Transmembrane movements of artificial redox mediators in relation to electron transport and ionic currents in chloroplasts

Electrochemical data obtained with TMPD⁺-sensitive electrodes indicate that ammonium-uncoupled chloroplasts retain TMPD (N,N,N',N'-tetramethyl- p -phenylenediamine) mainly in the reduced form during illumination, whereas uncoupled DCMU-treated chloroplasts accumulate TMPD in the oxidized form (TMPD⁺). This observation indicates that the reduced plastoquinol is the preferred electron donor for photosystem I (PSI) and TMPD can only compete efficiently when plastoquinone reduction is blocked. After adding DCMU the formation of a transmembrane gradient for TMPD⁺ is reflected by a slow-down of the electrogenic electron transport and by the emerging of the overshoot of the membrane current in the light-off response. A light-dependent increase in photoelectric current generated by chloroplasts in the presence of NH₄Cl and TMPD is observed and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.     

Role of mediators in electron transport from glucose oxidase redox centre to electrode surface in a covalently coupled enzyme paste electrode

We have studied the glucose oxidase immobilized carbon paste electrodes in the presence and absence of small mediator molecules. We have used p-benzoquinone and riboflavin as mediators in our studies. The effect of mediator molecules on the electron transfer between the enzyme redox centre and the electrode surface was explained from the cyclic voltammograms and rotating disk electrode data. In the absence of oxygen, we have noted that the mediators play a central role in the electron transfer. We have also proposed a possible mechanism for the electron transfer from enzyme active site to the electrode surface via mediators, based on our observations. Dedicated to the memory of Professor J Das     

Functional and topological aspects of pH-dependent regulation of electron and proton transport in chloroplasts in silico

In this work, we summarize results of computer simulation of electron and proton transport processes coupled to ATP synthesis in chloroplasts performed within the frames of a mathematical model developed as a system of differential equations for concentrations of electron carriers and hydrogen ion inside and outside the granal and stromal thylakoids. The model takes into account topological peculiarities and lateral heterogeneity of the chloroplast lamellar system. This allowed us to analyze the influence of restricted diffusion of protons inside small compartments of a chloroplast (e.g., in the narrow inter-thylakoid gap) on electron transport processes. The model adequately describes two modes of pH-dependent feedback control of electron transport associated with: (i) the acidification of the thylakoid lumen, which causes the slowing down of plastoquinol oxidation and stimulates an increase in dissipation of excess energy in PS2, and (ii) the alkalization of stroma, inducing the activation of the BBC (Bassham-Benson-Calvin) cycle and intensified consumption of ATP and NADPH. The influence of ATP on electron transport is mediated by modulation of the thylakoid membrane conductivity to protons through the ATP synthase complexes. We also analyze the contribution of alternative electron transport pathways to the maintenance of optimal balance between the energy donating and energy consuming stages of the light-induced photosynthetic processes.