Formate dehydrogenases

Formate dehydrogenases (FDHs) catalyze the interconversion of CO2 and formic acid through an oxidoreductive process (9.2) [75]. 

FDHs present in acetogens are known to take part in a carbon fixation metabolic pathway producing acetate (the Eastern branch of the Wood-Ljungdahl pathway), in which the first step involves reduction of CO2 to formate [76]. Several FDHs are known to catalyze CO2 reduction under appropriate conditions [77-81]. Those enzymes from acetogenic and related anaerobes, such as Moorella thermoacetica and Clostridium pasteurianum, are better than other FDHs as reduction catalysts but also show similar catalytic efficiency toward formate oxidation. 

The structure of Escherichia coli FDHh, as solved by multiple isomorphous replacement (MIR) and multiwavelength anomalous dispersion (MAD) methods, consists of four ap domains [82]. It has been prepared with MOLSCRIPT and RASTER3D [83-85]. 

Formate dehydrogenase H from E. coli contains selenocysteine (SeCys), molybdenum, two molybdopterin guanine dinucleotide (MGD) cofactors, and an Fe4S4 

The active form of selenium in M. vannielii has been identified as occurring as selenocysteine residues . In addition to selenium, the known selenium-dependent formate dehydrogenases contain molybdenum and iron-sulfur centers. The E. coli enzyme also contains cytochrome b subunits . It has been characterized as an approximately 

000 dalton protein which contains three different types of subunits (a a4fi y2(ot y ) structure). The a subunits which are 110,000 dalton subunits contain the 4 gram atoms of selenium (presumably selenocysteine) . Formate dehydrogenases are rapidly deactivated by oxygen and have proven difficult to isolate in pure, catalytically active form. Because of these problems, neither the exact composition nor the catalytic roles of selenium, molybdenum or the iron-sulfur centers are known at this time. 

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In order to broaden the field of biocatalysis in ionic liquids, other enzyme classes have also been screened. Of special interest are oxidoreductases for the enan-tioselective reduction of prochiral ketones [40]. Formate dehydrogenase from Candida boidinii was found to be stable and active in mixtures of [MMIM][MeS04] with buffer (Entry 12) [41]. So far, however, we have not been able to find an alcohol dehydrogenase that is active in the presence of ionic liquids in order to make use of another advantage of ionic liquids that they increase the solubility of hydrophobic compounds in aqueous systems. On addition of 40 % v/v of [MMIM][MeS04] to water, for example, the solubility of acetophenone is increased from 20 mmol to 200 mmol L ... 

Formamidine, N,N -di-2-anthraquinonyI-metal complexes, 2,275 Formamidine, IV.AT-diaryl-metal complexes, 2, 275 Formamidine, N, N -dibenzyl-metal complexes, 2,276 Formamidine, IV.N -diisopropyl-metal complexes, 2, 276 Formamidinesulfinic acid technetium complexes, 6, 974 Formate dehydrogenases bacteria... 

Figure 8.8 Reduction of carbon dioxide with formate dehydrogenase and porphyrin complex using light energy [6h].

The enzymes that utilize molybdenum can be grouped into two broad categories (1) the nitrogenases, where Mo is part of a multinu-clear metal center, or (2) the mononuclear molybdenum enzymes, such as xanthine oxidase (XO), dimethyl sulfoxide (DMSO) reductase, formate dehydrogenase (FDH), and sulfite oxidase (SO). The last... 

The three known crystal structures of molybdopterin-containing enzymes are from members of the first two families the aldehyde oxido-reductase from D. gigas (MOP) belongs to the xanthine oxidase family (199, 200), whereas the DMSO reductases from Rhodobacter (R.) cap-sulatus (201) and from/ , sphaeroides (202) and the formate dehydrogenase from E. coli (203) are all members of the second family of enzymes. There is a preliminary report of the X-ray structure for enzymes of the sulfite oxidase family (204). 

Sulfate reducers can use a wide range of terminal electron acceptors, and sulfate can be replaced by nitrate as a respiratory substrate. Molybdenum-containing enzymes have been discovered in SRB (also see later discussion) and, in particular, D. desulfuricans, grown in the presence of nitrate, generates a complex enzymatic system containing the following molybdenum enzymes (a) aldehyde oxidoreduc-tase (AOR), which reduces adehydes to carboxylic acids (b) formate dehydrogenase (FDH), which oxidizes formate to CO2 and (c) nitrate reductase (the first isolated from a SRB), which completes the enzy-... 

The molyhdopterin cofactor, as found in different enzymes, may be present either as the nucleoside monophosphate or in the dinucleotide form. In some cases the molybdenum atom binds one single cofactor molecule, while in others, two pterin cofactors coordinate the metal. Molyhdopterin cytosine dinucleotide (MCD) is found in AORs from sulfate reducers, and molyhdopterin adenine dinucleotide and molyb-dopterin hypoxanthine dinucleotide were reported for other enzymes (205). The first structural evidence for binding of the dithiolene group of the pterin tricyclic system to molybdenum was shown for the AOR from Pyrococcus furiosus and D. gigas (199). In the latter, one molyb-dopterin cytosine dinucleotide (MCD) is used for molybdenum ligation. Two molecules of MGD are present in the formate dehydrogenase and nitrate reductase. 

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

In two sulfate-reducing bacteria we found oxygen-tolerant formate dehydrogenases with different subunit composition. The formate dehydrogenase from D. desulfuricans is an af3y protein whereas the one from D. gigas is an afi protein. Both proteins contain two MGD cofactors but the protein from D. desulfuricans contains four heme c attached to the y subunit (16 kDa). 

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). 

Formate dehydrogenase can be said to catalyze a kind of decarboxylation reaction and is the most widely used in NADH regeneration. However, as the reaction does not include C—C bond fission, the studies on this enzyme are not described in this chapter. 

Boyington JC, VN Gladyshev, SV Khangulov, TC Stadtman, PD Sun (1997) Crystal structure of formate dehydrogenase H catalysis involving Mo, molybdopterin, selenocysteine, and an Fe4S4 cluster. Science 275 1305-1308. 

Graentzdoerffer A, D Rauh, A Pich, JR Andreesen (2003) Molecular and biochemical characterization of two tungsten-and selenium-containing formate dehydrogenases from Eubacterium acidamophilum that are associated with components of an iron-only hydogenase. Arch Microbiol 179 116-130. 

Yamamoto I, T Saiki, S-M Liu, LG Ljungdahl (1983) Purification and properties of NADP-dependent formate dehydrogenase from Clostridium thermoaceticum, a tungsten-selenium-iron protein. J Biol Chem 258 1826-1832. 

Jones JB, TC Stadtman (1981) Selenium-dependent and selenium-independent formate dehydrogenase of Methanococcus vannielii. Separation of the two forms and characterization of the purified selenium-independent form. J Biol Chem 256 656-663. 

Furthermore, a biological catalyst [formate dehydrogenase (FDH)] combined with an illuminated p-InP photocathode was... 

Figure 16. Scheme for the photoelectrochemical reduction of C02 at p-InP with formate dehydrogenase (FDH) as the catalyst and methyl viologen (MV2+) as the electron transfer mediator.163... 

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