Response of the central metabolism in Corynebacterium glutamicum to the use of an NADH-dependent glutamate dehydrogenase

The extensive use of 13C enrichments in precursor metabolites for flux quantification does not rely on NADPH stoichiometries and can therefore be used to quantify reducing power fluxes. As an application of this concept, the NADPH fluxes were quantified in an L-lysine producer of Corynebacterium glutamicum grown into metabolic and isotopic steady state with [1-13C]glucose. In this case, where the organism's NADPH-dependent glutamate dehydrogenase consumes reducing power, the NADPH flux generated is 210% (molar flux relative to glucose uptake rate) with its major part (72% of the total) generated via the pentose phosphate pathway activity. An isogenic strain in which the glutamate dehydrogenase of C. glutamicum was replaced by the NADH-dependent glutamate dehydrogenase of Peptostreptococcus asaccharolyticus was made and the metabolite fluxes were again estimated. The major response to this local perturbation is a drastically reduced NADPH generation of only 139%. Most of the NADPH (62% of the total) is now generated via the tricarboxylic acid cycle activity. This shows the extraordinary flexibility of the central metabolism and provides a picture of the global regulatory properties of the central metabolism. Furthermore, a detailed analysis of the fluxes and exchange fluxes within the anaplerotic reactions is given. It is hypothesized that these reactions might also serve to balance the total reducing power budget as well as the energy budget within the cell.