Evidence for role of transketolase function in the maintenance of pyridine nucleotide levels in Escherichia coli.
The transketolase (Tkt) activity provides reversible link between glycolysis and pentose phosphate pathway (PPP). Depending on the metabolic flux, it can catalyse synthesis of glycolytic intermediates, fructose-6-phosphate and glyceraldehyde-3-phosphate from xylulose-5-P and ribose-5-P (PPP intermediates) and synthesis of xylulose-5-P from the above mentioned glycolytic intermediates. Using E. coli, we addressed the physiological significance of this metabolic flexibility by studying the growth phenotypes and metabolic changes associated with depletion of transketolase activity and the genetic changes or growth conditions that rescued the growth phenotypes. Tkt function was needed for cell growth when glucose was catabolized solely through Oxidative-PPP. Under gluconeogenic growth conditions, either transketolase or UdhA transhydrogenase was needed for growth. Cells depleted of Tkt activity were more sensitive than wild type to genetic changes that perturb pyridine cofactor levels. In LB medium, Tkt function was needed to prevent growth arrest from the accumulation of ribose- 5-P and possibly other pentose phosphates. In cell free extracts, the activity of Zwf and Gnd enzymes that support NADPH synthesis was inhibited by ribose-5-P. These results suggested, Tkt function played an important role in the maintenance of pyridine cofactor pool and this was confirmed by quantification. Metabolomic changes associated with transketolase depletion supported the genetic data.