Some microorganisms, such as Escherichia coli, harbor transhydrogenases that catalyze the interconversion between NADPH and NADH. However, such transhydrogenase genes have not been found in the genome of a glutamic acid-producing bacterium Corynebacterium glutamicum. In this study, the E. coli transhydrogenase genes udhA and pntAB were introduced into the C. glutamicum wild-type strain ATCC 13032, and the metabolic characteristics of the recombinant strains under aerobic and microaerobic conditions were examined. No major metabolic changes were observed following the introduction of the E. coli transhydrogenase genes under aerobic conditions. Under microaerobic conditions, significant metabolic change was not observed following the introduction of the udhA gene. However, the specific production rates of lactic acid, acetic acid, and succinic acid, and the overall production levels of acetic acid and succinic acid were increased by introducing the E. coli pntAB gene. Moreover, the NADH/NAD + ratio was increased by introduction of pntAB. Our results suggest that the E. coli PntAB transhydrogenase enhances the conversion of NADPH to NADH in C. glutamicum under microaerobic conditions, and the increased NADH/NAD + ratio results in increased succinic acid production. In addition, acetic acid production might be enhanced to supply ATP to the anaplerotic reaction catalyzed by pyruvate carboxylase.Key words: acetic acid; Corynebacterium glutamicum; pntAB; redox balance; succinic acid; transhydrogenases
IntroductionMany oxidation and reduction reactions are involved in cellular metabolism, most of which require pyrimidine nucleotide cofactors, such as nicotinamide adenine dinucleotide (NAD + ) and nicotinamide adenine dinucleotide phosphate (NADP + ) and their reduced forms NADH and NADPH, to supply oxidizing and reducing equivalents. Generally, NADH is produced via catabolism, and is oxidized in the respiratory chain under aerobic conditions or by fermentation under anaerobic conditions. Anabolic reactions to produce cell biomass components, such as fatty acid and amino acid biosynthesis, require NADPH as a reducing equivalent. Therefore, maintaining the intracellular balance of NAD(P) + and NAD(P)H is very important for most intracellular metabolic reactions to occur.Some organisms contain the enzymes catalyzing the following interconversion of NADH and NADPH, called transhydrogenase:NADPH + NAD + NADP + + NADH. To date, two types of transhydrogenases have been identified; the cytoplasmic-soluble and membrane-bound types. Membrane-bound transhydrogenases have been found in the inner mitochondrial membrane of eukaryotic cells and in the cytoplasmic membrane of many bacteria, and simultaneously catalyze proton translocation across a membrane.
Full PaperEnhanced acetic acid and succinic acid production under microaerobic conditions by Corynebacterium glutamicum harboring Escherichia coli transhydrogenase gene pntAB (Received April 3, 2014; Accepted April 8, 2014) Yuto Yamauchi, None of the authors of this manuscript ha...