Catalyzes the activation of medium-chain fatty acids as acyl-coenzyme A (acyl-CoA) (PubMed:11690636, PubMed:19477415). Shows maximal activity with saturated fatty acids of medium-chain length between C6 and C12. Has lower activity with tridecanoic acid (C13), tetradecanoic acid (C14) and with unsaturated fatty acids like oleic acid (C18:1), linolenic acid (C18:3) and arachidonic acid (C20:4). Shows weak activity with some aromatic carbon acids (PubMed:11690636). Involved in the metabolism of fatty acid during mycobacterial survival in macrophages (PubMed:11690636). a medium chain fatty acid + ATP + CoA = a medium-chain fatty acyl-CoA + AMP + diphosphate ATP + CoA + hexanoate = AMP + diphosphate + hexanoyl-CoA ATP + CoA + heptanoate = AMP + diphosphate + heptanoyl-CoA ATP + CoA + octanoate = AMP + diphosphate + octanoyl-CoA ATP + CoA + decanoate = AMP + decanoyl-CoA + diphosphate ATP + CoA + dodecanoate = AMP + diphosphate + dodecanoyl-CoA ATP + CoA + tetradecanoate = AMP + diphosphate + tetradecanoyl-CoA (9Z)-octadecenoate + ATP + CoA = (9Z)-octadecenoyl-CoA + AMP + diphosphate (9Z,12Z,15Z)-octadecatrienoate + ATP + CoA = (9Z,12Z,15Z)-octadecatrienoyl-CoA + AMP + diphosphate (5Z,8Z,11Z,14Z)-eicosatetraenoate + ATP + CoA = (5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA + AMP + diphosphate Inhibited by 2-hydroxydodecanoic acid, a typical inhibitor of medium-chain acyl-CoA synthetases. Lipid metabolism; fatty acid metabolism. Expression is induced in human macrophages (PubMed:7507894, PubMed:9353032). Induced by acidity (PubMed:9353032). Transcription seems to be maximal between day 1 and day 5 after phagocytosis (PubMed:7507894). Belongs to the ATP-dependent AMP-binding enzyme family.