Enzymes formate dehydrogenase

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. 

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 ... 

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. 

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. 

The enzymes hydrogenase, nitrogenase, and formate dehydrogenase can be used to equilibrate reducing reagents with H2O/H2, N2NH3, and CO2/HCOOH, respectively.(53) In no case do the enzymes involve expensive noble metals as catalysts. 

As a consequence of the previous considerations Kieber et al. [75] have developed an enzymic method to quantify formic acid in non-saline water samples at sub-micromolar concentrations. The method is based on the oxidation of formate by formate dehydrogenase with corresponding reduction of /3-nicotinamide adenine dinucleotide (j6-NAD+) to reduced -NAD+(/3-NADH) jS-NADH is quantified by reversed-phase high performance liquid chromatography with fluorimetric detection. An important feature of this method is that the enzymic reaction occurs directly in aqueous media, even seawater, and does not require sample pre-treatment other than simple filtration. The reaction proceeds at room temperature at a slightly alkaline pH (7.5-8.5), and is specific for formate with a detection limit of 0.5 im (SIN = 4) for a 200 xl injection. The precision of the method was 4.6% relative standard deviation (n = 6) for a 0.6 xM standard addition of formate to Sargasso seawater. Average re-... 

SUZUKI, K., ITAI, R., SUZUKI, K., NAKANISHI, H., NISHIZAWA, N.K., YOSHIMURA, E., MORI, S., Formate dehydrogenase, an enzyme of anaerobic metabolism, is induced by iron deficiency in barley roots, Plant Physiol., 1998, 116, 725-732. 

It is possible to use isolated, partially purified enzymes (dehydrogenases) for the reduction of ketones to optically active secondary alcohols. However, a different set of complications arises. The new C H bond is formed by delivery of the hydrogen atom from an enzyme cofactor, nicotinamide adenine dinucleotide (phosphate) NAD(P) in its reduced form. The cofactor is too expensive to be used in a stoichiometric quantity and must be recycled in situ. Recycling methods are relatively simple, using a sacrificial alcohol, or a second enzyme (formate dehydrogenase is popular) but the real and apparent complexity of the ensuing process (Scheme 8)[331 provides too much of a disincentive to investigation by non-experts. 

The redox properties of Mo also make it useful in enzymes that catalyze reactions involving two-electron or oxygen-atom transfer (Frausto da Silva and Williams 2001). Such enzymes include nitrate reductase, sulfite oxidase, formate dehydrogenase and aldehyde oxidase (Hille 1996 Stiefel 1997 Kroneck and Abt 2002). Hence, while Mo is rarely a terminal electron... 

The one-carbon reactions and enzymes unique to the carbon dioxide reduction pathway are shown in Figure 11.2 and Table 11.2. Electrons required for the reductive reactions are derived from the oxidation of either hydrogen or formate, catalyzed by hydrogenase or formate dehydrogenase. 

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