– Catalyzes the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs. First enzyme of the fatty acid beta-oxidation pathway.; Shows highest activity against medium-chain fatty acyl-CoAs and activity decreases with increasing chain length.; Is active against a much broader range of substrates and shows activity towards very long-chain acyl-CoAs. Is twice as active as isoform 1 against 16-hydroxy-palmitoyl-CoA and is 25% more active against 1,16-hexadecanodioyl-CoA.