Pterin cofactor

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. 

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

In the first family, the metal is coordinated by one molecule of the pterin cofactor, while in the second, it is coordinated to two pterin molecules (both in the guanine dinucleotide form, with the two dinucleotides extending from the active site in opposite directions). Some enzymes also contain FejSj clusters (one or more), which do not seem to be directly linked to the Mo centers. The molybdenum hydroxylases invariably possess redox-active sites in addition to the molybdenum center and are found with two basic types of polypeptide architecture. The enzymes metabolizing quinoline-related compounds, and derivatives of nicotinic acid form a separate groups, in which each of the redox active centers are found in separate subunits. Those enzymes possessing flavin subunits are organized as a2jS2A2, with a pair of 2Fe-2S centers in the (3 subunit, the flavin in the (3 subunit, and the molybdenum in the y subunit. 

J. E. Gready, Theoretical studies on the activation of the pterin cofactor in the catalytic... 

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. 

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.)...

Xanthine oxidase, flavin, Fe S, Mo-pterin cofactors Massey (1973)... 

Some compounds of this type may have a high affinity for proteins that is not due to their binding to two thiol groups (35). In particular, arsenite also reacts with the molybdenum-pterin cofactor of many enzymes (35a-d). This usually inhibits the enzyme, but in particular cases (35e) the arsenite may be oxidized indeed the enzyme arsenite oxidase contains such a center (35f). 

PAH, TH and TPH are highly homologous enzymes. These enzymes catalyze a hydroxylation reaction of aromatic amino acids that requires reduced pterin cofactor 43, molecular oxygen, and iron (Scheme 28). Iron is present at the active sites of the enzymes. Ferrous iron (Fe(II)) is essential for the catalysis, although, the iron was found to be in the ferric form (Fe(III)) when the enzymes were purified from tissues or cells. The ferric iron at the active site of the enzymes was found to be reduced to the ferrous form by BH4 [125]. Thus, BH4 serves a bi-functional role for aromatic amino acid hydroxylases one is the reduction of iron at the active sites from the ferric form to the ferrous form and the other is an electron donor for the hydroxylation reaction. 

Figure 20 Proposed roles of the iron center and the pterin cofactor in the oxygen activation mechanism for pterin-dependent hydroxylases.

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).]...

Teigen K, Frpystein NA, Martinez A The structural basis of the recognition of phenylalanine and pterin cofactors by phenylalanine hydroxylase implications for the catalytic mechanism. / Mol Biol 294 807-823,1999. 

With the electrochemical detector, it is possible to quantitate the amount of Dopa present. The presence of pterins, cofactors required for activity, can also be detected. 

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