Nitrate reductase turnover

Okamoto, P.M., Fu, Y.-H. Marzluf, G.A. (1991). Nit-3, the structural gene of nitrate reductase in Neurospora crassa nucleotide sequence and regulation of mRNA synthesis and turnover. Molecular and General Genetics 227, 213-23. 

Remmler, J.L. Campbell, W.H. (1986). Regulation of com leaf nitrate reductase. II. Synthesis and turnover of the enzyme s activity and protein. Plant Physiology 80, 442-7. 

The sulfite oxidase enzymes are widespread in Nature, and are found in plants, bacteria (the sulfite dehydrogenases) and in birds and mammals. In addition, this family also includes the assimilatory plant nitrate reductases, which have essentially similar molybdenum coordination and differ structurally in lacking an active site arginine that is present in sulfite oxidase, and in showing somewhat different active site structures on turnover. We will focus here on the animal sulfite oxidase enzymes, of which chicken and human are the best studied. In animals the enzyme is responsible for the physiologically essential oxidation of sulfite to sulfate. It is a dimer of 52 kDa subunits and resides in the mitochondrial inner-membrane space. Each monomer contains Mo associated with one molybdopterin, plus a cytochrome heme. The enzymes catalyze the following reaction, which occurs at the Mo site which is reduced from Mo(vi) to Mo(iv) in the process ... 

Finally, and perhaps most importantly, for nitrate reductase and sulfite oxidase, we have to inquire about the number of electrons transferred from the enzyme to the substrate, or vice versa. This brings us to the question of the valence states between which molybdenum in the enzymes cycles in the turnover processes. For both of these enzymes, the overall reaction is a two-electron process, as is the xanthine oxidase reaction. For xanthine oxidase, the evidence (Olson et al, 1974) favors xanthine reducing Mo(VI) directly to... 

Yamanaka and co-workers (364-366) have crystallized a cytochrome oxidase from P. aeruginosa which oxidizes Pseudomonas ferrocytochrome c-551. It is also capable of nitrite reduction with a turnover number of 4000 moles nitrite reduced under anaerobic conditions to nitric oxide per minute at 37°. It is an adaptive enzyme, nitrate being essential for its biosynthesis. The enzyme has a molecular weight of 120,000, with two subunits of equivalent molecular weight, 2 heme c and 2 heme d groups per mole (Fig. 38) (366a). Nitrite reductase activity is 94% inhibited by 8 X 10 M KCN, but only by CO. The lack of CO inhibition appears to be related to the fact that the enzyme has a greater affinity for nitrite than for carbon monoxide. 

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