Sulfite oxidases molybdenum cofactor

Sulfite oxidase is a dimetallic enzyme that mediates the two-electron oxidation of sulfite by the one-electron reduction of cytochrome c. This reaction is physiologically essential as the terminal step in oxidative degradation of sulfur compounds. The enzyme contains a heme cofactor in the 10 kDa N-terminal domain and a molybdenum center in the 42 kDa C-terminal domain. The catalytic cycle is depicted in Fig. 9. 

Figure 17.2 The structure of the pterin cofactor (1) which is common to most molybdenum- and tungsten-containing enzymes and schematic active site structures for members of the xanthine oxidase (2,3), sulfite oxidase (4) and DMSO reductase (5-7) enzyme families. (From Enemark et al., 2004. Copyright (2004) American Chemical Society.)...

K. V. In vitro incorporation of nascent molybdenum cofactor into human sulfite oxidase, J Biol Chem 2001, 276, 1837-1844. 

METHOD OF CONTINUOUS VARIATION MOLYBDENUM COFACTOR (MoCo) Molybdenum-dependent reactions, ALDEHYDE OXIDASE MOLYBDOPTERIN NITRATE REDUCTASE NITROGENASE SULFITE OXIDASE XANTHINE DEHYDROGENASE MOLYBDOPTERIN... 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

Johnson JL, Duran M (2001) Molybdenum cofactor deficiency and isolated sulfite oxidase deficiency. In Scriver CR, Beaudet AL, Sly WS, Valle D (eds) The Metabolic and Molecular Bases of Inherited Disease, 8th edn. McGraw-Hill, New York, NY, pp 3163-3177... 

The first hint of an essential role of molybdenum in metabolism came from the discovery that animals raised on a diet deficient in molybdenum had decreased liver xanthine oxidase activity. There is no evidence that xanthine oxidase is essential for all life, but a human genetic deficiency of sulfite oxidase or of its molybdopterin coenzyme can be lethal.646,646a,b The conversion of molybdate into the molybdopterin cofactor in E. coli depends upon at least five genes.677 In Drosophila the addition of the cyanolyzable sulfur (Eq. 16-64) is the final step in formation of xanthine dehydrogenase.678 It is of interest that sulfur (S°) can be transferred from rhodanese (see Eq. 24-45), or from a related mercaptopyruvate sulfurtransferase679 into the desulfo form of xanthine oxidase to generate an active enzyme.680... 

The apocofactor is synthesized in the absence of molybdenum in E. coti and N. crassa. The cofactor from E. coli is soluble, but is isolated bound to a carrier protein from which it has to be separated.99 Tungstate competes with molybdate for the molybdenum site in the cofactor, leading either to the formation of a demolybdo species or to an inactive form containing tungsten. The tungsten protein has been characterized in the case of sulfite oxidase,1000 which, while inactive, is immunologically identical to the molybdenum cofactor. 

The assimilatory nitrate reductase from Chlorella contains the molybdenum cofactor, as evidenced by the ability of the enzyme to donate the cofactor to the nitrate reductase of the mutant nit-1 of N. crassa. Reduction of the enzyme with NADH gives the Mov ESR signal, which is abolished on reoxidation with nitrate. Line shape and g values of the signal show a pH dependence similar to those observed previously for sulfite oxidase. The signal observed at pH 7.0 shows evidence for interaction with a single exchangeable proton.1053... 

Protein sequence homology suggests that sulfite oxidase and assimilatory nitrate reductase are members of the same molybdenum enzyme subfamily [31]. Consistent with this classification, the cofactors of sulfite oxidase and assimilatory nitrate reductase differ significantly from those in dmso reductase, aldehyde oxido-reductase, xanthine oxidase (see Section IV.E.), and even respiratory nitrate reductase (Section IV.D). The EXAFS of both sulfite oxidase [132-136] and assimilatory nitrate reductase [131,137,138] and x-ray studies of sulfite oxidase (chicken liver) [116] confirm that the molybdenum center is coordinated by two sulfur atoms from a single MPT ligand and by the sulfur atom of a cysteine side chain. The Movl state is bis(oxido) coordinated (Figure 14). 

