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. a medium chain fatty acid + ATP + CoA = a medium-chain fatty acyl-CoA + AMP + diphosphate a long-chain fatty acid + ATP + CoA = a long-chain fatty acyl-CoA + AMP + diphosphate Lipid metabolism; fatty acid biosynthesis. Belongs to the ATP-dependent AMP-binding enzyme family.