METABOLIC CONTROL ANALYSIS OF GLYCOLYSIS AND BRANCHING TO ETHANOL-PRODUCTION IN CHEMOSTAT CULTURES OF SACCHAROMYCES-CEREVISIAE UNDER CARBON, NITROGEN, OR PHOSPHATE LIMITATIONS

Metabolic Control Analysis was applied to calculate the rate-controlli ng steps of the glycolytic flux of Saccharomyces cerevisiae growing ae robically in chemostat cultures subjected to carbon, nitrogen, or phos phate limitations. The control over the flux ratio at the branch point to ethanol formation and the tricarboxylic acid cycle as well as meta bolite concentration control coefficients were determined. Two sorts o f models were analyzed, essentially differing in the glucose uptake st ep which was considered either ATP-and extracellular glucose-dependent (model I) or glucose 6-phosphate-dependent (model II). Kinetic parame ters were optimized according to the experimentally determined metabol ite concentrations. In both models, glucose uptake was the main rate-c ontrolling step of the glycolytic flux. In model I the control coeffic ient of the uptake varied between 1.01 and 1.043. The control was shar ed by ATP-consuming and -producing processes (C-ATPase(J) = -0.080 to -0.0306; C-ADH(J) = -0.0069 to -0.0144; C-TCA(J) = 0.0044 to 0.0144). Meanwhile, in model II of the flux was almost exclusively controlled b y the uptake (C-IN(J) = 0.99). Independently of the nutrient limitatio n, growth rate and type of glucose breakdown metabolism, the rate-cont rolling steps were the same and only quantitative differences were not ed. All metabolite concentrations were controlled by the same rate-con trolling steps of the flux, and additionally, by the reaction steps ca talyzing the metabolite consumption. The results presented point to th e significant role of sugar uptake in yeast catabolism and the usefuln ess of control analysis to understand metabolic flux as applied to bio transformation processes.