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.

A needle in a haystack: The active site of the membrane‐bound complex cytochrome c nitrite reductase

“…In the presence of substrate, the catalytic process results in a steady-state electron flow whose detection allows instantaneous measurement of turnover rates. Catalytic electrochemical signals have been reported independently for the ccNiRs from D. desulfuricans [13] and E. coli [14,15]. Here we shall report on the former enzyme.…”

Direct Electrochemistry of Redox Enzymes as a Tool for Mechanistic Studies

“…230 Cytochrome c nitrite reductases also exhibit a shutdown of activity at low electrode potential, 149,150,189,346 and again, this was observed with enzymes from different organisms and using electrode surfaces as different as graphite and single-crystal gold. …”

The relationship between redox enzyme activity and electrochemical potential—cellular and mechanistic implications from protein film electrochemistry

“…Certainly, activity from the E. coli penta-heme nitrite reductase, NrfA, a MQH 2 -dependent system is only detected below ca. À0.07 V. 41,42 In this light it is striking that potentials below ca. À0.30 V are required to see any appreciable activity from the assimilatory nitrate reductase (Nas) of Synechococcus elongatus.…”