Tungsten enzymes

As mentioned above, tungsten enzymes are found in place of molybdenum enzymes in thermophilic bacteria and hyperhermophilic archaebacteria. Like the molybdenum enzymes, they can be classified into three broad families, all of which contain two pterin cofactor molecules per Mo. In this respect, they are similar to the DMSO reductase 

FIGURE 17.8 Proposed oxidation-state changes occurring at the Mo and Fe centres of animal SO during the catalytic oxidation of sulfite and the concomitant reduction of (cyt c)ox- (Adapted from Johnson-Winters, Tollin, Enemark, 2010.) 

FIGURE 17.10 Active site structures of W-containing enzymes. (Adaptedfrom Hille, 2002.) 

A number of organisms appear to be able to use either molybdenum or mngsten, as a function of their bioavailability, as mentioned earlier. 

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Bioinorganic Chemistry of Pterin-Containing Molybdenum and Tungsten Enzymes John H. Enemark and Charles G. Young... 

White H, G Strobl, R Feicht, H Simon (1989) Carboxylic acid reductase a new tungsten enzyme catalyses the reduction of non-activated carboxylic acids to aldehydes. Eur J Biochem 184 89-96. 

Me Master, J. and Enemark, J.H. (1998). The active sites of molybdenum and tungsten enzymes. Curr. Opin. Chem. Biol., 2, 201-207... 

Another factor that characterizes molybdenum and tungsten enzymes is that instead of using the metal itself, directly coordinated to amino acid side-chains of the protein, an unusual pterin cofactor, Moco, is involved in both molybdenum- and tungsten-containing enzymes. The cofactor (pyranopterin-dithiolate) coordinates the metal ion via a dithiolate side-chain (Figure 17.2). In eukaryotes, the pterin side-chain has a terminal phosphate group, whereas in prokaryotes, the cofactor (R in Figure 17.2) is often a dinucleotide. 

Brondino, C.D., Romao, M.J., Moura, I. and Moura, J J.G. (2006) Molybdenum and tungsten enzymes the xanthine oxidase family, Curr. Opin. Chem. Biol., 10, 109-114. 

ScHiNDELiN, H., Kisker, C., and Rajagopalan, K. V. Molybdopterin from molybdenum and tungsten enzymes, Adv Protein Chem 2001, 58,... 

Pterin, see also Tungsten, enzymes, pterin-containing... 

I. INTRODUCTION THE MOLYBDENUM AND TUNGSTEN ENZYMES, A BIOLOGICAL PERSPECTIVE... 

In the last 5 years, 20 new enzymes have been confirmed to contain either molybdenum or tungsten, and in the last 3 years representive members of both molybdenum and tungsten enzyme families have been crystallographically characterized. 

There are several recent reviews of the molybdenum and tungsten enzymes [4-6,23,26-36], In this chapter, we first define the metallocofactors and offer a compilation of the enzymes and their diverse activities. We then focus on the active-site structures, highlighting the confluence of crystallographic and spectroscopic studies. This is followed by a discussion of pertainent spectroscopic, structural, reactivity, and theoretical model studies. We then turn our attention to the mechanisms of catalytic activity of the molybdenum and tungsten enzymes. 

From recent x-ray crystallographic studies of both molybdenum and tungsten enzymes, MPT is now known to be a C(6)-substituted 5,6-dihydropterin that is covalently alkoxylated at the C(7) position by the alcohol of the 1,2-enedithio-late side chain (Eq. 1) [39],... 

Figure 2 The structures ofthe MPT cofactor subfamilies. The boxes represent subfamilies of molybdenum and tungsten enzymes with high sequence homology. Cofactor structure type determined aby x-ray crystallography from the listed source bby EXAFS.

Figure 3 The reconstitution of aponitrate reductase from the Nit-1 Neurospora crassa mutant by MPT-containing extracts from the molybdenum and tungsten enzymes.

The enzymes that catalyze substrate oxidation are oxidases, hydroxylases, dehydrogenases, and oxidoreductases. Most of the substrate oxidations catalyzed by molybdenum and tungsten enzymes involve the net transfer of an oxygen atom... 

Table 1c Molybdenum and Tungsten Enzyme Catalyzed Oxidations and Reductions of Oxygen-Bearing Carbon Centers...

Table 2e Non-redox Processes Catalyzed by Molybdenum and Tungsten Enzymes...

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