Improvement of the Redox Balance Increases l-Valine Production by Corynebacterium glutamicum under Oxygen Deprivation Conditions

Abstract

ABSTRACTProduction ofl-valine under oxygen deprivation conditions byCorynebacterium glutamicumlacking the lactate dehydrogenase geneldhAand overexpressing thel-valine biosynthesis genesilvBNCDEwas repressed. This was attributed to imbalanced cofactor production and consumption in the overalll-valine synthesis pathway: two moles of NADH was generated and two moles of NADPH was consumed per mole ofl-valine produced from one mole of glucose. In order to solve this cofactor imbalance, the coenzyme requirement forl-valine synthesis was converted from NADPH to NADH via modification of acetohydroxy acid isomeroreductase encoded byilvCand introduction ofLysinibacillus sphaericusleucine dehydrogenase in place of endogenous transaminase B, encoded byilvE. The intracellular NADH/NAD+ratio significantly decreased, and glucose consumption andl-valine production drastically improved. Moreover,l-valine yield increased and succinate formation decreased concomitantly with the decreased intracellular redox state. These observations suggest that the intracellular NADH/NAD+ratio, i.e., reoxidation of NADH, is the primary rate-limiting factor forl-valine production under oxygen deprivation conditions. Thel-valine productivity and yield were even better and by-products derived from pyruvate further decreased as a result of a feedback resistance-inducing mutation in the acetohydroxy acid synthase encoded byilvBN. The resultant strain produced 1,470 mMl-valine after 24 h with a yield of 0.63 mol mol of glucose−1, and thel-valine productivity reached 1,940 mM after 48 h.