Catalyzes the activation of fatty acids by CoA to produce an acyl-CoA, the first step in fatty acid metabolism (PubMed:10434065, PubMed:12616642). Capable of activating medium-chain fatty acids (e.g. butyric (C4) to decanoic (C10) acids), and certain carboxylate-containing xenobiotics, e.g. benzoate (PubMed:10434065, PubMed:12616642). a medium chain fatty acid + ATP + CoA = a medium-chain fatty acyl-CoA + AMP + diphosphate ATP + benzoate + CoA = AMP + benzoyl-CoA + diphosphate ATP + CoA + hexanoate = AMP + diphosphate + hexanoyl-CoA ATP + butanoate + CoA = AMP + butanoyl-CoA + diphosphate ATP + CoA + octanoate = AMP + diphosphate + octanoyl-CoA ATP + CoA + decanoate = AMP + decanoyl-CoA + diphosphate Activated by monovalent cations, such as potassium, rubidium or ammonium. Optimum pH is 8.8. Monomer. Detected in liver. Belongs to the ATP-dependent AMP-binding enzyme family.