Branched-chain alcohol formation by thermophilic bacteria within the genera of Thermoanaerobacter and Caldanaerobacter

A Versatile Aldehyde: Ferredoxin Oxidoreductase from the Organic Acid Reducing Thermoanaerobacter sp. Strain X514

Aldehyde:ferredoxin oxidoreductases (AORs) have been isolated and biochemically-characterized from a handful of anaerobic or facultative aerobic archaea and bacteria. They catalyze the ferredoxin (Fd)-dependent oxidation of aldehydes to acids. Recently, the involvement of AOR in the reduction of organic acids to alcohols with electrons derived from sugar or synthesis gas was demonstrated, with alcohol dehydrogenases (ADHs) carrying out the reduction of the aldehyde to the alcohol (AOR-ADH pathway). Here, we describe the biochemical characterization of an AOR of the thermophilic fermentative bacterium Thermoanaerobacter sp. strain X514 (AORX514). The putative aor gene (Teth514_1380) including a 6x-His-tag was introduced into the genome of the genetically-accessible, related species Thermoanaerobacter kivui. The protein was purified to apparent homogeneity, and indeed revealed AOR activity, as measured by acetaldehyde-dependent ferredoxin reduction. AORX514 was active over a wide temperature (10 to 95 °C) and pH (5.5 to 11.5) range, utilized a wide variety of aldehydes (short and branched-chained, aliphatic, aromatic) and resembles archaeal sensu stricto AORs, as the protein is active in a homodimeric form. The successful, recombinant production of AORX514 in a related, well-characterized and likewise strict anaerobe paves the road towards structure-function analyses of this enzyme and possibly similar oxygen-sensitive or W/Mo-dependent proteins in the future.

Biotransformation of Carboxylic Acids to Alcohols: Characterization of Thermoanaerobacter Strain AK152 and 1-Propanol Production via Propionate Reduction

Thermoanaerobacter strains have recently gained interest because of their ability to convert short chain fatty acids to alcohols using actively growing cells. Thermoanaerobacter thermohydrosulfuricus strain AK152 was physiologically investigated for its ethanol and other alcohol formation. The temperature and pH optimum of the strain was 70 °C and pH 7.0 and the strain degraded a variety of compounds present in lignocellulosic biomass like monosaccharides, disaccharides, and starch. The strain is highly ethanologenic, producing up to 86% of the theoretical ethanol yield form hexoses. Strain AK152 was inhibited by relatively low initial substrate (30 mM) concentration, leading to inefficient degradation of glucose and levelling up of all end-product formation. The present study shows that the strain produces alcohols from most of the tested carboxylic acids, with the highest yields for propionate conversion to propanol (40.7%) with kinetic studies demonstrating that the maximum conversion happens within the first 48 h of fermentation. Various physiological tests were performed to maximize the acid conversion to the alcohol which reveals that the optimum pH for propionate conversion is pH 6.7 which affords a 57.3% conversion. Kinetic studies reveal that propionate conversion is rapid, achieving a maximum conversion within the first 48 h of fermentation. Finally, by using ¹³C NMR, it was shown that the addition of propionate indeed converted to propanol.

Decolorization of Acid Orange 7 by extreme-thermophilic mixed culture

Although a large amount of textile wastewater is discharged at high temperatures, azo dye reduction under extreme-thermophilic conditions by mixed cultures has gained little attention. In this study, Acid Orange 7 (AO7) was used as the model azo dye to demonstrate the decolorization ability of an extreme-thermophilic mixed culture. The results showed that a decolorization efficiency of over 90% was achieved for AO7. The neutral red (NR, 0.1 mM) could promote AO7 decolorization, in which the group of Cell + NR offered the highest decolorization rate of 1.568 1/h and t_1/2 was only 0.44 h, whereas after CuCl₂ addition, the decolorization rate (0.141 1/h) was lower and t_1/2 (4.92 h) was much longer. Thus, CuCl₂ notably inhibited this process. Caldanaerobacter (64.0%) and Pseudomonas (25.4%) were the main enriched bacteria, which were not reported to have the ability for dye decolorization. Therefore, this study extends the application of extreme-thermophilic biotechnology.

