Nitrate reductase structural studies

NiFe hydrogenase, see NiFe hydrogenase nitrate reductase, 47 3, 5,13-14, 396,403-406, 472, 475 NMR studies, 4na5 -Tll electron relaxation times, 47 252-257 polypeptide folding, 47 271-276 reduction potential, 47 265-266 solution structure, 47 266-271 valence delocalization, XJOSl, 259, 261-265... 

Study by X-ray absorption spectroscopy of the extended X-ray absorption fine structure (EXAFS) has provided estimates of both the nature and the number of the nearest neighboring atoms around the Mo. The EXAFS spectra of xanthine dehydrogenase and of nitrate reductase from Chlorella confirmed the... 

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

Lu, G., Lindqvist, Y., Schneider, G., Dwivedi, U., and Campbell, W. H., 1995, Structural studies on com nitrate reductase refined structure of the cytochrome b reductase fragment at 2.5 A, its ADP complex and an active-site mutant and modeling of the cytochrome b domain, J. Mol. Biol. 248 9319948. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

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 ... 

However, some of these structures show unusual features and a crowded active site. EXAFS studies of Rhodobacter capsulatus DM SO reductase show the expected four Mo-S ligands and one Mo-O bond arising from a serine residue plus one Mo=0 in the reduced Mo (IV) state of the active site. This oxo ligand is removed upon oxidation of the metal ion to Mo(VI). In the oxidized structme, an aquo ligand is postulated to coordinate the molybdenum ion (Fig. 11.17c) [126-128]. An equivalent restdt has been reported for BSO that catalyzes the reduction of D-biotin D-sulfoxide to D-biotin [129]. A similar overall catalytic mechanism is expected for nitrate reductases (Fig. 11.18), which catalyze the following reaction ... 

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