Spectroelectrochemical Characterization of a Pentaheme Cytochrome in Solution and as Electrocatalytically Active Films on Nanocrystalline Metal-Oxide Electrodes

摘要

Redox enzymes are numerous and ubiquitous. To date the majority of these enzymes have been found to catalyze two-electron transformations of their substrates. However, a pentaheme containing cytochrome, NrfA, from Escherichia coli catalyzes the six-electron reduction of nitrite, in addition to the five-electron reduction of nitric oxide, to achieve transformations that would otherwise require four, or three, distinct enzymes respectively. The search for an understanding of the NrfA reaction mechanism(s) is usefully inspired by several structures. In addition, protein film voltammetry (PFV) has provided insight into rate-limiting events during the NrfA-catalyzed reduction of various substrates. However, unusually for such a well-studied enzyme, the thermodynamic descriptions of its redox activity are incomplete. Here we rectify this situation through simultaneous electrodic manipulation and spectroscopic characterization of NrfA both in solution and as electrocatalytically active films on mesoporous SnO_2 electrodes.