Branched-chain amino acid catabolism of Thermoanaerobacter pseudoethanolicus reveals potential route to branched-chain alcohol formation

The fermentation of branched-chain amino acids (BCAAs) to branched-chain fatty acids (BCFAs) and branched-chain alcohols (BCOHs) is described using Thermoanaerobacter pseudoethanolicus. BCAAs were not degraded without an electron scavenging system but were degraded to a mixture of their BCFA (major) and BCOH (minor) when thiosulfate was added to the culture. Various environmental parameters were investigated using isoleucine as the substrate which ultimately demonstrated that at higher liquid-gas phase ratios the formation of 2-methyl-1-butanol from isoleucine achieved a maximal titer of 3.4 mM at a 1:1 liquid-gas ratio suggesting that higher partial pressure of hydrogen influences the BCOH/BCFA ratio but did not increase further with higher L-G phase ratios. Alternately, increasing the thiosulfate concentration decreased the BCOH to BCFA ratio. Kinetic monitoring of BCAA degradation revealed that the formation of BCOHs occurs slowly after the onset of BCFA formation. ¹³C2-labeled studies of leucine confirmed the production of a mixture of 3-methyl-1-butyrate and 3-methyl-1-butanol, while experiments involving ¹³C1-labeled 3-methyl-1-butyrate in fermentations containing leucine demonstrated that the carboxylic acid is reduced to the corresponding alcohol. Thus, the role of carboxylic acid reduction is likely of importance in the production of BCOH formation during the degradation of BCAA such as leucine.

Branched-chain amino acid catabolism of Thermoanaerobacter strain AK85 and the influence of culture conditions on branched-chain alcohol formation

The bioprocessing of amino acids to branched-chain fatty acids and alcohols is described using Thermoanaerobacter strain AK85. The amino acid utilization profile was evaluated without an electron scavenger, with thiosulfate, and in a co-culture with a methanogen. There was an emphasis on the production of branched-chain alcohols and fatty acids from the branched-chain amino acids, particularly the influence of culture conditions which was investigated using isoleucine, which revealed that the concentration of thiosulfate was of great importance for the branched-chain alcohols/fatty acid ratio produced. Kinetic studies show that branched-chain amino acid fermentation is relatively slow as compared to glucose metabolism with the concentrations of the branched-chain alcohol increasing over time. To understand the flow of electrons and to investigate if the branched-chain fatty acid was being converted to branched-chain alcohol, enzyme assays and fermentation studies using ¹³C-labeled leucine and 3-methyl-1-butyrate were performed which indeed suggest that carboxylic acid reduction is a source of branched-chain alcohols when Thermoanaerobacter strain AK85 was cultivated with thiosulfate as an electron scavenger.

Biotransformation of organic acids to their corresponding alcohols by Thermoanaerobacter pseudoethanolicus

Higher order alcohols, such as 1-butanol and 1-hexanol, have a large number of applications but are currently prepared from non-renewable feedstocks. Here, the ability of Thermoanaerobacter pseudoethanolicus to reduce short-chain fatty acids to their corresponding alcohols using reducing potential generated by glucose catabolism with yields between 21.0 and 61.0%. ¹³C-labelled acetate, 1-propionate and 1-butyrate demonstrates that exogenously added fatty acids are indeed reduced to their corresponding alcohols. This mode of producing primary alcohols from fatty acids using a thermophilic anaerobe opens the door for the production of such alcohols from low-value materials using an inexpensive source of reducing potential.

Amino Acid Metabolism of Thermoanaerobacter Strain AK90: The Role of Electron-Scavenging Systems in End Product Formation

The catabolism of the 20 amino acids by Thermoanaerobacter strain AK90 (KR007667) was investigated under three different conditions: as single amino acids without an electron-scavenging system, in the presence of thiosulfate, and in coculture with a hydrogenotrophic methanogen. The strain degraded only serine without an alternative electron acceptor but degraded 11 amino acids (alanine, cysteine, isoleucine, leucine, lysine, methionine, phenylalanine, serine, threonine, tyrosine, and valine) under both of the electron-scavenging systems investigated. Acetate was the dominant end product from alanine, cysteine, lysine, serine, and threonine under electron-scavenging conditions. The branched-chain amino acids, isoleucine, leucine, and valine, were degraded to their corresponding fatty acids under methanogenic conditions and to a mixture of their corresponding fatty acids and alcohols in the presence of thiosulfate. The partial pressure of hydrogen seems to be of importance for the branched-chain alcohol formation. This was suggested by low but detectable hydrogen concentrations at the end of cultivation on the branched-chain amino acid in the presence of thiosulfate but not when cocultured with the methanogen. A more detailed examination of the role of thiosulfate as an electron acceptor was performed with Thermoanaerobacter ethanolicus (DSM 2246) and Thermoanaerobacter brockii (DSM 1457).