Catalyzes the dehydrogenation of acyl-coenzymes A (acyl-CoAs) to 2-enoyl-CoAs, the first step of the beta-oxidation cycle of fatty acid degradation. Is required for the utilization of medium- and long-chain fatty acids as sole carbon sources for growth. a medium-chain 2,3-saturated fatty acyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = a medium-chain (2E)-enoyl-CoA + reduced [electron-transfer flavoprotein] a long-chain 2,3-saturated fatty acyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = a long-chain (2E)-enoyl-CoA + reduced [electron-transfer flavoprotein] Lipid metabolism; fatty acid beta-oxidation. Belongs to the acyl-CoA dehydrogenase family. Extended N-terminus. Extended N-terminus.