Overview of the main metabolic reactions in central carbon metabolism. A) Xylose can be assimilated through two pathways: the fungal oxidoreductive pathway consisting of XR and XDH or the bacterial isomerization pathway via an XI. The xylulose formed by the two pathways is phosphorylated by XK and channelled through the non-oxidative PPP yielding 2/3 moles F6P and 1/3 moles GAP per mole substrate. These metabolites enter glycolysis to generate ATP in the conversion of PEP to pyruvate and the oxidative PPP to regenerate NADPH. Pyruvate is converted into ethanol, acetate and oxaloacetate, which is transported into the mitochondria for amino acid synthesis. Under anaerobic conditions succinate dehydrogenase is not operational which results in a branched TCA-cycle with succinate as the end product. The twelve precursor metabolites which are required for biosynthesis of macromolecules are highlighted in orange colour. Metabolites in capital letters have been measured in the current study. B) Glycolysis is activated through the induction of the Ras/cAMP/PKA pathway. cAMP formed by adenylate cyclase (CYR1) interacts with the regulatory subunits (R) of PKA which releases the catalytic subunits (C). The active PKA phosphorylates PFK26 which subsequently produces F26BP from F6P and ATP. F26BP is an essential activator of PFK1 which phosphorylates F6P to FBP. The production of FBP in turn activates PYK and ATP generation. Production of ATP and ethanol depends on a high catalytic activity of PYK which requires the simultaneous presence of G6P, F6P and FBP. PKA and F26BP also efficiently inactivate gluconeogenesis by inhibiting FBP1.