Vitreoscilla hemoglobin enhances ethanol production by Escherichia coli in a variety of growth media

Abstract

Abstract In a previous study, we showed that expression of Vitreoscilla hemoglobin (VHb) in ethanologenic Escherichia. coli strain FBR5 enhanced ethanol production in a limited range of nutrient concentrations. Here enhancement of ethanol production in the VHb expressing strain (TS3) was investigated in a wider variety of growth media. For nutrient (LB) concentrations from 0.25X to 2X (supplemented with 80 g dm −3 xylose), ethanol production was always better in TS3 than in FBR5. The greatest relative advantage of TS3 over FBR5 occurred at 0.25X LB, while the greatest yield for both strains relative to LB concentration occurred at 0.5X LB. In these experiments, the enhancement in ethanol production of TS3 was determined to be due to both an increased ethanol metabolic efficiency and increased cell growth. 2X LB was also supplemented with purified biomass hydrolysate produced by acid treatment and overliming; additional treatment with activated carbon decreased the fermentability of the hydrolysate. Strain TS3 outperformed strain FBR5 in 2X LB at hydrolysate concentrations of 0.2, 0.4 and 0.8 m 3 m −3 ; the greatest yield by TS3 relative to the level of hydrolysate occurred at 0.4 m 3 m −3 . The concentrations of NAD, NADH, and ATP were measured in both strains to investigate the biochemical basis of the ethanol production enhancement by VHb. The most significant difference found was in the NAD/NADH ratio; this was generally lower for TS3 than for FBR5 earlier in the fermentation, while this pattern was reversed later in the fermentation.