Molybdenum is a component of at least three enzymes aldehyde oxidase, xanthine dehydrogenase, and sulfite oxidase. The first two contain FAD, whereas the last is a heme protein similar to cytochrome c. Xanthine dehydrogenase can also act as an oxidase, that is, it can use 02 as an electron acceptor. Physiologically, however, it uses NAD+ as an electron acceptor when it converts hypoxanthine to xanthine and the latter to uric acid (see Chapter 10). Aldehyde and sulfite oxidases are true oxidases physiologically they both use 02 as an electron acceptor. Molybdenum in all three enzymes is associated with a pterinlike cofactor whose structure is shown in Figure 6.11. The Mo cofactor cannot be... 

Figure 6.11 Structure of the pterin cofactor, which binds molybdenum in aldehyde oxidase, xanthine oxidase, and sulfite oxidase. [Reproduced by permission from Rajago-palan, KV. Molybdenum, an essential trace element. Nutr. Rev., 45 321-328 (1987).]...

Figure 16. Consensus oxidized active-site structures of the xanthine oxidase (XO), sulfite oxidase (SO), and DMSO reductase (DMSOR), and aldehyde oxidoreductase (AOR) families of mononuclear molybdenum and tungsten enzymes and the structure of the common ppd cofactor (41, 42). The question mark in the AOR structure indicates uncertainty in the presence of a coordinated water molecule.

Deistung and Bray (29) have described a procedure for anaerobic isolation of active intact molybdenum cofactor from xanthine oxidase. The molecular mass by gel filtration was about 610. Hawkes and Bray (30) have reported that Mo-co from xanthine oxidase and sulfite oxidase can be stabilized under anaerobic conditions in the presence of dithio-nite and that oxidation in the presence of thiophenol results in EPR signals characteristic of Mo(V) and little loss of cofactor activity (31). However, to date intact Mo-co has not been structurally characterized. The possible coordination about the molybdenum atom in Mo-co in enzymes is discussed in Section III. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

These clusters are each composed of eight iron atoms and seven sulfide ions. In the reduced form, each cluster takes the form of two 4Fe-3S partial cubes linked by a central sulfide ion. Each cluster is linked to the protein through six cysteinate residues. Electrons flow from the P cluster to the FeMo cofactor, a very unusual redox center. Because molybdenum is present in this cluster, the nitrogenase component is also called the molybdenum-iron protein (MoFe protein). The FeMo cofactor consists of two M-3Fe-3S clusters, in which molybdenum occupies the M site in one cluster and iron occupies it in the other. The two clusters are joined by three sulfide ions. The FeMo cofactor is also coordinated to a homocitrate moiety and to the a subunit through one histidine residue and one cysteinate residue. This cofactor is distinct from the molybdenum-containing cofactor found in sulfite oxidase and apparently all other molybdenum-containing enzymes except nitrogenase. 

Molybdenum is required in the diet. It is required by three enzymes in mammals sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. Molybdenum occurs in these enzymes as part of the molybdenum cofactor (Figure 10.52). This cofactOT is biosynthes z.ed in the body with GTP as the starling material. All known Mo mclalloenzymes, with the exception of nitrogenase (a plant enzyme), use Mo in the form of the molybdenum cofactor. 

Several important mammalian enzymes, such as sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase, require molybdenum as a cofactor. This organic component is a molybdopterin complex.Sulfite oxidase is probably the most important enzyme in relation to human health. This enzyme catalyzes the last step in the degradation of sulfur amino acids, oxidizing sulfite to sulfate and transferring electrons to cytochrome c. Xanthine dehydrogenase and aldehyde oxidase hydroxylate a number of heterocyclic substances, such as purines, pteridines, and others. ... 

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