Electrochromic absorption changes of a chloroplast suspension induced by an external electric field

Torque generation by the Fo motor of the sodium ATPase

Based on recent structural and functional findings, we have constructed a mathematical model for the sodium-driven Fo motor of the F1Fo-ATPase from the anaerobic bacterium Propionigenium modestum. The model reveals the mechanochemical principles underlying the Fo motor's operation, and explains all of the existing experimental data on wild-type and mutant Fo motors. In particular, the model predicts a nonmonotonic dependence of the ATP hydrolysis activity on the sodium concentration, a prediction confirmed by new experiments. To explain experimental observations, the positively charged stator residue (R227) must assume different positions in the ATP synthesis and hydrolysis directions. This work also illustrates how to extract a motor mechanism from dynamical experimental observations in the absence of complete structural information.

The proton-driven rotor of ATP synthase: ohmic conductance (10 fS), and absence of voltage gating

The membrane portion of F(0)F(1)-ATP synthase, F(0), translocates protons by a rotary mechanism. Proton conduction by F(0) was studied in chromatophores of the photosynthetic bacterium Rhodobacter capsulatus. The discharge of a light-induced voltage jump was monitored by electrochromic absorption transients to yield the unitary conductance of F(0). The current-voltage relationship of F(0) was linear from 7 to 70 mV. The current was extremely proton-specific (>10(7)) and varied only slightly ( approximately threefold) from pH 6 to 10. The maximum conductance was approximately 10 fS at pH 8, equivalent to 6240 H(+) s(-1) at 100-mV driving force, which is an order-of-magnitude greater than of coupled F(0)F(1). There was no voltage-gating of F(0) even at low voltage, and proton translocation could be driven by deltapH alone, without voltage. The reported voltage gating in F(0)F(1) is thus attributable to the interaction of F(0) with F(1) but not to F(0) proper. We simulated proton conduction by a minimal rotary model including the rotating c-ring and two relay groups mediating proton exchange between the ring and the respective membrane surface. The data fit attributed pK values of approximately 6 and approximately 10 to these relays, and placed them close to the membrane/electrolyte interface.

Functional mechanism of water splitting photosynthesis

A personal account is given on physico-chemical aspects of photosynthesis. The article starts with the way I entered the field of photosynthesis. Then, selected results from our research group are discussed. Three methods used for functional analysis in our laboratory are described: the repetitive flash spectroscopy; the electrochromic volt- and ammeter; and the membrane energization by a battery. Our subsequent studies deal with the two photoreaction centers, the primary charge separation, the plastoquinones as a transmembrane link between the two centers and the vectorial electron- and proton pathways. The results led to a picture of the elementary functional mechanism of the molecular machinery in the thylakoid membrane. The perspective then focuses on the coupling between the electric field, protons and phosphorylation. This section is followed by our observations and analysis of the mechanism of water cleavage and its coupling with the functioning of reaction center II. Finally, information is provided on structural aspects of the two reaction centers. The article ends with a retrospect.

Relation between the initial kinetics of ATP synthesis and of conformational changes in the chloroplast ATPase studied by external field pulses

ATP formation and the energy-dependent release of tightly bound [14C]-adenine nucleotides from the chloroplast coupling factor CF1 has been studied as a function of the time of energization of the membrane in the range of 500 mus up to 60 ms. The high time resolution was achieved because the energization was generated artificially by external electric field pulses. Applying external electric field pulses to a chloroplast suspension induces an electric potential difference across the thylakoid membrane. The following results were obtained: (1) The amount of ATP generated increases linearly with the time of energization. The steady-state rate of ATP formation is reached in less than 500 mus. (2) A fraction of the adenine nucleotides tightly bound to CF1 is released on energization with a half-rise-time of about 2 ms. The size of the fraction, i.e., the amplitude of the fast phase of the release, increases with the magnitude of the induced transmembrane electric potential difference. A further slow release is superimposed. (3) The initial rate of the release of adenine nucleotides is practically identical with the rate of ATP formation. It is concluded that the release of tightly bound nucleotides monitors an initial conformational change by which the ATPase turns from an inactive into an activated state. For the explanation of the results a reaction scheme is proposed which takes into account a preceding activation of the ATPase.