    4.1.1.74 | synthesis |
engineering of a strain of Corynebacterium glutamicum, based on inactivation of the pyruvate dehydrogenase complex, pyruvate:quinone oxidoreductase, transaminase B, and additional overexpression of the ilvBNCD genes, encoding acetohydroxyacid synthase, acetohydroxyacid isomeroreductase, and dihydroxyacid dehydratase, for the production of isobutanol from glucose under oxygen deprivation conditions by inactivation of L-lactate and malate dehydrogenases, implementation of ketoacid decarboxylase from Lactococcus lactis, alcohol dehydrogenase 2 (ADH2) from Saccharomyces cerevisiae, and expression of the pntAB transhydrogenase genes from Escherichia coli. The resulting strain produces isobutanol with a substrate-specific yield (YP/S) of 0.60 mol per mol of glucose. Chromosomally encoded alcohol dehydrogenase AdhA rather than the plasmid-encoded ADH2 from S. cerevisiae is involved in isobutanol formation, and overexpression of the corresponding AdhA gene increases the YP/S to 0.77 mol of isobutanol per mol of glucose. Inactivation of the malic enzyme significantly reduces the YP/S, indicating that the metabolic cycle consisting of pyruvate and/or phosphoenolpyruvate carboxylase, malate dehydrogenase, and malic enzyme is responsible for the conversion of NADH + H+ to NADPH + H+. In fed-batch fermentations with an aerobic growth phase and an oxygen-depleted production phase, the most promising strain produces about 175 mM isobutanol, with a volumetric productivity of 4.4 mM per h, and shows an overall YP/S of about 0.48 mol per mol of glucose in the production phase |
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