Elongation of polyunsaturated fatty acids in trypanosomatids

Leishmania major synthesizes polyunsaturated fatty acids by using Δ6, Δ5 and Δ4 front-end desaturases, which have recently been characterized [Tripodi KE, Buttigliero LV, Altabe SG & Uttaro AD (2006) FEBS J273, 271–280], and two predicted elongases specific for C18 Δ6 and C20 Δ5 polyunsaturated fatty acids, respectively. Trypanosoma brucei and Trypanosoma cruzi lack Δ6 and Δ5 desaturases but contain Δ4 desaturases, implying that trypanosomes use exogenous polyunsaturated fatty acids to produce C22 Δ4 fatty acids. In order to identify putative precursors of these C22 fatty acids and to completely describe the pathways for polyunsaturated fatty acid biosynthesis in trypanosomatids, we have performed a search in the three genomes and identified four different elongase genes in T. brucei, five in T. cruzi and 14 in L. major. After a phylogenetic analysis of the encoded proteins together with elongases from a variety of other organisms, we selected four candidate polyunsaturated fatty acid elongases. Leishmania major CAJ02037, T. brucei AAX69821 and T. cruzi XP_808770 share 57–52% identity, and group together with C20 Δ5 polyunsaturated fatty acid elongases from algae. The predicted activity was corroborated by functional characterization after expression in yeast. T. brucei elongase was also able to elongate Δ8 and Δ11 C20 polyunsaturated fatty acids. L. major CAJ08636, which shares 33% identity with Mortierella alpinaΔ6 elongase, showed a high specificity for C18 Δ6 polyunsaturated fatty acids. In all cases, a preference for n6 polyunsaturated fatty acids was observed. This indicates that L. major has, as predicted, Δ6 and Δ5 elongases and a complete pathway for polyunsaturated fatty acid biosynthesis. Trypanosomes contain only Δ5 elongases, which, together with Δ4 desaturases, allow them to use eicosapentaenoic acid and arachidonic acid, a precursor that is relatively abundant in the host, for C22 polyunsaturated fatty acid biosynthesis.