Carbon Fixation: “Let Things Flow Naturally Forward in Whatever Way They Like”

Abstract : Mixed-acid fermentation generates H2 and CO2 from formate. As shown in a recent study, the formate oxidation reaction can be driven backwards when sufficiently high partial pressures of the gases are applied, suggesting potentially interesting biotechnological applications. Escherichia coli grown in the presence of glucose — but in the absence of electron acceptors — ferments the sugar to a mix of organic acids; specifically, lactic, succinic and formic acids. The formic acid produced in this 'mixed-acid fermentation' is further reacted with protons to yield two gases, CO2 and H2. In this redox process, the electrons derived from the oxidation of formate to CO2 are used for the reduction of two protons to yield molecular hydrogen. Oxidising formate to CO2 (which escapes into the gas phase) by using ubiquitously available protons as electron acceptors is a clever way to prevent the main fermentation reaction from stalling due to product inhibition. All this was elucidated almost a century ago [1,2], and the process has since become a standard entry in textbooks on microbial physiology. As a sorted-out mechanism, it was further studied only by a fringe community of microbiologists in more recent years. That changed dramatically when molecular details of the enzyme carrying out the final step (oxidation of formate to CO2) started to emerge [3,4]. The so-called 'formate hydrogenlyase' (FHL) turned out to be a textbook example for the construction-kit strategy that life almost exclusively applies during evolutionary innovation and diversification [5]. FHL revealed itself to be composed of protein subunits belonging to the vast superfamily of
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Wolfgang Nitschke. Carbon Fixation: “Let Things Flow Naturally Forward in Whatever Way They Like”. Current Biology - CB, Elsevier, 2018, 28 (3), pp.R110 - R112. ⟨10.1016/j.cub.2017.12.039⟩. ⟨hal-01709233⟩

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