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The periplasmic cytochrome c nitrite reductase (Nrf) system of Escherichia coli utilizes nitrite as a respiratory electron acceptor by reducing it to ammonium. Nitric oxide (NO) is a proposed intermediate in this six-electron reduction and NrfA can use exogenous NO as a substrate. This chapter describes the method used to assay Nrf-catalyzed NO reduction in whole cells of E. coli and the procedures for preparing highly purified NrfA suitable for use in kinetic, spectroscopic, voltammetric, and crystallization studies.

The recent crystallographic characterization of NrfAs from Sulfurospirillum deleyianum, Wolinella succinogenes, Escherichia coli and Desulfovibrio desulfuricans allows structurally conserved regions to be identified. Comparison of nitrite and sulphite reductase activities from different bacteria shows that the relative activities vary according to organism. By comparison of both amino acid sequences and structures, differences can be identified in the monomer-monomer interface and the active-site channel; these differences could be responsible for the observed variance in substrate activity and indicate that subtle changes in the NrfA structure may optimize the enzyme for different roles.

Escherichia coli cytochrome c nitrite reductase is one of a large family of homologous enzymes that are particularly prevalent in pathogenic enterobacteria. The enzymes are periplasmic and in vivo may find themselves challenged by molecules that could enhance or compromise their performance. In the present study, we describe protein film voltammetry in which the activity of E. coli cytochrome c nitrite reductase is challenged by the presence of a number of small molecules. These results are discussed in light of the environment(s) that the enzyme may face before and after colonization of a human host.

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.