Nitrogen oxidases

Unfortunately, many enzymes which fall clearly within the scope of oxidation-reduction catalysis will have to be neglected because of limitations of space and the inadequate amount of information available about them. In this group are the hydrogenases, the sulfur oxidases, the nitrogen oxidases, and a host of related enzymes found in autotrophic bacteria, as well as in plant and animal organisms. 

Many key protein ET processes have become accessible to theoretical analysis recently because of high-resolution x-ray stmctural data. These proteins include the bacterial photosynthetic reaction centre [18], nitrogenase (responsible for nitrogen fixation), and cytochrome c oxidase (the tenninal ET protein in mammals) [19, 20]. Although much is understood about ET in these molecular machines, considerable debate persists about details of the molecular transfonnations. 

In animal metabolism, oxomolybdoenzymes catalyse a number of oxidation processes. These oxidases contain Mo coordinated to terminal O and S atoms, and their action appears to involve loss of an O or S atom along with reduction to Mo or Mo". It is, however, the role of molybdenum in nitrogen fixation which has received most attention. 

Amino Acid Oxidases Also Remove Nitrogen as Ammonia... 

BUN, blood urea nitrogen CBC, complete blood cell count CNS, central nervous system CYP, cytochrome P-450 isoenzyme LFT, liver function test MAO, monoamine oxidase QTc, Q-T interval corrected for heart rate SCr, serum creatinine TMP-SMX, trimethoprim-sulfamethoxazole. 

Several lupin alkaloids have been derived from the unsaturated quinalozidine 433, that was obtained in the treatment of amine 431 with ortho-quinone 432. This quinone behaves as a model of topaquinone, the cofactor of copper-containing amine oxidases. The cyclization step involved a nucleophilic attack of the piperidine nitrogen of 431 onto a side-chain aldehyde function that is unmasked by the oxidative deamination. Quinolizine 433, when treated with dehydropiperidine, gave the oxime ether 434 that, on ozonolysis followed by reduction, afforded sparteine 10, presumably via the bis(iminium) system 435 (Scheme 102) <1996JOC5581>. 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

Iron, Fe2+ (d6) Iron, Fe2+ (d6) 4, tetrahedral 6, octahedral N-Thiolate O-Carboxylate, alkoxide, oxide, phenolate Electron transfer, nitrogen fixation in nitrogenases, electron transfer in oxidases... 

Molybdenum is essential for a number of enzymes, for example xanthine oxidases in mammals and nitrogenases in nitrogen-fixing bacteria. 

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