Catalyzes the activation of fatty acids by CoA to produce an acyl-CoA, the first step in fatty acid metabolism (PubMed:11772874). Capable of activating medium-chain fatty acids with a preference for isobutyrate among fatty acids with 2-6 carbon atoms (By similarity). a medium chain fatty acid + ATP + CoA = a medium-chain fatty acyl-CoA + AMP + diphosphate ATP + CoA + propanoate = AMP + diphosphate + propanoyl-CoA ATP + butanoate + CoA = AMP + butanoyl-CoA + diphosphate 2-methylpropanoate + ATP + CoA = 2-methylpropanoyl-CoA + AMP + diphosphate 2-methylbutanoate + ATP + CoA = 2-methylbutanoyl-CoA + AMP + diphosphate ATP + CoA + octanoate = AMP + diphosphate + octanoyl-CoA Belongs to the ATP-dependent AMP-binding enzyme family. It is uncertain whether Met-1 or Met-9 is the initiator.