L-aspartate oxidase provides new insights into fumarate reduction in anaerobic darkness in Synechocystis sp. PCC6803.

Cyanobacteria are promising microbial hosts for production of succinate, a central metabolite of the tricarboxylic acid (TCA) cycle, which cyanobacteria can secrete during anoxic fermentation. To investigate the route for succinate formation, we generated strains of Synechocystis PCC6803 lacking subunits of the succinate dehydrogenase complex (SDH). During dark anaerobic fermentation, the deletion strains show higher succinate accumulation than the wild type, indicating succinate dehydrogenase activity of SDH rather than fumarate reduction. Another potential route for succinate formation from fumarate is as a byproduct of the L-aspartate oxidase (Laspo) reaction. Using purified SynLaspo, we show that in addition to L-aspartate oxidation, the enzyme exhibits L-aspartate-fumarate oxidoreductase activity, generating succinate. This work contributes to the understanding of the cyanobacterial TCA cycle.