Involved in the production of polyhydroxyalkonic acids (PHAs), composed primarily of 3-hydroxybutyric acid (3HB) and 3-hydroxyvaleric acid (3HV). 2 acetyl-CoA = acetoacetyl-CoA + CoA Metabolic intermediate biosynthesis; (R)-mevalonate biosynthesis; (R)-mevalonate from acetyl-CoA: step 1/3. Belongs to the thiolase-like superfamily. Thiolase family.