Catalyzes the activation of fatty acids by CoA to produce an acyl-CoA, the first step in fatty acid metabolism (By similarity). Capable of activating medium-chain fatty acids with a preference for C6-12 fatty acids (By similarity). a medium chain fatty acid + ATP + CoA = a medium-chain fatty acyl-CoA + AMP + diphosphate ATP + CoA + hexanoate = AMP + diphosphate + hexanoyl-CoA ATP + CoA + octanoate = AMP + diphosphate + octanoyl-CoA ATP + CoA + decanoate = AMP + decanoyl-CoA + diphosphate ATP + CoA + dodecanoate = AMP + diphosphate + dodecanoyl-CoA Belongs to the ATP-dependent AMP-binding enzyme family.