Carbon metabolism and the sign of control coefficients in metabolic adaptations underlying K-ras transformation.

Metabolic adaptations are associated with changes in enzyme activities. These adaptations are characterized by patterns of positive and negative changes in metabolic fluxes and concentrations of intermediate metabolites. Knowledge of the mechanism and parameters governing enzyme kinetics is rarely available. However, the signs-increases or decreases-of many of these changes can be predicted using the signs of metabolic control coefficients. These signs require the only knowledge of the structure of the metabolic network and a limited qualitative knowledge of the regulatory dependences, which is widely available for carbon metabolism. Here, as a case study, we identified control coefficients with fixed signs in order to predict the pattern of changes in key enzyme activities which can explain the observed changes in fluxes and concentrations underlying the metabolic adaptations in oncogenic K-ras transformation in NIH-3T3 cells. The fixed signs of control coefficients indicate that metabolic changes following the oncogenic transformation-increased glycolysis and oxidative branch of the pentose-phosphate pathway, and decreased concentration in sugar-phosphates-could be associated with increases in activity for glucose-6-phosphate dehydrogenase, pyruvate kinase and lactate dehydrogenase, and decrease for transketolase. These predictions were validated experimentally by measuring specific activities. We conclude that predictions based on fixed signs of control coefficients are a very robust tool for the identification of changes in enzyme activities that can explain observed metabolic adaptations in carbon metabolism.