Molybdopterin dinucleotides

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. 

In recent years several bacterial proteins have been isolated that contain a pterin that is similar but not identical to molybdopterin from eucaryotic organisms (1) (33, 41-43). These bactopterins (44) have been shown to be molybdopterin dinucleotides (e.g., 6 is the oxidized... 

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,... 

The degradation of isoquinoline by Pseudomonas diminuta strain 7 is initiated by an oxidoreductase that contains [2Fe-2S] centers and the cofactor molybdopterin cytosine dinucleotide (Lehmann et al. 1994). 

They have a molecular mass of 300-360 kDa, and contain per molecule, eight atoms of Fe, eight atoms of acid-labile S, two atoms of Mo, and two molecules of FAD. The organic component of the pterin molybdenum cofactor is generally molybdopterin cytosine dinucleotide (Hetterich et al. 1991 Schach et al. 1995). 

Reichenbecher W, A Rudiger, PMH Kroneck, B Schink (1996) One molecule of molybdopterin guanine dinucleotide is associated with each subunit of the heterodimeric Mo-Fe-S protein transhydroxylase of Pelobacter acidigalUci as determined by SDS (PAGE) and mass spectrometry. Eur J Biochem 237 406-413. 

Johnson JL, Bastian NR, Schauer NL, et al. 1991. Identification of molybdopterin guanine dinucleotide in formate dehydrogenase from Methanobacterium formi-cicum. FEMS Microbiol Lett 77 2-3. 

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 recognition that the Mo in the molybdoproteins exists in organic cofactor forms came from studies of mutants of Aspergillus and Neurospora.650 In 1964, Pateman and associates discovered mutants that lacked both nitrate reductase and xanthine dehydrogenase. Later, it was shown that acid-treated molybdoenzymes released a material that would restore activity to the inactived nitrate reductase from the mutant organisms. This new coenzyme, a phosphate ester of molybdopterin (Fig. 15-17), was characterized by Rajagopalan and coworkers.650 651 A more complex form of the coenzyme, molybdopterin cytosine dinucleotide... 

Johnson, J.L., Bastian, N.R. Rajagopalan, K.V. (1990). Molybdop-terin guanine dinucleotide a modified form of molybdopterin identified in the molybdenum cofactor of dimethyl sulfoxide reductase from Rhodobacter sphaeroides forma specialis denitrificans. Proceedings of the National Academy of Sciences (USA) 87, 3190-4. 

What is known about the biosynthesis of MPT and the MPT dinucleotides has been reviewed elsewhere from both a biochemical and a genetic perspective [7,8,62-64], Here we only discuss the hnal steps in MPT biosynthesis, which involve the enzyme molybdopterin synthase [65,66],... 

MPT, molybdopterin mononucleotide MPTpG, molybdopterin guanine dinucleotide MPTpC, molybdopterin cytosine dinucleotide. b Assignment based on the abstraction of S by CN. ... 

Abbreviations used in this table MPT designates molybdopterin MGD designates MPT guanine dinucleotide MCD designates MPT cytosine dinucleotide N-His, O-Ser, S-Cys, Se-Cys designate attachments from protein residues histidine, serine, cysteine and selenocysteine. b Flavin adenine dinucleotide — FAD. c Dimethyl sulfoxide — DMSO. 

Molybdopterin guanine dinucleotide (MGD) Flavin adenine dinucleotide (FAD)... 

Molybdopterin has another function besides participating in electron transfer between the site of catalysis and other electron-acceptor groups. It serves as an anchor for the active site where a multitude of hydrogen bonds between the pterin (and, if present, the dinucleotide) and the protein provide a secure tether for the reactive metal site (17). Evidence for the immobility conferred by the pterin(s) embedded in the protein is found in a comparsion of the DMSOR structures from both Rhodobacter sources. Regardless of the Mo coordination environment, the MGD ligands are nearly superimposable (75). This similarity of pterin structure is most clearly observed in the 1.3-A structure, where the Mo atom dissociated and shifted away from one pterin ligand, which otherwise was unaffected. The nucleotide tails on MGD, MCD, and other derivatives of molybdopterin also contribute to locking the molybdenum catalyst in position. 

MCD molybdopterin cytosine dinucleotide or magnetic circular dichroism... 

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