Phegnon, L.; Perochon, J.; Uttenweiler, S.; Cahoreau, E.; Millard, P.; Letisse, F.

The metabolic networks of microorganisms are remarkably robust to genetic and environmental perturbations. This robustness stems from redundancies such as gene duplications, isoenzymes, alternative metabolic pathways, and also from non-enzymatic reactions. In the oxidative branch of the pentose-phosphate pathway (oxPPP), 6-phosphogluconolactone hydrolysis into 6-phosphogluconate is catalysed by 6-phosphogluconolactonase (Pgl) but in the absence of the latter, the oxPPP flux is thought to be maintained by spontaneous hydrolysis. However, in {Delta}pgl Escherichia coli, an extracellular pathway can also contribute to pentose-phosphate synthesis. This raises question as to whether the non-enzymatic reaction can compensate for the absence of 6-phosphogluconolactonase and, ultimately, on the role of 6-phosphogluconolactonase in central metabolism. Our results indicate that in the absence of Pgl, this bypass pathway accounts for the entire flux into the oxPPP, suggesting that non-enzymatic hydrolysis does not compensate for the absence of Pgl and demonstrating that Pgl is critical for an efficiently functioning oxPPP.