Biocatalytic reduction of short‐chain carboxylic acids into their corresponding alcohols with syngas fermentation

Abstract

Short-chain carboxylic acids generated by various mixed- or pure-culture fermentation processes have been considered valuable precursors for production of bioalcohols. While conversion of carboxylic acids into alcohols is routinely performed with catalytic hydrogenation or with strong chemical reducing agents, here, a biological conversion route was explored. The potential of carboxydotrophic bacteria, such as Clostridium ljungdahlii and Clostridium ragsdalei, as biocatalysts for conversion of short-chain carboxylic acids into alcohols, using syngas as a source of electrons and energy is demonstrated. Acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, and n-caproic acid were converted into their corresponding alcohols. Furthermore, biomass yields and fermentation stoichiometry from the experimental data were modeled to determine how much metabolic energy C. ljungdahlii generated during syngas fermentation. An ATP yield of 0.4-0.5 mol of ATP per mol CO consumed was calculated in the presence of hydrogen. The ratio of protons pumped across the cell membrane versus electrons transferred from ferredoxin to NAD(+) via the Rnf complex is suggested to be 1.0. Based on these results, we provide suggestions how n-butyric acid to n-butanol conversion via syngas fermentation can be further improved.

Trash to treasure: production of biofuels and commodity chemicals via syngas fermenting microorganisms

“…Yet, the reversibility of this and similar alcohol forming enzymatic reactions was exploited by providing external short-chain carboxylic acids and syngas to C. ljungdahlii for the production of alcohols [58].…”

“…Thus, it is extremely important to detail the exact mechanisms of energy conservation, including the stoichiometry of the ATPase reaction, in the metabolic network reconstruction for more realistic phenotype predictions. Another model of the WL pathway in C. ljungdahlii was used to determine the ATP yield per mol CO consumed and the proton translocation per electron transfer of the Rnf complex [58]. Lastly, metabolic modeling has been used to optimize media formulations, for instance, based on energy demands [82].…”

A Two-Stage Continuous Fermentation System for Conversion of Syngas into Ethanol

“…For estimation of the amount of acetic acid produced by a certain amount of cells, it is helpful to note that the concentration of acetic acid in stage one was always ~100 mM per OD 600 . Knowing that the coefficient for milligram dry weight per Liter per OD 600 [mg DW/(L·OD 600 )] is 242 for C. ljungdahlii ERI-2 [17], one can estimate that 1 g DW of cell mass produced about 0.4 mol, or 24 g acetic acid.…”

“…ljungdahlii ERI-2 (ATCC 55380) was used as a biocatalyst, since it had proven to be a good ethanol producer [17]. Bacteria were always grown anaerobically at 35 °C in medium designed for efficient syngas fermentation [22], which is referred to here as 1× medium.…”

A review of conversion processes for bioethanol production with a focus on syngas fermentation

“…It was recently reported that mono-cultures of C. ljungdahlii and C. ragsdalei as well as a mixed culture of A. bacchi and C. propionicum were able to convert added acids such as propionic, butyric, and hexanoic acids to their respective alcohols (Liu et al, 2014b;Perez et al, 2013). Additional development and optimization of biological gas conversion processes are expected to result in production of various products besides biofuels at commercial scale in the near future.…”