Fig. 1

Metabolic engineering strategies for redistributing the carbon flux to 2,3-BD by manipulating NADH levels. 2,3-BD is an NADH-dependent product; the NADH availability and its proportion in the active form play important roles in 2,3-BD production. But NADH oxidase catalyzes the oxidation of NADH to NAD⁺. Lactate, one of the main by-products, competes with 2,3-BD for pyruvate as a metabolic intermediate and NADH as a cofactor. To block a pathway that competitively consumes NADH and to redirect carbon flux towards 2,3-BDO production, the NADH oxidase was disrupted by insertion of a formate dehydrogenase gene fdh, and then the pathway to lactate was blocked using the insertional mutation technique to disrupt the lactate dehydrogenase gene ldhA. (3-PGA 3-Phosphate glyceraldehyde, 1,3-BPG 1,3-Bisphosphoglycerate, Red arrows, oxidation of NADH to NAD⁺; Green arrows, reduction of NAD⁺ to NADH; , blocking the pathway; The blue dotted box, introducing extrinsic pathway)