Reduction of CO2 to Carbon Monoxide or Formate

In some prokaryotes the FDHs are complex enzymes which contain molybdenum or tungsten cofactors to transfer the electrons from formate oxidation to an independent active site, to reduce quinone, protons, or NAD(P) [82, 92]. These enzymes are suitable for adsorption onto an electrode, so that the electrode accepts the electrons from formate oxidation, and it may also donate electrons and drive 

Tungstoenzymes catalyze low-potential reactions [93] so tungsten-containing FDHs, in which the tungsten is coordinated by two pyranopterin guanosine dinucleotide cofactors and a selenocysteine [92], are those most associated with CO2 reduction. 

One of the tungsten-containing FDHs, namely FDHl, isolated by Syntrophobacter fumaroxidans, an anaerobic bacterium, oxidizes propionate to acetate, CO2, and six reducing equivalents [94]. The reducing equivalents are used to reduce protons to hydrogen or to reduce CO2 to formate. 

The NADH-independent FDH enzymes from S. fumaroxidans are highly unstable and inactive in the presence of O2, limiting their practical application. Conversely, the NADH-dependent FDH from Candida boidinii (CbsEDH) was sufficiently stable for commercial use and has been used to enzymatically regenerate NADH. CbsFDH requires NADH, protons, and electrons to convert CO2 into formate, which can be supplied by the electrochemical system of the Cu electrode 

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