Protein film voltammetry and spectroelectrochemistry of the electron acceptor site in Photosystem II

Abstract

The key reaction of oxygenic photosynthesis, the light-driven oxidation of water, is carried out by Photosystem II (PSII), a light-driven water-plastoquinone oxidoreductase. Photosystem II is a multi-cofactor protein and its energetic characterization is quite a difficult undertaking. Electrochemical approaches play an important role as a tool for completing the energetic picture and for carrying out fundamental studies of the enzyme. In this work, two electrochemical methods have been applied to study the electron acceptor site of the Photosystem II: spectroelectrochemistry and protein film voltammetry. The first part focuses on the re-measurement of the midpoint potential of the primary quinone electron acceptor, QA, in PSII core complexes isolated from Thermosynechococcus elongatus and in PSII enriched membranes from spinach using an optical transparent thin layer (OTTLE) cell. The obtained results show that the bicarbonate anion, ligated to the non-heme iron at the electron acceptor site in close proximity to QA, plays a significant role in controlling the redox properties of the QA/QA--couple. This finding explains various controversies about existing literature values of the QA/QA-midpoint potential. The second part describes investigations of photocurrents generated by Photosystem II in metal oxide hybrid systems. PSII isolated from T. elongatus was immobilized onto nanostructured titanium dioxide/indium tin oxide electrodes (TiO2/ITO) and the origin of photocurrent upon illumination was studied. Using conditions in which PSII was immobilized as a monolayer, it was demonstrated that direct electron transfer occurs from the redox cofactor QA to the electrode surface, but that the electron transfer through the metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the TiO2 surface to the ITO and not from PSII. Furthermore, the origin of cathodic photocurrents (i.e. electron flow from the electrode) was investigated. The results indicate that a one-electron reduction of oxygen to the superoxide anion radical (O2•-) occurs at the ITO surface in darkness when an external bias lower than +300 mV vs NHE was applied. The findings can explain the light-driven and catalytic nature of this current by the fact that the reduction of O2•- occurs at the non-heme iron, which is driven by the photo-reduction of QA.