(9Z)-hexadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + O2 + 2 H(+) => (9Z,12Z)-hexadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O
(9z)-hexadecenoyl-coa + 2 fe(ii)-[cytochrome b5] + o2 + 2 h(+) => (9z,12z)-hexadecadienoyl-coa + 2 fe(iii)-[cytochrome b5] + 2 h2o