Fatty acids are incorporated into membranes and signaling molecules and have roles in energy storage and metabolism. These essential functions require activation of the fatty acid by acyl-coenzyme A (CoA) synthetases, such as ACSM5, which form an activating thioester linkage between the fatty acid and CoA. Has medium-chain fatty acid:CoA ligase activity with broad substrate specificity (in vitro). Acts on acids from C4 to C11 and on the corresponding 3-hydroxy- and 2,3- or 3,4-unsaturated acids (in vitro).