title: Diode or tunnel-diode characteristics? Resolving the catalytic consequences of proton coupled electron transfer in a multi-centered oxidoreductase.
abstract: Protein film voltammetry has been employed to define multiple catalytic consequences of proton coupled electron transfer (PCET) in a cytochrome c nitrite reductase. Current-potential profiles reflecting the steady-state rate of nitrite-limited reduction have been defined from pH 4 to 8. Lowering the electrode potential at pH 8 causes the catalytic current to increase and then decrease before it takes a value independent of any further lowering of electrode potential. By comparison, at pH 4, catalysis is initiated at more positive electrode potentials in an approximately sigmoidal fashion with no attenuation of the catalytic rate evident at more negative electrode potentials. The results show that activity is turned on by the coupled transfer of two electrons and one proton to the enzyme. The decreased rate of catalysis at lower electrode potentials under more alkaline conditions shows that this rate attenuation occurs only when reduction is not coupled to compensating protonation(s) of the enzyme. Sites within the enzyme whose reduction and/or protonation may contribute to the definition of these activities are discussed.
authors.affiliation: Centre for Metalloprotein Spectroscopy and Biology, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, NR4 7TJ, UK.
authors.forename: James D
authors.initials: JD
authors.lastname: Gwyer
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson
authors.forename: Julea N
authors.initials: JN
authors.lastname: Butt
mesh_terms: D002384:Catalysis; D003574:Cytochrome c Group / Q000378:metabolism; D045263:Cytochromes a1 / Q000378:metabolism; D003575:Cytochromes c1 / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004926:Escherichia coli / Q000201:enzymology*; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D009565:Nitrate Reductases / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D011522:Protons
chemical_list: D003574:Cytochrome c Group; D011522:Protons; D045263:Cytochromes a1; D003575:Cytochromes c1; D010088:Oxidoreductases; D009565:Nitrate Reductases; C054690:nitrate reductase (cytochrome)