A bifunctional Δ12, Δ15-desaturase from Acanthamoeba castellanii directs the synthesis of highly unusual n-1 series unsaturated fatty acids

The free-living soil protozoon Acanthamoeba castellanii synthesizes a range of polyunsaturated fatty acids, the balance of which can be altered by environmental changes. We have isolated and functionally characterized in yeast a microsomal desaturase from A. castellanii, which catalyzes the sequential conversion of C-16 and C-18 Delta 9-monounsaturated fatty acids to di- and tri-unsaturated forms. In the case of C-16 substrates, this bifunctional A. castellanii Delta 12, Delta 15-desaturase generated a highly unusual fatty acid, hexadecatrienoic acid (16:3 Delta(9,12,15)(n1)). The identification of a desaturase, which can catalyze the insertion of a double bond between the terminal two carbons of a fatty acid represents a new addition to desaturase functionality and plasticity. We have also co-expressed in yeast the A. castellanii bifunctional Delta 12, Delta 15-desaturase with a microsomal Delta 6-desaturase, resulting in the synthesis of the highly unsaturated C16 fatty acid hexadecatetraenoic acid (16:4 Delta(6,9,12,15)(n-1)), previously only reported in marine microorganisms. Our work therefore demonstrates the feasibility of the heterologous synthesis of polyunsaturated fatty acids of the n-1 series. The presence of a bifunctional Delta 12, Delta 15-desaturase in A. castellanii is also considered with reference to the evolution of desaturases and the lineage of this protist.