Alpha-ketoglutarate utilization in Saccharomyces cerevisiae: transport, compartmentation and catabolism

alpha -Ketoglutarate (alpha KG) is a metabolite of the tricarboxylic acid cycle, important for biomass synthesis and a precursor for biotechnological products like 1,4-butanediol. In the eukaryote Saccharomyces cerevisiae alpha KG is present in different compartments. Compartmentation and (intra-)cellular transport could interfere with heterologous product pathways, generate futile cycles and reduce product yields. Batch and chemostat cultivations at low pH (<= 5) showed that alpha KG can be transported, catabolized and used for biomass synthesis. The uptake mechanism of alpha KG was further investigated under alpha KG limited chemostat conditions at different pH (3, 4, 5, and 6). At very low pH (3, 4) there is a fraction of undissociated alpha KG that could diffuse over the periplasmic membrane. At pH 5 this fraction is very low, and the observed growth and residual concentration requires a permease/facilitated uptake mechanism of the mono-dissociated form of alpha KG. Consumption of alpha KG under mixed substrate conditions was only observed for low glucose concentrations in chemostat cultivations, suggesting that the putative alpha KG transporter is repressed by glucose. Fully C-13-labeled alpha KG was introduced as a tracer during a glucose/alpha KG co-feeding chemostat to trace alpha KG transport and utilization. The measured C-13 enrichments suggest the major part of the consumed C-13 alpha KG was used for the synthesis of glutamate, and the remainder was transported into the mitochondria and fully oxidized. There was no enrichment observed in glycolytic intermediates, suggesting that there was no gluconeogenic activity under the co-feeding conditions. C-13 based flux analysis suggests that the intracellular transport is bi-directional, i.e. there is a fast exchange between the cytosol and mitochondria. The model further estimates that most intracellular alpha KG (88%) was present in the cytosol. Using literature reported volume fractions, the mitochondria/cytosol concentration ratio was 1.33. Such ratio will not require energy investment for transport towards the mitochondria (based on thermodynamic driving forces calculated with literature pH values). Growth on alpha KG as sole carbon source was observed, suggesting that S. cerevisiae is not fully Krebs-negative. Using C-13 tracing and modelling the intracellular use of alpha KG under co-feeding conditions showed a link with biomass synthesis, transport into the mitochondria and catabolism. For the engineering of strains that use cytosolic alpha KG as precursor, both observed sinks should be minimized to increase the putative yields.