Functional expression of Spirulina-Delta6 desaturase gene in yeast, Saccharomyces cerevisiae.

Spirulina-acyl-lipid desaturases are membrane-bound enzymes found in thylakoid and plasma membranes. These enzymes carry out the fatty acid desaturation process of Spirulina to yield gamma-linolenic acid (GLA) as the final desaturation product. In this study, Spirulina-Delta(6) desaturase encoded by the desD gene was heterologously expressed and characterized in Saccharomyces cerevisiae. We then conducted site-directed mutagenesis of the histidine residues in the three histidine boxes to determine the role of these amino acid residues in the enzyme function. Our results showed that while four mutants showed complete loss of Delta(6)-desaturase activity and two mutants showed only trace of the activity, the enzyme activity could be partially restored by chemical rescue using exogenously provided imidazole. This study reveals that the histidine residues (which have imidazole as their functional group) in the conserved clusters play a critical role in Delta(6)-desaturase activity, possibly by providing a di-iron catalytic center. In our previous study, this enzyme was expressed in Escherichia coli. The results reveal that the enzyme can function only in the presence of an exogenous cofactor, ferredoxin, provided in vitro. This evidence suggests that baker's yeast has a cofactor that can complement ferredoxin, thought to act as an electron donor for the Delta(6) desaturation in cyanobacteria, including Spirulina. The electron donor of the Spirulina-Delta(6) desaturation in yeast is more likely to be cytochrome b(5), which is absent in E. coli. This means that the enzyme expressed in S. cerevisiae can catalyze the biosynthesis of the product, GLA, in vivo.