Abstract 1718 DGAT tales: DGAT1 and DGAT2 enzymes across species differ in their specificity towards diacylglycerol enantiomers

Acyl-CoA: diacylglycerol acyltransferases (DGATs) catalyze the final rate-limiting step in the biosynthesis of triacylglycerol (TAG, e.g. fat, oil) by transesterification of a fatty acid from acyl-CoA to the free hydroxyl of diacylglycerol (DAG). The composition of TAG produced depends mainly on DGAT specificity towards the fatty acid moieties attached to the acyl donor and acyl acceptor substrates and is largely responsible for the vast diversity in fat or oil compositions among the tree of life. DAG is synthesized de novo from glycerol-3-phosphate where the acyl groups are localized to the sn-1/2 positions, but the sn-2/3-DAG stereoisomer can also be formed through partial TAG degradation. The specificities of different DGATs towards each stereoisomer and their biological significance are yet to be explored. Recently, we discovered a novel TAG biosynthesis pathway in the plant Physaria fendleri in which differential DAG enantiomer specificities of the non-homologous DGAT1 and DGAT2 isoenzymes play a major role in remodeling the triacylglycerol molecules to accumulate high levels of industrially valuable unusual fatty acids. We carried out an enantiomer-specific DGAT assay utilizing DGAT1 and DGAT2 isoenzymes from different plants, humans, mice, yeast, and algae. The results demonstrate that DGAT1 and DGAT2 isoenzymes from humans, mice, and yeast can use both sn-1/2 and sn-2/3 stereoisomers of DAG. Interestingly, the DGAT1 and DGAT2 isoenzymes from various plant and microalgae species were highly selective towards either sn-1/2-DAG or sn-2/3-DAG. Together these results point toward different roles of TAG synthesis/utilization within lipid metabolism between animals and plants, especially for the accumulation of membrane-incompatible unusual fatty acids. Overall, our results enhance our understanding of the unique specificities of DGAT isoenzymes and identify novel DGATs that could be exploited for biotechnological control of designer oils and fats for the food and oleochemical industries.