초록
<P><B>Background</B></P><P>Acetoin is an important bio-based platform chemical. However, it is usually existed as a minor byproduct of 2,3-butanediol fermentation in bacteria.</P><P><B>Results</B></P><P>The present study reports introducing an exogenous NAD<SUP>+</SUP> regeneration sysytem into a 2,3-butanediol producing strain <I>Klebsiella pneumoniae</I> to increse the accumulation of acetoin. Batch fermentation suggested that heterologous expression of the NADH oxidase in <I>K. pneumoniae</I> resulted in large decreases in the intracellular NADH concentration (1.4 fold) and NADH/NAD<SUP>+</SUP> ratio (2.0 fold). Metabolic flux analysis revealed that fluxes to acetoin and acetic acid were enhanced, whereas, production of lactic acid and ethanol were decreased, with the accumualation of 2,3-butanediol nearly unaltered. By fed-batch culture of the recombinant, the highest reported acetoin production level (25.9 g/L) by <I>Klebsiella</I> species was obtained.</P><P><B>Conclusions</B></P><P>The present study indicates that microbial production of acetoin could be improved by decreasing the intracellular NADH/NAD<SUP>+</SUP> ratio in <I>K. pneumoniae</I>. It demonstrated that the cofactor engineering method, which is by manipulating the level of intracellular cofactors to redirect cellular metabolism, could be employed to achieve a high efficiency of producing the NAD<SUP>+</SUP>-dependent microbial metabolite.</P>