Abstract
BackgroundWith the discovery of interspecies hydrogen transfer in the late 1960s (Bryant et al. in Arch Microbiol 59:20–31, 1967), it was shown that reducing the partial pressure of hydrogen could cause mixed acid fermenting organisms to produce acetate at the expense of ethanol. Hydrogen and ethanol are both more reduced than glucose. Thus there is a tradeoff between production of these compounds imposed by electron balancing requirements; however, the mechanism is not fully known.ResultsDeletion of the hfsA or B subunits resulted in a roughly 1.8-fold increase in ethanol yield. The increase in ethanol production appears to be associated with an increase in alcohol dehydrogenase activity, which appears to be due, at least in part, to increased expression of the adhE gene, and may suggest a regulatory linkage between hfsB and adhE. We studied this system most intensively in the organism Thermoanaerobacterium saccharolyticum; however, deletion of hfsB also increases ethanol production in other thermophilic bacteria suggesting that this could be used as a general technique for engineering thermophilic bacteria for improved ethanol production in organisms with hfs-type hydrogenases.ConclusionSince its discovery by Shaw et al. (JAMA 191:6457–64, 2009), the hfs hydrogenase has been suspected to act as a regulator due to the presence of a PAS domain. We provide additional support for the presence of a regulatory phenomenon. In addition, we find a practical application for this scientific insight, namely increasing ethanol yield in strains that are of interest for ethanol production from cellulose or hemicellulose. In two of these organisms (T. xylanolyticum and T. thermosaccharolyticum), the ethanol yields are the highest reported to date.
Highlights
With the discovery of interspecies hydrogen transfer in the late 1960s (Bryant et al in Arch Microbiol 59:20–31, 1967), it was shown that reducing the partial pressure of hydrogen could cause mixed acid fermenting organisms to produce acetate at the expense of ethanol
We address the effect of mutations in hfs hydrogenase subunits on ethanol production in T. saccharolyticum as well as other thermophilic anaerobes
It has been previously observed that T. saccharolyticum can be engineered for increased ethanol yield by deleting pathways for lactate and acetate production [17, 18], but that ethanol production increases a few generations after the deletion of phosphotransacetylase, acetate kinase, and/or lactate dehydrogenase rather than immediately
Summary
With the discovery of interspecies hydrogen transfer in the late 1960s (Bryant et al in Arch Microbiol 59:20–31, 1967), it was shown that reducing the partial pressure of hydrogen could cause mixed acid fermenting organisms to produce acetate at the expense of ethanol. Thermophilic bacteria have long been studied for their potential use in biofuel production from cellulose and Eminoğlu et al Biotechnol Biofuels (2017) 10:282 ethanol, H2, and C O2 It is, stoichiometrically possible to produce two moles of ethanol per mole of C 6 sugar (C6H12O6 → 2 C2H6O + 2 CO2) at 97% thermodynamic efficiency (based on heat of combustion, [1]). Stoichiometrically possible to produce two moles of ethanol per mole of C 6 sugar (C6H12O6 → 2 C2H6O + 2 CO2) at 97% thermodynamic efficiency (based on heat of combustion, [1]) To achieve this conversion, all of the electrons initially present in the C6 sugar must be transferred to ethanol and not diverted to organic acids or H2. Support for this hypothesis is found in experiments where increasing the partial pressure of H2 led to an increase in ethanol production [2, 3], or a decrease in ethanol production when the partial pressure of H2 was decreased, by introduction of a syntrophic H2-consuming methanogen [4, 5]
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