Abstract
BackgroundThe industrial production of various alcohols from organic carbon compounds may be performed at high rates and with a low risk of contamination using thermophilic microorganisms as whole-cell catalysts. Thermoanaerobacter species that thrive around 50–75 °C not only perform fermentation of sugars to alcohols, but some also utilize different organic acids as electron acceptors, reducing them to their corresponding alcohols.ResultsWe purified AdhE as the major NADH- and AdhB as the major NADPH-dependent alcohol dehydrogenase (ADH) from the cell extract of the organic acid-reducing Thermoanaerobacter sp. strain X514. Both enzymes were present in high amounts during growth on glucose with and without isobutyrate, had broad substrate spectra including different aldehydes, with high affinities (< 1 mM) for acetaldehyde and for NADH (AdhE) or NADPH (AdhB). Both enzymes were highly thermostable at the physiological temperature of alcohol production. In addition to AdhE and AdhB, we identified two abundant AdhA-type ADHs based on their genes, which were recombinantly produced and biochemically characterized. The other five ADHs encoded in the genome were only expressed at low levels.ConclusionsAccording to their biochemical and kinetic properties, AdhE and AdhB are most important for ethanol formation from sugar and reduction of organic acids to alcohols, while the role of the two AdhA-type enzymes is less clear. AdhE is the only abundant aldehyde dehydrogenase for the acetyl-CoA reduction to aldehydes, however, acid reduction may also proceed directly by aldehyde:ferredoxin oxidoreductase. The role of the latter in bio-alcohol formation from sugar and in organic acid reduction needs to be elucidated in future studies.
Highlights
The industrial production of various alcohols from organic carbon compounds may be performed at high rates and with a low risk of contamination using thermophilic microorganisms as whole-cell catalysts
Efforts towards genetic modification of Thermoanaerobacter sp. strain X514 Nine genes encoding for putative alcohol dehydrogenase (ADH) and one aor gene (Teth514_1380) are present in the genome of Thermoanaerobacter sp. strain X514, but it remains unclear which of those genes play a role in alcohol production, from sugar as well as in organic acid reduction[9]
Since we recently reported that Thermoanaerobacter sp. strain X514 reduced externally added organic acids to alcohols, we studied the biochemical properties of the purified enzymes, which may shed light on their putative role in organic acid reduction via aldehydes
Summary
The industrial production of various alcohols from organic carbon compounds may be performed at high rates and with a low risk of contamination using thermophilic microorganisms as whole-cell catalysts. Thermoanaerobacter species that thrive around 50–75 °C perform fermentation of sugars to alcohols, but some utilize different organic acids as electron acceptors, reducing them to their corresponding alcohols. Species of the genus Thermoanaerobacter have been described as thermophilic, chemoorganotrophic sugar-utilizing microorganisms, producing a wide range of fermentation products such as acetate, lactate, CO2, H2 or ethanol [2], with the notable exception of the (homo)acetogen T. kivui [3,4,5]. We recently found that a variety of Thermoanaerobacter species produced ethanol from sugars, and took up and subsequently reduced organic acids to their corresponding alcohols, e.g., isobutyrate to isobutanol [9]. More evidence of organic acid reduction to alcohols has since been put forward for other Thermoanaerobacter strains (AK85 and AK152) [11, 12]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
