Catalyzes the activation of medium/long-chain fatty acids as acyl-coenzyme A (acyl-CoA), which are then transferred to the multifunctional polyketide synthase (PKS) type III for further chain extension. Also involved in the degradation of cholesterol via the degradation of the side chains of C-24 branched-chain sterols. Catalyzes the ATP-dependent CoA thioesterification of the sterol 3-oxocholest-4-en-26-oate to yield 3-oxocholest-4-en-26-oyl-CoA