Abstract
SummaryHydrogen-dependent reduction of carbon dioxide to formic acid offers a promising route to greenhouse gas sequestration, carbon abatement technologies, hydrogen transport and storage, and the sustainable generation of renewable chemical feedstocks [1]. The most common approach to performing direct hydrogenation of CO2 to formate is to use chemical catalysts in homogeneous or heterogeneous reactions [2]. An alternative approach is to use the ability of living organisms to perform this reaction biologically. However, although CO2 fixation pathways are widely distributed in nature, only a few enzymes have been described that have the ability to perform the direct hydrogenation of CO2 [3, 4, 5]. The formate hydrogenlyase (FHL) enzyme from Escherichia coli normally oxidizes formic acid to carbon dioxide and couples that reaction directly to the reduction of protons to molecular hydrogen [6]. In this work, the reverse reaction of FHL is unlocked. It is established that FHL can operate as a highly efficient hydrogen-dependent carbon dioxide reductase when gaseous CO2 and H2 are placed under pressure (up to 10 bar). Using intact whole cells, the pressurized system was observed to rapidly convert 100% of gaseous CO2 to formic acid, and >500 mM formate was observed to accumulate in solution. Harnessing the reverse reaction has the potential to allow the versatile E. coli system to be employed as an exciting new carbon capture technology or as a cell factory dedicated to formic acid production, which is a commodity in itself as well as a feedstock for the synthesis of other valued chemicals.
Highlights
Al pH, the behavior of CO2 in solution is known to be complex [11], and substrate availability to the formate hydrogenlyase (FHL) enzyme is likely to be a limiting parameter
Henry’s law states that the amount of dissolved gas is proportional to the applied pressure [12]; to predict what relative concentrations of dissolved H2 and CO2 might be attainable by applying headspace pressure to a 1:1 mixture of these gases, a nonrandom two-liquid (NRTL) activity coefficient model [13] with Henry’s law for H2 and CO2 derived from isothermal datasets at 308 K/35C was devised (Figure S1)
It was considered that the close standard redox potentials of the two half-reactions of FHL, and evidence that the enzyme activity was not coupled to other biochemical processes such as generation of electrochemical gradients [25], should allow the correct environmental conditions to be found that would drive the reverse reaction: i.e., increased pH, increased gas pressure/substrate concentrations, and rapid removal of the product from the vicinity of the enzyme
Summary
Al pH, the behavior of CO2 in solution is known to be complex [11], and substrate availability to the formate hydrogenlyase (FHL) enzyme is likely to be a limiting parameter. Henry’s law states that the amount of dissolved gas is proportional to the applied pressure [12]; to predict what relative concentrations of dissolved H2 and CO2 might be attainable by applying headspace pressure to a 1:1 mixture of these gases, a nonrandom two-liquid (NRTL) activity coefficient model [13] with Henry’s law for H2 and CO2 derived from isothermal datasets at 308 K/35C was devised (Figure S1). The model, consistent with Henry’s law, predicts CO2 could reach $120 mmol,LÀ1 in solution, and H2 $4 mmol,LÀ1, when mixed together at 10 bar pressure
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