Molybdenum Moco 222

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

Fig. 5.10. The formula of one of the mononuclear molybdenum cofactors, Moco. Others have a nucleotide phosphate extension (see references to these elements in Further Reading). In sulfide-rich environments, tungsten replaced molybdenum. In some coenzymes, two pterins are bound to the metal ions.

Molybdenum co-factor (Moco), 77 33 Molybdenum complexes, 26 927-929, 949 Molybdenum(III) complexes, 17 26-27 Molybdenum compounds, 17 19-43 in advanced structural and heating materials, 17 38-39 in anticorrosion agents, 17 39 biological aspects of, 17 31-34 biological uses for, 17 39-40 biomedical uses for, 17 40 catalytic applications of, 17 38 chemistry of, 17 29-31... 

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. 

The molybdenum cofactor (Moco) is the essential component of a group of redox enzymes [20-22], which are diverse not only in terms of their phylogenetic distri-... 

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

The molybdenum cofactor is often assumed to be involved in dimerisa-tion of NR, either as a bridge as in an early structural model (Pan Nason, 1978), or more indirectly. This hypothesis is no longer valid, since it has been shown that in bakers yeast, an organism which lacks the MoCo biosynthetic pathway, a transgenic, MoCo-less tobacco NR is nevertheless dimeric (Truong et al., 1991). This is in agreement with the observa-... 

CoMo-124 Alumina was impregnated first with cobalt stepwise. The sample was dried at 120°C and calcined at 650 °C after each impregnation step. Afterwards the catalyst was impregnated with molybdenum. Final calcination temperature 650°C. The composition was the same as for MoCo-124. 

MoCo-153 Alumina was impregnated first with molybdenum, dried and calcined at 650°C. Then the cobalt was brought hereupon. Two final calcination temperatures were applied, 480 and 650°C. The composition was 15 wt% M0O3 and 3 wt% CoO. 

However, the molybdenum-alumina and the high calcined cobalt-molybdenum-alumina samples still show an important difference. The pyridine spectra of MoCo-124 indicate a second Lewis acid site, characterized by the 1612 cm-1 band. This band differs from the weak Lewis acid sites of the alumina support (1614 cm- ) because the position is significantly different. It also appears that the strength of the bond between pyridine and the catalyst is stronger, for the 1612 cm-1 band is still present after evacuation at 250°C, while the weak Lewis band (1614 cm-1) of the alumina has disappeared at this desorption temperature. Obviously the second Lewis band for the MoCo-124 catalyst is introduced by the interaction of cobalt with the surface molybdate layer. This interaction is... 

Of the complexes studied, those with ancillary ligands that contain sulfur atoms have received the most attention. Among the most studied are the bis(dial-kyldithiocarbamate) molybdenum oxido complexes [196,197], Unfortunately, although these complexes react with certain enzyme substrates, their common problem as models for Moco active sites involves the propensity of the Movl dioxo and MoIV monoxo complexes to form a comproportionated Mov p-oxido dimer (Eq. 13 dtc = diethyldithiocarbamate). 

Synthetic and structural aspects about the extremely labile molybdenum cofactor (Moco) have been summarized <93H(35)l55l> and indicate fundamental difficulties in building up the molybdenum enedithiole complex. 

The nomenclature used by scientists working in the mononuclear molybdenum enzyme field has been a cause of confusion and contention (1, 59). For many years, the molybdenum atom and its ligands at the catalytic site has been referred to as the molybdenum cofactor and Moco or Mo-co (1, 60, 61). 

The idea that the same cofactor species operated in all Mo enzymes originated from a reconstitution assay. In this assay method, the isolated Moco from one enzyme, such as XO, is inserted into a cofactor-free mutant (Nit-1) of nitrate reductase from Neuraspora crassa, where it can reactivate or reconstitute normal nitrate reductase catalytic activity. It is now recognized that the Mo at the active site has many different coordination environments, as has been illustrated for the three Mo families in Fig. 1. In this context, the mutant nitrate reductase assay experiment is interpreted as involving some reprocessing of the inserted molybdenum cofactor from foreign enzymes to obtain the correct form of the cofactor for nitrate reductase catalysis. The Moco designation, if it is to be used, must refer to the family of sites present in Moco enzymes. 

The term molybdenum cofactor (or Moco) refers to the metal center and its inner coordination sphere. Moco is not a single, unique, moiety, rather it is a diverse collection of protein-bound sites that have certain common features. Thus, one or two MPTs are coordinated to the metal via the dithiolene group and the remainder of the metal s coordination sphere is taken up by non-protein ligands (e.g., oxo, hydroxo, water, or sulfido groups) and, in some cases, an amino acid side chain is coordinated. Hille (1) has shown that the mononuclear Mo—MPT enzymes of molybdenum can be classified into three families, on the basis of the nature of the inner coordination sphere of the oxidized form of the enzyme (Fig. 2). 

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