The valuable pharmaceutical polymer, hyaluronic acid, is produced industrially using the gram-positive bacterium Streptococcus zooepidemicus. Synthesis of this polymer is a significant energetic burden upon the microorganism hence the native NADH oxidase gene was cloned and overexpressed to increase the energy yield of catabolism during aerobic cultivation on glucose. Elevated NADH oxidase levels led to a decline in lactic acid generation and prevented ethanol formation, leaving acetate as the main fermentation product. Biomass yield increased due to the energy gained from the formation of acetate. Evaluation of the acetate flux control coefficient over a range of NADH oxidase expression levels revealed that acetate production was sensitive to the NADH oxidase level. However, at high NADH oxidase levels, the acetate flux was mainly influenced by another factor. The concomitant excretion of pyruvate at high NADH oxidase levels suggested that the flux through the pyruvate dehydrogenase enzyme complex was limiting the conversion of pyruvate to acetate.
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