The Front-end Desaturase: Structure,Function, Evolution and Biotechnological Use

Very long chain polyunsaturated fatty acids such as arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), docosapentaenoic acid (DPA, 22:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are essential components of cell membranes, and are precursors for a group of hormone-like bioactive compounds (eicosanoids and docosanoids) involved in regulation of various physiological activities in animals and humans. The biosynthesis of these fatty acids involves an alternating process of fatty acid desaturation and elongation. The desaturation is catalyzed by a unique class of oxygenases called front-end desaturases that introduce double bonds between the pre-existing double bond and the carboxyl end of polyunsaturated fatty acids. The first gene encoding a front-end desaturase was cloned in 1993 from cyanobacteria. Since then, front-end desaturases have been identified and characterized from a wide range of eukaryotic species including algae, protozoa, fungi, plants and animals including humans. Unlike front-end desaturases from bacteria, those from eukaryotes are structurally characterized by the presence of an N-terminal cytochrome b ₅-like domain fused to the main desaturation domain. Understanding the structure, function and evolution of front-end desaturases, as well as their roles in the biosynthesis of very long chain polyunsaturated fatty acids offers the opportunity to engineer production of these fatty acids in transgenic oilseed plants for nutraceutical markets.     

Genomic Analysis of Genes Involved in the Biosynthesis of Very Long Chain Polyunsaturated Fatty Acids in Thraustochytrium sp. 26185

Thraustochytrium sp. 26185 is a marine protist that can produce a large amount of docosahexaenoic acid (DHA, 22:6n‐3), an ω3 very long chain polyunsaturated fatty acid (VLCPUFA) of nutritional importance. However, the mechanism of how this fatty acid is synthesized and assembled into the storage lipid triacylglycerol is unclear. Here we report sequencing of the whole genome and genomic analysis of genes involved in the biosynthesis and assembly of the fatty acids in this species. Genome sequencing produced a total of 2,418,734,139 bp clean sequences with about 62 fold genome coverage. Annotation of the genome sequences revealed 10,797 coding genes. Among them, 10,216 genes could be assigned into 25 KOG classes where 451 genes were specifically assigned to the group of lipid transport and metabolism. Detailed analysis of these genes revealed co‐existence of both aerobic pathway and anaerobic pathways for the biosynthesis of DHA in this species. However, in the aerobic pathway, a key gene encoding stearate Δ9 desaturase introducing the first double bond to long chain saturated fatty acid 18:0 was missing from the genome. Genomic survey of genes involved in the acyl trafficking among glycerolipids showed that, unlike plants, this protist did not possess phosphatidylcholine:diacylglycerol cholinephosphotransferase, an important enzyme in bridging two types of glycerolipids, diacylglycerols (DAG) and phosphatidylcholines (PtdCho). These results shed new insight on the biosynthesis and assembly of VLCPUFA in the Thraustochytrium.