Can function as a Delta(12)/Delta(15) bifunctional desaturase and behaves as a nu +3' desaturase. Introduces a double bond in the fatty acid chain three carbons away from an existing double bond to biosynthesize polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes). Acts on a number of substrates like oleoyl-CoA ((9Z)-octadecenoyl-CoA, 18:1n-9), palmitoleoyl-CoA ((9Z)-hexadecenoyl-CoA, 16:1n-7), and gamma-linolenoyl-CoA ((6Z,9Z,12Z)-octadecatrienoyl-CoA, 18:3n-6), to generate linoleoyl-CoA ((9Z,12Z)-octadecadienoyl-CoA, 18:2n-6), (9Z,12Z)-hexadecadienoyl-CoA (16:2n-4) and (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA (18:4n-3) respectively (PubMed:10775428, PubMed:11972048, PubMed:22041902, PubMed:26806391). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (Probable). (9Z)-octadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-hexadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-hexadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z,15Z)-octadecatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-heptadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-heptadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-pentadecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-pentadecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (6Z,9Z,12Z)-octadecatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (6Z,9Z,12Z,15Z)-octadecatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (9Z)-tetradecenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z)-tetradecadienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. The fat-2(wa17) mutant leads to high levels of C18:1n-9 and low levels of PUFAs. HPO-19 and T05H4.4 are cytochrome b5 reductases required for PUFA desaturation in Caenorhabditis elegans. HPO-19 knockdown or mutation alters FAT-2 desaturase activity. Although FAT-2 lacks a cytochrome b5 domain, its N-terminal contains a DUF3474 domain that may possess some oxidoreductase activity for electron transfer, obviating the need for an extra cytochrome b5 enzyme, but still requiring cytochrome b5 reductase HPO-19/T05H4.4 to become activated. Belongs to the fatty acid desaturase type 1 family.