Although the elongation enzymes are sometimes called fatty acid synthase II and III, they consist of discrete enzyme components loosely held together in a membrane bound complex, and not a single multicatalytic polypeptide. Since desaturase removes two hydrogens from the substrate, in this case both atoms of O end up as H 2 O . The action of a relatively strong reductant such as NADH is necessary because of the large energy barrier for introducing the first electron to the O 2 molecule. Because this system is on the ER, not mitochondrial associated, it has nothing to do with oxidative phosphorylation. Fatty acyl CoA desaturases act at specific positions in the acyl chain.
Higher plants contain the necessary far-chain desaturases.