Acyl-Lipid Δ⁶-Desaturase May Act as a First FAD in Cyanobacteria

Fatty acid desaturases (FADs) play important roles in various metabolic and adaptive pathways in all living organisms. They represent a superfamily of oxygenases that introduce double bonds into the acyl chains of fatty acids (FAs). These enzymes are highly specific to the length of the carbon chain, position of double bonds formation, etc. The modes by which FADs "count" the position of the double bond formation may differ. In cyanobacteria, the first double bond is formed between 9th and 10th carbons (position Δ⁹), counting from the carboxylic end of an FA. Other FADs that produce polyunsaturated FAs may introduce double bonds counting from the carboxyl (Δ) or methyl (ω) terminus, or from a pre-existing double bond towards carboxyl or methyl terminus of an FA chain. Here, we expressed the desD gene for the Δ6-FAD from Synechocystis sp. PCC 6803 in Synechococcus elongatus PCC 7942 (which is capable of synthesizing only monoenoic FAs desaturated mainly at position Δ⁹) and observed the appearance of unusual monoenoic FAs desaturated at position Δ⁶, as well as Δ^6,9 dienoic FAs. Exogenously added cis-10-heptadecenoic acid (17:1Δ¹⁰) was converted into cis-6,10-heptadecadienoic (17:2Δ^6,10). These data demonstrate the ability of Δ6-FAD to introduce the first double bond into the unsaturated substrates and suggests that it "counts" from the carboxyl end, irrespective of the absence or presence of a previous double bond in an FA chain.