Bacillus subtilis produces acetoin as a major product along with several NADH-dependent byproducts, especially 2,3-butanediol. In this study, the down-regulation of the NADH pool and the redistribution of NADH/NADPH were targeted using external and genetic processes, as a means by which to redistribute the metabolic flux in favor of acetoin synthesis. First, it was found that the use of carbon sources of different oxidation states resulted in very different intracellular NADH/NAD(+) ratios that dictated the total process yield of acetoin. A mixture of glucose and gluconate as substrate produced a relatively low NADH/NAD(+) ratio, and resulted in an increase in acetoin production while byproducts significantly decreased. Metabolic engineering methods using glucose as a substrate could yield a similar effect. Acetoin production was significantly enhanced by overexpression of the oxidative pentose phosphate pathway: increased expression of glucose-6-phosphate dehydrogenase resulted in a decrease in the intracellular NADH/NADPH ratio (1.9-fold) and NADH/NAD(+) ratio (1.7-fold). In fed-batch culture the engineered strain yielded an acetoin concentration of 43.3 g L(-1) , while the production of 2,3-butanediol was only 1.7 g L(-1) . The concept of the manipulation of cofactor levels to redistribute carbon flux by external and genetic means as explored in this paper provides a novel strategy for improving industrial acetoin fermentation.
Keywords: Acetoin; Bacillus subtilis; Gluconate; Glucose-6-phosphate dehydrogenase; NADH/NAD+.
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