Description
The enzymes from several algae introduce a cis double bond at the 4-position in 22-carbon polyunsaturated fatty acids that contain a Δ7 double bond. The enzyme from the fresh water alga Chlamydomonas reinhardtii acts on the 16 carbon fatty acid (7Z,10Z,13Z)-hexadeca-7,10,13-trienoate. The enzyme contains an N-terminal cytochrome b5 domain that acts as the direct electron donor to the active site of the desaturase, and does not require an external cytochrome.
Reaction
(1) a (7Z,10Z,13Z,16Z,19Z)-docosa-7,10,13,16,19-pentaenoyl-[glycerolipid] + 2 ferrocytochrome b5 + O2 + 2 H+ = a (4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoyl-[glycerolipid] + 2 ferricytochrome b5 + 2 H2O; (2) a (7Z,10Z,13Z,16Z)-docosa-7,10,13,16-tetraenoyl-[glycerolipid] + 2 ferrocytochrome b5 + O2 + 2 H+ = a (4Z,7Z,10Z,13Z,16Z)-docosa-4,7,10,13,16-pentaenoyl-[glycerolipid] + 2 ferricytochrome b5 + 2 H2O
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