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Respiratory and photosynthetic electron transfer chains are dependent on vectorial electron transfer through a series of redox proteins. Examples include electron transfer from NapC to NapAB nitrate reductase in Paracoccus denitrificans and from CymA to Fcc3 (flavocytochrome c3) fumarate reductase in Shewanella oneidensis MR-1. In the present article, we demonstrate that graphite electrodes can serve as surfaces for the stepwise adsorption of NapC and NapAB, and the stepwise adsorption of CymA and Fcc3. Aspects of the catalytic properties of these assemblies are different from those of NapAB and Fcc3 adsorbed in isolation. We propose that this is due to the formation of NapC-NapAB and of CymA-Fcc3 complexes that are capable of supporting vectorial electron transfer.

Shewanella species are isolated from the oxic/anoxic regions of seawater and aquatic sediments where redox conditions fluctuate in time and space. Colonization of these environments is by virtue of flexible respiratory chains, many of which are notable for the ability to reduce extracellular substrates including the Fe(III) and Mn(IV) contained in oxide and phyllosilicate minerals. Shewanella oneidensis MR-1 serves as a model organism to consider the biochemical basis of this flexibility. In the present paper, we summarize the various systems that serve to branch the respiratory chain of S. oneidensis MR-1 in order that electrons from quinol oxidation can be delivered the various terminal electron acceptors able to support aerobic and anaerobic growth. This serves to highlight several unanswered questions relating to the regulation of respiratory electron transport in Shewanella and the central role(s) of the tetrahaem-containing quinol dehydrogenase CymA in that process.

PFV (protein film voltammetry) allows kinetic analysis of redox and coupled-chemical events. However, the voltammograms report on the electron transfer through a flow of electrical current such that simultaneous spectroscopy is required for chemical insights into the species involved. Mesoporous nanocrystalline SnO(2) electrodes provide opportunities for such 'spectroelectrochemical' analyses through their high surface area and optical transparency at visible wavelengths. Here, we illustrate kinetic and mechanistic insights that may be afforded by working with such electrodes through studies of Escherichia coli NrfA, a pentahaem cytochrome with nitrite and nitric oxide reductase activities. In addition, we demonstrate that the ability to characterize electrocatalytically active protein films by MCD (magnetic circular dichroism) spectroscopy is an advance that should ultimately assist our efforts to resolve catalytic intermediates in many redox enzymes.