Molybdoprotein

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... 

The cofactor appears to include a novel pterin.996-998 The properties of the pterin depend upon the nature of the side-chain in the 6-position. The structure shown in Figure 39 has been proposed997 on the basis that molybdopterin is related to urothione, oxidized to pterin-6-carboxylic acid, and contains in the side-chain two sulfur groups, a double bond, a hydroxyl function and a terminal phosphate group. Two stable fluorescent derivatives of molybdopterin have been characterized,999 which may be of value in view of the extreme instability of the native molybdoprotein when released from the enzyme. 

It is obvious that x-ray cyrstallographic methods will be the final arbiter of the structural features of molybdoproteins, but until such structures are obtained, and even afterwards as far as dynamic features are concerned, spectroscopic methods must be used to gain insight into the nature of these catalysts. Electronic spectroscopy so far has been of little use here since molybdenum complexes in general appear to exhibit broad weak absorptions. In proteins these are always buried under absorptions from hemes, flavins, and iron-sulfur centers. Massey et al., (15) discovered that pyrazolo [3,4-d] pyrimidines will bind Mo (IV) in milk xanthine oxidase that had been reduced with xanthine... 

NapB. NapB is a subunit of the heterodimeric periplasmic nitrate reductase (NapAB) and transfers electrons to the catalytic NapA molybdoprotein. Nap systems are found in a number of bacteria, iucluding enterobacteria, aerobic denitrifiers, and nonsulfur purple photo synthetic bacteria. Their physiological function is different in these groups of bacteria and includes redox balancing using nitrate as an electron sink to dispose of excess rednctant, aerobic denitrification, and nitrate scavenging in nitrate-limited environments. 

The known redox roles of tetrahydropterins in biochemistry led Rajagopalan to investigate the redox behavior of the pterin unit of Moco. They titrated Moco within molybdoproteins (XO and SO) with two different oxidants, ferrocyanide and the redox dye dichlorophenol indophenol (DCIP), and obtained unexpected results two electron equivalents of either oxidant produced the spectral signature of a fully oxidized pterin (Scheme 2.1), a result only consistent with the pterin in Moco starting at the dihydro oxidation state rather than the tetrahydro state as initially proposed. The interpretation at the time was that the pterin in Moco, instead of the initially proposed tetrahydropterin structure, was a dihydropterin in an unusual tautomeric form. 

By now it is well established that the assimilatory reduction of nitrate to ammonia proceeds in two separate and well-defined steps (1) The reduction of nitrate to nitrite, catalyzed by the molybdoprotein nitrate reductase, and (2) the reduction of nitrite to ammonia, catalyzed by the heme-protein nitrite reductase ... 

C. Molybdoproteins Xanthine oxidase 2Mo, 8Fe-8S Mammals (milk, liver) 275... 

Keywords Flavoprotein Molybdoprotein Quinoportein Quinate Respiratory chain Shikimate 4-Keto-D-aldopentonate o-Aldopentonate... 

We take a quick look on the series of membrane-bound dehydrogenase of acetic acid bacteria. We try to categorize them into five groups, that is, quinoprotein-cytochrome complex, molybdoprotein-cytochrome complex, flavoprotein-cytochrome complex, membrane-bound quinoprotein, and others, in terms of their primary structure. 

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