Publications

Charge exchange is the fundamental process that sustains cellular respiration and photosynthesis by shuttling electrons in a cascade of electron transfer (ET) steps between redox cofactors. While intraprotein charge exchange is well characterized in protein complexes bearing multiple redox sites, interprotein processes are less understood due to the lack of suitable experimental approaches and the dynamic nature of the interactions. Proteins constrained between electrodes are known to support electron transport (ETp) through the protein matrix even without redox cofactors, as the charges housed by the redox sites in ET are furnished by the electrodes. However, it is unknown whether protein ETp mechanisms apply to the interprotein medium present under physiological conditions. We study interprotein charge exchange between plant photosystem I (PSI) and its soluble redox partner plastocyanin (Pc) and address the role of the Pc copper center. Using electrochemical scanning tunneling spectroscopy (ECSTS) current-distance and blinking measurements, we quantify the spatial span of charge exchange between individual Pc/PSI pairs and ETp through transient Pc/PSI complexes. Pc devoid of the redox center (Pcapo) can exchange charge with PSI at longer distances than with the copper ion (Pcholo). Conductance bursts associated with Pcapo/PSI complex formation are higher than in Pcholo/PSI. Thus, copper ions are not required for long-distance Pc/PSI ETp but regulate its spatial span and conductance. Our results suggest that the redox center that carries the charge in Pc is not necessary to exchange it in interprotein ET through the aqueous solution and question the canonical view of tight complex binding between redox protein partners.

JTD Keywords: azurin, binding, blinking, crystal-structure, cupredoxin, current distance spectroscopy, electrochemical tunneling microscopy, proteinconductance, reduction, single metalloprotein, single molecule measurements, site, spectroscopy, Blinking, Cupredoxin, Current distance spectroscopy, Electrochemical tunneling microscopy, Interprotein electron transfer, Protein conductance, Single molecule measurements, State electron-transport

Photosynthesis is a fundamental process that converts photons into chemical energy, driven by large protein complexes at the thylakoid membranes of plants, cyanobacteria, and algae. In plants, water-soluble plastocyanin (Pc) is responsible for shuttling electrons between cytochrome b6f complex and the photosystem I (PSI) complex in the photosynthetic electron transport chain (PETC). For an efficient turnover, a transient complex must form between PSI and Pc in the PETC, which implies a balance between specificity and binding strength. Here, we studied the binding frequency and the unbinding force between suitably oriented plant PSI and Pc under redox control using single molecule force spectroscopy (SMFS). The binding frequency (observation of binding-unbinding events) between PSI and Pc depends on their respective redox states. The interaction between PSI and Pc is independent of the redox state of PSI when Pc is reduced, and it is disfavored in the dark (reduced P700) when Pc is oxidized. The frequency of interaction between PSI and Pc is higher when at least one of the partners is in a redox state ready for electron transfer (ET), and the post-ET situation (PSIRed-PcOx) leads to lower binding. In addition, we show that the binding of ET-ready PcRed to PSI can be regulated externally by Mg2+ ions in solution.

JTD Keywords: architecture, binding-site, complexes, ferredoxin, force spectroscopy, induced structural-changes, interprotein electron transfer, light-dependent interaction, mg2+ concentration, photosystem i, plastocyanin, probe, recognition, reduction, Force spectroscopy, Interprotein electron transfer, Light-dependent interaction, Photosynthetic reaction-center, Photosystem i, Plastocyanin, Single molecule measurements

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current-distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current-distance spectroscopy, it is observed that the distance-decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance-decay constant ?) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.© 2021 Wiley-VCH GmbH.

JTD Keywords: azurin, current distance decay spectroscopy, cytochrome c(6), electrochemical scanning tunneling microscopy (ecstm), electrochemistry, photosystem i, photosystem-i, plastocyanin, protein electron transfer, recognition, single metalloprotein, single molecules, structural basis, tunneling spectroscopy, 'current, Amino acids, Charge transfer, Chlorine compounds, Current distance decay spectroscopy, Decay spectroscopies, Distance decay, Electrochemical scanning tunneling microscopy, Electrochemical scanning tunneling microscopy (ecstm), Electrodes, Electron transfer, Electron transport properties, Gold compounds, Photosystem i, Photosystems, Protein electron transfer, Protein electron-transfer, Proteins, Scanning tunneling microscopy, Silver halides, Single molecule, Single molecules