Regulation of NAD(H) pool and NADH/NAD⁺ ratio by overexpression of nicotinic acid phosphoribosyltransferase for succinic acid production in Escherichia coli NZN111

Succinic acid is not the dominant fermentation product from glucose in wild-type Escherichia coli W1485. To reduce byproduct formation and increase succinic acid accumulation, pyruvate formate-lyase and lactate dehydrogenase, encoded by pflB and ldhA genes, were inactivated. However, E. coli NZN111, the ldhA and pflB deletion strain, could not utilize glucose anaerobically due to the block of NAD⁺ regeneration. To restore glucose utilization, overexpression of nicotinic acid phosphoribosyltransferase, a rate limiting enzyme of NAD(H) synthesis encoded by the pncB gene, resulted in a significant increase in cell mass and succinic acid production. Furthermore, the results indicated a significant increase in NAD(H) pool size, and decrease in the NADH/NAD⁺ ratio from 0.64 to 0.13, in particular, the concentration of NAD⁺ increased 6.2-fold during anaerobic fermentation. In other words, the supply of enough NAD⁺ for NADH oxidation by regulation of NAD(H) salvage synthesis mechanism could improve the cell growth and glucose utilization anaerobically. In addition, the low NADH/NAD⁺ ratio also change the metabolite distribution during the dual-phase fermentation. As a result, there was a significant increase in succinic acid production, and it is provided further evidence that regulation of NAD(H) pool and NADH/NAD⁺ ratio was very important for succinic acid production.