Freely diffusing versus adsorbed protein: Which better mimics the cellular state of a redox protein?

Dynamic electrochemistry of adsorbed proteins, often termed protein film electrochemistry (PFE), is widely used for the characterisation of redox proteins. The method provides a powerful alternative to spectroscopic studies that interrogate protein solutions. The reduction potential and electron stoichiometry of redox couples can be defined. The rates of catalytic redox transformations can also be quantified. Often it is considered that the behaviour of the adsorbed protein should match that displayed in solution studies if it is to be relevant to understanding the biological role of the protein. However, the environment of the protein in PFE is fundamentally different from that when it is freely diffusing in solution. As a consequence different behaviours may be expected. This raises the question, which approach is more relevant when aiming to provide insight into the cellular role of the protein? We consider this question here taking as an example the properties of a penta-heme cytochrome NrfB from Escherichia coli. The redox properties of NrfB containing solutions were presented previously (Clarke et al., Journal of Biological Chemistry (2004)). Here we present PFE of NrfB adsorbed on graphite and optically transparent mesoporous, nanocrystalline SnO2 electrodes.