The enzyme introduces a cis double bond at carbon 5 of acyl-CoAs that contain a double bond at position 8. The enzymes from algae, mosses, mammals and the protozoan Leishmania major catalyse the desaturation of dihomo-¦Ã-linoleate ((8Z,11Z,14Z)-icosa-8,11,14-trienoate) and (8Z,11Z,14Z,17Z)-icosa-8,11,14,17-tetraenoate to generate arachidonate and (5Z,8Z,11Z,14Z,17Z)-icosa-5,8,11,14,17-pentaenoate, respectively. The enzyme contains a cytochrome b5 domain that acts as the direct electron donor to the desaturase active site and does not require an external cytochrome. cf. EC 1.14.19.37 acyl-CoA 5-desaturase.