Molybdopterin-containing enzymes

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). 

Three human redox enzymes, and a variety of bacterial enzymes, contain molybdenum chelated by two sulfur atoms in a modified pterin molybdopterin (see Figure 10.1). In sulfite oxidase, the other two chelation sites of the molybdenum are occupied by oxygen in xanthine oxidase / dehydrogenase (Section 7.3.7) and aldehyde oxidase, one site is occupied by oxygen and one by sulfur. In some bacterial enzymes, molybdopterin occurs as a guanine dinucleotide rather than free. In others, tungsten rather than molybdopterin is the chelated metal there is no evidence that any mammalian enzymes contain tungsten. 

R. S. Pilato, K. A. Eriksen, M. A. Greaney, E. I. Stiefel, S. Goswami, L. Kilpatrick, T. G. Spiro, E. C. Taylor and A. L. Rheingold, Model complexes for molybdopterin-containing enzymes preparation and crystallographic characterization of a molybdenum ene-l-perthi-olate-2-thiolate (trithiolate) complex,/. Am. Chem. Soc., 1991, 113, 9372-9374. 

In terms of the reaction catalyzed, molybdopterin-containing enzymes can be divided in two groups those that mediate oxygen atom transfer, such as dimethyl sulfoxide (DMSO) reductase and sulfite oxidase (SO), and those that catalyze hydroxylation reactions of aromatic heterocyclic compounds and aldehydes [116], for instance xanthine oxidoreductase (XOR) and aldehyde oxidoreductase (AOR). However, this functional classification does not coincide with structural properties that suggest that the enzymes should be grouped into five families, whose most representative members are (1) DMSO reductase (2) XOR (3) SO (4) aldehyde-... 

The properties of CODH/acetyl-CoA synthase are summarized in Table II. The enzyme has been isolated from eight species. There exist three types of CODH. One, which lacks nickel and acetyl-CoA synthase activity, contains a molybdopterin active site and will not be... 

The protein from D. desulfuricans has been characterized by Mbss-bauer and EPR spectroscopy 224). The enzyme has a molecular mass of approximately 150 kDa (three different subunits 88, 29, and 16 kDa) and contains three different types of redox-active centers four c-type hemes, nonheme iron arranged as two [4Fe-4S] centers, and a molybdopterin site (Mo-bound to two MGD). Selenium was also chemically detected. The enzyme specific activity is 78 units per mg of protein. 

Molybdopterin is a component of four enzyme families all of which contain Mo(VI) the xanthine oxidase and the sulfite oxidase families with one molybdopterin and the DMSO family with two molybdopterins. There are a number of tungsten-containing enzymes with structures analogous... 

Tungsten-containing enzymes have been found to mediate a variety of reactions both in aerobic and anaerobic bacteria, and their structure may plausibly be assumed to be analogous to the molybdopterins ... 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

Figure 16-31 (A) Structure of molybdopterin cytosine dinucleotide complexed with an atom of molybdenum. (B) Stereoscopic ribbon drawing of the structure of one subunit of the xanthine oxidase-related aldehyde oxidoreductase from Desulfo-vibrio gigas. Each 907-residue subunit of the homodimeric protein contains two Fe2S2 clusters visible at the top and the molybdenum-molybdopterin coenzyme buried in the center. (C) Alpha-carbon plot of portions of the protein surrounding the molybdenum-molybdopterin cytosine dinucleotide and (at the top) the two plant-ferredoxin-like Fe2S2 clusters. Each of these is held by a separate structural domain of the protein. Two additional domains bind the molybdopterin coenzyme and there is also an intermediate connecting domain. In xanthine oxidase the latter presumably has the FAD binding site which is lacking in the D. gigas enzyme. From Romao et al.633 Courtesy of R. Huber.

The active site structures of the three classes of molybdenum-containing enzymes are compared in Fig. 16-32. In the DMSO reductase family there are two identical molybdopterin dinucleotide coenzymes complexed with one molybdenum. However, only one of these appears to be functionally linked to the Fe2S2 center. 

Formate dehydrogenases from many bacteria contain molybdopterin and also often selenium (Table 15-4).664/665 A membrane-bound Mo-containing formate dehydrogenase is produced by E. coli grown anaerobically in the presence of nitrate. Under these circumstances it is coupled to nitrate reductase via an electron-transport chain in the membranes which permits oxidation of formate by nitrate (Eq. 18-26). This enzyme is also a multisubunit protein.665 666 Two other Mo- and Se- containing formate dehydrogenases are produced... 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

The cofactor appears to include a novel pterin.996-998 The properties of the pterin depend upon the nature of the side-chain in the 6-position. The structure shown in Figure 39 has been proposed997 on the basis that molybdopterin is related to urothione, oxidized to pterin-6-carboxylic acid, and contains in the side-chain two sulfur groups, a double bond, a hydroxyl function and a terminal phosphate group. Two stable fluorescent derivatives of molybdopterin have been characterized,999 which may be of value in view of the extreme instability of the native molybdoprotein when released from the enzyme. 

Molybdopterin synthase is a 27 kDa protein (in its inactive resting state) that is comprised of 16 and 10 kDa subunits [67], The protein has a tendency to dissociate and the exact stoichiometry of the active synthase remains uncertain. Activation of the protein requires that the smaller subunit undergo a single sulfur-for-oxygen exchange, a finding that has been confirmed by mass spectral analysis of both active and inactive 10 kDa subunits. Since conversion of the precursor to MPT requires two sulfur atoms, it has been postulated that the active form of the synthase contains two 10 kDa subunits [67], The synthase not only provides sulfur atoms for MPT precursor conversion but also transports MPT to the apo-molybdenum (or presumably apotungsten) enzyme. 

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