Tungsten formate dehydrogenase 1 enzyme

D. gigas formate dehydrogenase seems to be quite different in terms of subunit composition. It does not contain a y subunit and no heme c was detected (225). Also, two MGD were identified, but surprisingly, the enzyme contains tungsten instead of molybdenum. Mossbauer and EPR studies confirmed the presence of two [4Fe-4S] + + clusters with similar properties to the ones found in D. desulfuricans FDH (247). 

Recently, interest has been expressed in natural enzymes that effect the reduction of C02 to various products (see Section 11.4). For example, Reda et al. used a tungsten-containing formate dehydrogenase 1 enzyme derived from Syntropho-bacterjumaroxidans in the mediated electroreduction of C02 to formate [103]. The enzyme, which is either adsorbed onto the graphite electrode surface or is free in solution, was observed to reduce C02 to formate with near-100% faradaic efficiency. Although a minimal overpotential for the process was required (-0.4V of applied bias), the current densities were rather low. 

Most of the substrate reactions catalyzed by the molybdenum and tungsten enzymes involve either incorporation or removal of an oxygen atom. For a CEPT process to apply to these reactions, transfer of protons and electrons must occur concomitantly with either the addition or elimination of water (see Section VLD). However, the substrate reactions of polysulfide reductase and formate dehydrogenase [122,228] (Eq. 14 and 15)... 

Formate dehydrogenase and poly sulfide reductase function by what could be called simple CEPT (see Section VI.B.2). However, if CEPT is an important component of substrate reactivity in most of the molybdenum and tungsten enzymes, it must involve not only substrate activation but also water addition or elimination. Involvement of the metal in these processes could rely on oxidation state-coupled pA a changes of aqua, hydroxido, or hydrosulfido ligands. Whether the metal is directly involved in substrate binding and water activation or whether... 

Acetogenic bacteria such as Clostridium (Cl.) ther-moaceticum or Cl. formicoaceticum catalyze an NADPH2-driven CO2 reduction to formate, which is further reduced via the tetrahydrofolate pathway to the -CH3 oxidation state, as the methyl is finally incorporated into acetic acid. This anabolic formate dehydrogenase has long been known to depend on the presence of Se in the medium for its formation. The enzyme is an ai 2 heterooligomer of 340 kDa the complex contains two selenocysteine residues, two moles of a tungsten cofactor, and Fe/S centers. 

Active Sites of Tungsten Enzyme Families Formate Dehydrogenase Family 0... 

Fermentation of hexose yields two pyruvates, and pyruvate is further oxidized to two acetyl CoA and two CO2 by the enzyme pyruvate ferredoxin oxidoreductase. Then, the two acetyd CoAs are converted to two acetates. The 4 moles ATP per hexose produced are used for further metabolism. The oxidation-reduction balance is achieved by reduction of both CO2 molecules to a third acetyl CoA and finally to acetate by the Wood-Ljungdahl pathway. The NADH and reduced ferredoxin generated from the oxidized forms during fermentation of hexose are the source of reducing power. One CO2 is reduced to 5-methyltetrahydrofolate. The first reaction in this pathway (reaction 4, O Fig. 1-4) is the formation of formate catalyzed by an unusual tungsten-selenoprotein formate dehydrogenase (Yamamoto et al. 1983). For C. thermoaceticum, the electron donor is NADPH. 

The growth of Methanobacterium wolfei is dependent on the presence of molybdenum or tungsten, and one of the two formylmethanofuran dehydrogenases from this bacterium is a tungsten enzyme (285). The reversible reaction catalyzed by these enzymes (Eq. (29)) is the first step in methane formation from CO2 in all methanogenic Archaea (286). 

---