Dynamic metabolomics differentiates between carbon and energy starvation in recombinant Saccharomyces cerevisiae fermenting xylose

Background:The concerted effects of changes in gene expression due to changes in the environment areultimately reflected in the metabolome. Dynamics of metabolite concentrations under a certain condition cantherefore give a description of the cellular state with a high degree of functional information. We used thispotential to evaluate the metabolic status of two recombinant strains ofSaccharomyces cerevisiaeduringanaerobic batch fermentation of a glucose/xylose mixture. Two isogenic strains were studied, differing only inthe pathways used for xylose assimilation: the oxidoreductive pathway with xylose reductase (XR) and xylitoldehydrogenase (XDH) or the isomerization pathway with xylose isomerase (XI). The isogenic relationshipbetween the two strains ascertains that the observed responses are a result of the particular xylose pathwayand not due to unknown changes in regulatory systems. An increased understanding of the physiologicalstate of these strains is important for further development of efficient pentose-utilizing strains for bioethanolproduction.Results:Using LC-MS/MS we determined the dynamics in the concentrations of intracellular metabolites incentral carbon metabolism, nine amino acids, the purine nucleotides and redox cofactors. The generalresponse to the transition from glucose to xylose was increased concentrations of amino acids and TCA-cycleintermediates, and decreased concentrations of sugar phosphates and redox cofactors. The two strainsinvestigated had significantly different uptake rates of xylose which led to an enhanced response in theXI-strain. Despite the difference in xylose uptake rate, the adenylate energy charge remained high and stablearound 0.8 in both strains. In contrast to the adenylate pool, large changes were observed in the guanylatepool.Conclusions:The low uptake of xylose by the XI-strain led to several distinguished responses: depletion ofkey metabolites in glycolysis and NADPH, a reduced GTP/GDP ratio and accumulation of PEP and aromaticamino acids. These changes are strong indicators of carbon starvation. The XR/XDH-strain displayed few suchtraits. The coexistence of these traits and a stable adenylate charge indicates that xylose supplies energy tothe cells but does not suppress a response similar to carbon starvation. Particular signals may play a role inthe latter, of which the GTP/GMP ratio could be a candidate as it decreased significantly in both strains.ISSN:1754-683