Catalyzes the condensation of two acetyl-coA units to form acetoacetyl-CoA (PubMed:4198758). Is involved in the biosynthesis of polyhydroxybutyrate (PHB), which is accumulated as an intracellular energy reserve material when cells grow under conditions of nutrient limitation (PubMed:2670936, PubMed:4198758). Also catalyzes the reverse reaction, i.e. the cleavage of acetoacetyl-CoA, and is therefore also involved in the reutilization of PHB (PubMed:4198758, PubMed:25152395). 2 acetyl-CoA = acetoacetyl-CoA + CoA The condensation reaction is inhibited by free CoA. The cleavage reaction is characterized by substrate inhibition by acetoacetyl-CoA, which is partially relieved by free CoA. Optimum pH is 7.8 for the condensation reaction and 8.1 for the reverse cleavage reaction. Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Homotetramer. Belongs to the thiolase-like superfamily. Thiolase family.