Fig. 1

Cyanobacterial pathways of 18- and 20-carbon polyunsaturated fatty acid (PUFA) synthesis. Introduction of double bonds into stearic acid (18:0) involves a series of acyl-lipid desaturases designated DesC, DesA, DesD and DesB in cyanobacteria which catalyze desaturation at distinct sites of the carbon chain, ultimately producing stearidonic acid (SDA) if all four desaturase steps occur. Addition of two more carbons by an elongase can then form eicosatetraenoic acid (ETA), the ω3 isomer of arachidonic acid. The major three n-3 (omega-3) polyunsaturated fatty acids observed in cyanobacteria are shown [alpha-linolenic acid (ALA), SDA and ETA]. The structures shown represent a monogalactosyldiacylglycerol (MGDG) backbone and typical 16:0 saturated fatty acid (palmitic acid) at the sn-2 position in addition to the unsaturated fatty acid at sn-1. LA and GLA refer to linoleic acid and gamma-linolenic acid, respectively