In Rhodobacter sphaeroides respiratory nitrate reductase, the kinetics of substrate binding favors intramolecular electron transfer.
The respiratory nitrate reductase (NapAB) from Rb. sphaeroides is a periplasmic molybdenum-containing enzyme which belongs to the DMSO reductase family. We report a study of NapAB by protein film voltammetry (PFV), and we present the first quantitative interpretation of the complex redox-state dependence of activity that has also been observed with other related enzymes. The model we use to fit the data assumes that binding of substrate partly limits turnover and is faster and weaker when the Mo ion is in the V oxidation state than when it is fully reduced. We explain how the presence in the catalytic cycle of such slow chemical steps coupled to electron transfer to the active site decreases the driving force required to reduce the MoV ion and makes exergonic the last intramolecular electron-transfer step (between the proximal cubane and the Mo cofactor). Importantly, comparison is made with all Mo enzymes for which PFV data are available, and we emphasize general features of the energetics of the catalytic cycles in enzymes of the DMSO reductase family.
Frangioni
Bettina
B
Laboratoire de Bioénergétique et Ingénierie des Protéines, CNRS, 31 chemin J. Aiguier, 13402 Marseille, France.
Arnoux
Pascal
P
Sabaty
Monique
M
Pignol
David
D
Bertrand
Patrick
P
Guigliarelli
Bruno
B
Léger
Christophe
C
D004563:Electrochemistry / Q000379:methods; D004583:Electrons; D007700:Kinetics; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D012242:Rhodobacter sphaeroides / Q000201:enzymology*
D009565:Nitrate Reductases; D050901:Nitrate Reductase