Desaturase involved in the biosynthesis of (5Z)-icos-5-enoate, an unusual monounsaturated fatty acid that makes up to 60% of the total fatty acids in Limnanthes sp. seed oil. Only acts on saturated fatty acids. eicosanoyl-CoA + 2 Fe(II)-[cytochrome b5] + 2 H(+) + O2 = (5Z)-eicosenoyl-CoA + 2 Fe(III)-[cytochrome b5] + 2 H2O Lipid metabolism; monounsaturated fatty acid biosynthesis. The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.