Protein characterization of the soybean malic enzyme family to select metabolic targets for seed oil improvement

Seeds are of great economic interest due to their organic composition. The metabolic pathways leading to the deposition of seed storage compounds, such as oil and proteins, are complex in terms of the number of enzymes involved, the regulatory context, and the variability among species. Previous studies indicated that mitochondrial NAD-dependent and plastidial NADP-dependent malic enzymes (MEs; EC 1.1.1.39 and 1.1.1.40, respectively) participate in providing reducing power and citrate or acetyl-CoA, respectively, to fatty acid biosynthesis in soybean (Glycine max L. Merr.) embryos. In this work, we analyzed the biochemical properties of enzymes belonging to the soybean ME family; which were obtained by recombinant expression and subsequent affinity purification. Our results show that NADP-ME1.1 has high affinity for the substrates malate and NADP and is specifically activated by glutamine, the main nitrogenous compound imported from maternal tissues to support the filling process. In addition, NAD-MEs exhibit differential kinetic parameters and regulation by key metabolites, making the isoforms better suited to fulfill non-redundant functions under certain physiological conditions. Activation by intermediates of glycolysis, the citric acid cycle, and lipogenesis indicated coordination between these pathways and malate metabolism. In turn, NAD-ME activity in soybean mitochondria would be regulated by the alternative association between linage 1 and 2 subunits. These findings provide new insights into intermediary metabolism in oilseeds and nominate specific soybean ME isoforms as biotechnological tools to be used in breeding programs.