Formate dehydrogenases molybdenum

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... 

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. 

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). 

Gladyshev VN, Khangulov SV, Axley MJ, StadtmanTC. 1994b. Coordination of selenium to molybdenum in formate dehydrogenase H from Escherichia coli. Proc Natl Acad Sci USA 91 7708-11. 

A molybdenum cofactor has been isolated from Proteus mirabilis 047, and has a molecular weight greater than 1000. The molybdoenzymes of E. coli, in addition to the formate dehydrogenases described above and the nitrate reductase (Section 62.1.9.6), also include the membrane-bound trimethylamine oxidase1044 and the soluble biotin sulfoxide reductase.1045... 

A range of redox centres have been found in the membranes from cells grown on glycerol and fumarate including menaquinone, ubiquinone, the iron-sulfur centres of fumarate reductase, cytochromes d, b and a, other iron-sulfur centres and molybdenum. Some of these centres may be assignable to specific enzymes, such as molybdenum to formate dehydrogenase. Other dehydrogenases (for NADH and lactate) are also present, and are linked to the cytochromes by menaquinone. However, there have been relatively few studies on the function of these components. 

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

IV.C.1. The Molybdenum Cofactor of Formate Dehydrogenase (Escherichia coli), L2Mo-SeCys... 

Figure 9 The coordination geometry around molybdenum as suggested from the x-ray crystallographic studies of formate dehydrogenase (Escherichia coli) (a) oxidized form (b) reduced form. The dotted line and Se—S bond distance are suggested from EXAFS [124] but are not seen in the x-ray structure [111,124],...

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)... 

The x-ray crystallographic results for formate dehydrogenase [111,122] show that the cofactor is des(oxido) but coordinated by selenocysteine in both the MoVI and MoIV oxidation states. The lack of an oxido is consistent with the CEPT role for the cofactor with the selenocysteine [3,111,122] serving as the proton acceptor while modulating the redox behavior of the molybdenum center (Figure 23). 

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... 

Mechanisms of action for the metal centers in acetylene hydratase, polysulfide reductase, and formate dehydrogenase have been briefly described in Sections VI.A and VLB. The discussion, in each case, was relatively straightforward insofar as the natures of these reactions lend themselves to simple mechanistic proposals. The mechanism by which the metal centers function in most of the other Mo and W enzymes is not as obvious. We elect to discuss mechanistic roles for the molybdenum centers in xanthine oxidase, sulfite oxidase, and dmso reductase. These enzymes are representative members of each large class of molybdenum enzymes, and the large body of literature on each enzyme makes detailed discussion possible. 

This chemistry may be relevant to the nature and function of the molybdenum center of E. coli formate dehydrogenase. The Mo K-edge EXAFS of both the oxidized and reduced form of this enzyme were found to be very similar, each inolving a des-oxo-molybdenum site with four Mo S bonds at 2.35 A, (probably) one Mo O bond at 2.1 A, and one Mo—Se interaction at 2.62 A. The Se K-edge EXAFS showed clear evidence for a S Se contact of 2.19 A, presumably indicative of a novel seleno-sulfide ligand to the molybdenum (121). 

The final family of molybdenum enzymes consists of enzymes from bacterial and archaeal sources that also catalyze oxygen atom transfer reactions for the most part, although formate dehydrogenase and polysulfide... 

As for the reactions catalyzed by members of this third family of molybdenum enzymes, there are several variations from the principal theme of oxygen atom transfer. Formate dehydrogenase from E. coli catalyzes the oxidation of formate to CO2, a reaction that isotope studies have shown does not pass through a bicarbonate intermediate (Khangulov et al., 1998). Instead, it appears likely that C02is formed by direct hydride transfer from substrate to the molybdenum center. Polysulfide reductase is another molybdenum enzyme that catalyzes a non-canonical reaction the... 

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