Abstract
CO2 and formate are rapidly, selectively, and efficiently interconverted by tungsten-containing formate dehydrogenases that surpass current synthetic catalysts. However, their mechanism of catalysis is unknown, and no tractable system is available for study. Here, we describe the catalytic properties of the molybdenum-containing formate dehydrogenase H from the model organism Escherichia coli (EcFDH-H). We use protein film voltammetry to demonstrate that EcFDH-H is a highly active, reversible electrocatalyst. In each voltammogram a single point of zero net current denotes the CO2 reduction potential that varies with pH according to the Nernst equation. By quantifying formate production we show that electrocatalytic CO2 reduction is specific. Our results reveal the capabilities of a Mo-containing catalyst for reversible CO2 reduction and establish EcFDH-H as an attractive model system for mechanistic investigations and a template for the development of synthetic catalysts.
Highlights
The rapid, reversible, and specific electrochemical reduction of CO2 to formate by a tungsten-containing formate dehydrogenase from the anaerobic bacterium Syntrophobacter fumaroxidans (Sf FDH1) provided the paradigm case for a formate/CO2 catalyst.[10]
The catalytic mechanism of CO2 reduction by the W-center in Sf FDH1 is a valuable source of information to aid the design of improved synthetic catalysts
Sf FDH1 itself is intractable for mechanistic studies: cell cultures of S. fumaroxidans take several months to achieve low cell densities that provide only minuscule amounts of enzyme; no genetic manipulation is possible; and the enzyme contains an extensive cohort of iron−sulfur (FeS) centers to transfer electrons to and from the active site, making overexpression strategies untenable.[10,12,13]
Summary
Cleanly through unique zero-current points, traced out as points of intersection as the current decays during successive scans. The zero-current points denote the thermodynamic reduction potential for the CO2/formate interconversion (net formate oxidation occurs at more positive potentials and net CO2 reduction at more negative potentials), demonstrating that the electrocatalytic reaction is thermodynamically reversible.[11] Both the oxidation and reduction currents increase rapidly as the overpotential is increased but do not reach potentialindependent limiting currents within the accessible potential range This behavior is typical of highly catalytically active enzymes, for which the electrocatalytic rate is limited by interfacial electron transfer.[11]. Iron, manganese, and copper based catalysts are able potentials to are reduce CO2 electrochemically, but typically required and their efficiency large overis low.[2−9] In contrast, formate dehydrogenases catalyze the two electron reduction of CO2 directly to energy-rich formic acid with high selectivity, under mild conditions, with little overpotential requirement, and elucidation of their catalytic mechanism may inform the development of improved synthetic catalysts.
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