Functional characterization of front-end desaturases from trypanosomatids depicts the first polyunsaturated fatty acid biosynthetic pathway from a parasitic protozoan

A survey of the three kinetoplastid genome projects revealed the presence of three putative front-end desaturase genes in Leishmania major, one in Trypanosoma brucei and two highly identical ones (98%) in T. cruzi. The encoded gene products were tentatively annotated as Delta 8, Delta 5 and Delta 6 desaturases for L. major, and Delta 6 desaturase for both trypanosomes. After phylogenetic and structural analysis of the deduced proteins, we predicted that the putative Delta 6 desaturases could have Delta 4 desaturase activity, based mainly on the conserved HX3HH motif for the second histidine box, when compared with Delta 4 desaturases from Thraustochytrium, Euglena gracilis and the microalga, Pavlova lutheri, which are more than 30% identical to the trypanosomatid enzymes. After cloning and expression in Saccharomyces cerevisiae, it was possible to functionally characterize each of the front-end desaturases present in L. major and T. brucei. Our prediction about the presence of Delta 4 desaturase activity in the three kinetoplastids was corroborated. In the same way, Delta 5 desaturase activity was confirmed to be present in L. major. Interestingly, the putative Delta 8 desaturase turned out to be a functional Delta 6 desaturase, being 35% and 31% identical to Rhizopus oryzae and Pythium irregulare Delta 6 desaturases, respectively. Our results indicate that no conclusive predictions can be made about the function of this class of enzymes merely on the basis of sequence homology. Moreover, they indicate that a complete pathway for very-long-chain polyunsaturated fatty acid biosynthesis is functional in L. major using Delta 6, Delta 5 and Delta 4 desaturases. In trypanosomes, only Delta 4 desaturases are present. The putative algal origin of the pathway in kinetoplastids is discussed.