Involved in both the production of very long-chain fatty acids for sphingolipid synthesis and the degradation of the sphingosine moiety in sphingolipids through the sphingosine 1-phosphate metabolic pathway (PubMed:25049234). Catalyzes the last of the four reactions of the long-chain fatty acids elongation cycle (PubMed:12482854). This endoplasmic reticulum-bound enzymatic process, allows the addition of 2 carbons to the chain of long- and very long-chain fatty acids/VLCFAs per cycle (PubMed:12482854). This enzyme reduces the trans-2,3-enoyl-CoA fatty acid intermediate to an acyl-CoA that can be further elongated by entering a new cycle of elongation (PubMed:12482854). Thereby, it participates in the production of VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators (PubMed:12482854). Catalyzes the saturation step of the sphingosine 1-phosphate metabolic pathway, the conversion of trans-2-hexadecenoyl-CoA to palmitoyl-CoA (PubMed:25049234). a very-long-chain 2,3-saturated fatty acyl-CoA + NADP(+) = a very-long-chain (2E)-enoyl-CoA + H(+) + NADPH NADP(+) + octadecanoyl-CoA = (2E)-octadecenoyl-CoA + H(+) + NADPH (2E,7Z,10Z,13Z,16Z)-docosapentaenoyl-CoA + H(+) + NADPH = (7Z,10Z,13Z,16Z)-docosatetraenoyl-CoA + NADP(+) (2E,7Z,10Z,13Z,16Z,19Z)-docosahexaenoyl-CoA + H(+) + NADPH = (7Z,10Z,13Z,16Z,19Z)-docosapentaenoyl-CoA + NADP(+) (2E,8Z,11Z,14Z)-eicosatetraenoyl-CoA + H(+) + NADPH = (8Z,11Z,14Z)-eicosatrienoyl-CoA + NADP(+) (2E)-hexadecenoyl-CoA + H(+) + NADPH = hexadecanoyl-CoA + NADP(+) Lipid metabolism; fatty acid biosynthesis. Lipid metabolism; sphingolipid metabolism. Interacts with ELOVL1 and LASS2. Expressed in most tissues tested. Highly expressed in skeletal muscle. Glycosylated. The disease is caused by variants affecting the gene represented in this entry. Belongs to the steroid 5-alpha reductase family.