Omega-3 fatty acid desaturase that recognizes a range of 18- and 20-carbon omega-6 substrates. Introduces a double bond in the fatty acid chain three carbons away from terminal methyl group to biosynthesize n-3 (omega-3) polyunsaturated fatty acids (PUFAs) endogenously (PUFAs are essential for membrane structure and many cellular and physiological processes). Acts on a number of substrates like linoleoyl-CoA ((9Z,12Z)-octadecadienoyl-CoA, 18:2n-6), dihomo-gamma-linolenoyl-CoA ((8Z,11Z,14Z)-eicosatrienoyl-CoA, 20:3n-6), and arachidonoyl-CoA ((5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA, 20:4n-6), to generate alpha-linolenoyl-CoA ((9Z,12Z,15Z)-octadecatrienoyl-CoA, 18:3n-3), (8Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA (20:4n-3) and (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-CoA (20:5n-3) respectively (PubMed:9037020, PubMed:11972048, PubMed:14765186, PubMed:22041902, PubMed:24391980, PubMed:26806391). Unlike plants, Caenorhabditis elegans desaturases seem to use fatty acyl-CoAs as substrates (By similarity). (9Z,12Z)-octadecadienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (9Z,12Z,15Z)-octadecatrienoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (8Z,11Z,14Z)-eicosatrienoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (8Z,11Z,14Z,17Z)-eicosatetraenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (5Z,8Z,11Z,14Z)-eicosatetraenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z,8Z,11Z,14Z,17Z)-eicosapentaenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O (7Z,10Z,13Z,16Z)-docosatetraenoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (7Z,10Z,13Z,16Z,19Z)-docosapentaenoyl-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 Lipid metabolism; polyunsaturated fatty acid biosynthesis. The fat-1 mutant displays a reduction in the n-3 PUFAs, the homozygous strain is incapable of producing the normal levels of C18 or C20 PUFAs. HPO-19 and T05H4.4 are cytochrome b5 reductases required for PUFA desaturation in Caenorhabditis elegans. HPO-19 knockdown or mutation alters FAT-1 desaturase activity. Although FAT-1 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.