ABSTRACT
We studied the physiological effect of the interconversion between the NAD(H) and NADP(H) coenzyme systems in recombinantSaccharomyces cerevisiae expressing the membrane-bound transhydrogenase from Escherichia coli. Our objective was to determine if the membrane-bound transhydrogenase could work in reoxidation of NADH to NAD+ in S. cerevisiaeand thereby reduce glycerol formation during anaerobic fermentation. Membranes isolated from the recombinant strains exhibited reduction of 3-acetylpyridine-NAD+ by NADPH and by NADH in the presence of NADP+, which demonstrated that an active enzyme was present. Unlike the situation in E. coli, however, most of the transhydrogenase activity was not present in the yeast plasma membrane; rather, the enzyme appeared to remain localized in the membrane of the endoplasmic reticulum. During anaerobic glucose fermentation we observed an increase in the formation of 2-oxoglutarate, glycerol, and acetic acid in a strain expressing a high level of transhydrogenase, which indicated that increased NADPH consumption and NADH production occurred. The intracellular concentrations of NADH, NAD+, NADPH, and NADP+were measured in cells expressing transhydrogenase. The reduction of the NADPH pool indicated that the transhydrogenase transferred reducing equivalents from NADPH to NAD+.
Membrane-bound pyrophosphatase of human gut microbe Clostridium methylpentosum confers improved salt tolerance in Escherichia coli, Saccharomyces cerevisiae and tobacco
