-268, adjacent to a coenzyme A binding site. Desuccinylated by SIRT5 (By similarity). The disease is caused by variants affecting the gene represented in this entry. Belongs to the thiolase-like superfamily. Thiolase family.