Effect of temperature and surface potential on the electrogenic proton uptake in the Q(B) site of the Rhodobacter sphaeroides photosynthetic reaction center: Q(A)(-center dot)Q(B)(-center dot)-> Q(A)Q(B)H(2) transitionстатья

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[1] Effect of temperature and surface potential on the electrogenic proton uptake in the q(b) site of the rhodobacter sphaeroides photosynthetic reaction center: Q(a)(-center dot)q(b)(-center dot)-> q(a)q(b)h(2) transition / O. A. Gopta, D. A. Cherepanov, A. Y. Mulkidjanian et al. // Photosynthesis Research. — 1998. — Vol. 55, no. 2-3. — P. 309–316. Direct electrometry was used to study the light-induced voltage changes in the Rhodobacter sphaeroides chromatophores adsorbed to a phospholipid-impregnated nitrocellulose film. After the second laser flash, a fast increase in the voltage associated with charge separation was followed by a slower increase attributed to the proton uptake in the QB site of the photosynthetic reaction centers. Kinetics and relative amplitudes of these voltage changes attributed to the Q(A)(-.)Q(B)(-.) –> Q(A)Q(B)H(2) transition, were measured as a function of pH and temperature between +4 and +40 degrees C. The kinetics can be approximated by a single exponent above +23 degrees C (100 mu s at +25 degrees C, pH 7.2), whereas below this temperature, it was a good fit of two exponential approximation (65 mu s and 360 mu s with similar contributions at +10 degrees C, pH 7.2). The faster component diminished with an apparent pK similar to 8.5, whereas the slower one was maintained at a constant level until pH similar to 9.5 and then decreased. The calculated activation energy from the temperature dependence of the slower component (55 - 65 kJ/mol) was much higher than that of the faster component (< 10 kJ/mol). The two voltage components can be attributed to the transfer of the first (faster component) and the second (slower component) proton from the reaction center surface to QB. We suggested that higher activation energy of the slower component was due to a conformational change in the reaction center kinetically coupled to the second proton transfer to Q(B)H(-). The faster component diminished in the presence of 1 M KCl, with an apparent pK similar to 7.5. To explain this observation, we assume that: (i) the midpoint potential of the Q(A)/Q(A)(-.) redox pair was higher in 1 M KCl because of the reduced surface potential of chromatophores; (ii) the midpoint potential of the Q(B)(-.)/Q(B)H(-) redox pair was insensitive to the surface potential change; (iii) the equilibrium constant of the reaction Q(A)(-.)Q(B)(-.) <-> Q(A)Q(B)H(-) decreased at high ionic strength. [ DOI ]

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