Pterins sulfite oxidase

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.)...

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).]...

The reduction state of the pterin was a point of uncertainty throughout these studies of molybopterin derivatives. The absence of fluorescence in anaerobic molybdopterin samples suggested a reduced pterin. Redox titration of XO and SO both indicated that the pterin could undergo a two-electron oxidation reaction (73, 74). Sulfite oxidase, for example, produced the fluorescence characteristic of an oxidized pterin after addition of 2 equiv of ferricyanide. However, titrating XO was problematic due to interfering redox processes of the iron-sulfur clusters. 

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. 

The molybdenum-containing oxidoreductases that catalyze Eq. (1) have been variously termed molybdenum hydroxylases (6), oxotransferases (7), and oxo-type molybdenum enzymes (8). Molybdenum hydroxylase aptly describes the conversion of xanthine to uric acid, but the name seems less appropriate for the reactions catalyzed by sulfite oxidase and nitrate reductase oxotransferase implies that the function of these enzymes is to transfer oxo groups, even though relatively little is known about their actual mechanism of action and the name oxo-type molybdenum enzyme recognizes both the apparent oxo transfer chemistry of Eq. (1) and the fact that the molybdenum atom in each of these enzymes contains at least one terminal oxo group. In this chapter, we shall refer to these enzymes as pterin-containing molybdenum enzymes because a 6-substituted pterin appears to be a common chemical feature of all of the enzymes. 

The chapter consists of nine sections. Sections II through VII deal with the pterin-containing molybdenum enzymes. Biochemical and model studies of molybdopterin, Mo-co, and related species are described in Section II. In Section III, we briefly survey physical and spectroscopic techniques employed in the study of the enzymes, and consider their impact upon the current understanding of the coordination about the molybdenum atom in sulfite oxidase and xanthine oxidase. Model studies are described in Sections IV and V. Section IV concentrates on structural and spectroscopic models, whereas Section V considers aspects of the reactivity of model and enzyme systems. The xanthine oxidase cycle (Section VI) and facets of intramolecular electron transfer in molybdenum enzymes (Section VII) are then treated. Section VIII describes the pterin-containing tungsten enzymes and the evolving model chemistry thereof Future directions are addressed in Section IX. 

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). 

The postulated catalytic cycles for pterin-containing molybdenum enzymes involve a two-electron change at the molybdenum atom (Mo(VI) Mo(IV)). Microcoulometric titrations of nitrate reductase Chlorella vulgaris) (76), milk xanthine oxidase (77), and sulfite oxidase (78) show that their molybdenum centers are reduced by two electrons. The reduction potentials for the molybdenum center of chicken liver sulfite oxidase are strongly dependent upon pH and upon anion concentration (78). 

Molybdenum-containing enzymes can be divided into three families, the xanthine oxidase (XO), sulfite oxidase (SO), and the DMSO reductase (DMR) families. They each have a characteristic active site structure (Figure 17.2(a)) and catalyse a particular type of reaction (see below). Whereas in eukaryotes, the pterin side chain has a terminal phosphate group, in prokaryotes, the cofactor (R in Figure 17.2(b)) it is often a dinucleotide. 

Enzymes of the sulfite oxidase family coordinate a single equivalent of the pterin cofactor with an MPT-Mo 02 core in its oxidized state (54, Figure 16), and usually an additional cysteine ligand, which is provided by the polypeptide. Members of this family catalyze the transfer of an oxygen atom either to or from the substrate. Among the members of this family are sulfite oxidase, sulfite dehydrogenase, assimilatory nitrate reductases, and the YedY protein, the catalytic subunit of a sulfite oxidase homologue in E. coli So far, all members of this family contain the MPT-form of Moco without an additional dinucleotide. 

Garrett RM and Rajagopaian KV (1984) Molecular cloning of rat liver sulfite oxidase. Expression of a eukaryotic Mo-pterin-containing enzyme in Escherichia coli. J Biol Chem 269 272 - 276. 

Liver (0.6 ngg ) amounts in bile aldehyde oxidase, xanthine oxidase/ dehydrogenase and sulfite oxidase in which molyidenum exists as a small nonprotein factor containing a pterin nucleus molybdate ion (MoOl" ), the form that exists in blood and urine oxidize and detoxify various pyrimidines, purines, and pteridines catalyze the transformation of hypoxanthine to xanthine and xanthine to uric acid and catalyze the conversion of sulfite to sulfate... 

Mo 0.15-0.5 mg Xanthine dehydrogenase, sulfite, aldehyde oxidases Mo seems to be always bound to a cofactor— a compound containing a pterin ring... 

---