J. Biol. Chem., Vol. 256, Issue 22, 11471-11476, Nov, 1981
Light-induced changes in H+ binding to the purple membrane. Effect of pH, light, temperature, and ionic strength
R Renthal
Under continuous illumination, isolated planar sheets of purple membrane
from Halobacterium halobium acidify the surroundings at alkaline pH. This
light-induced change in H+ binding to the purple membrane (delta h) was
studied by differential titration under varying conditions of pH,
temperature, ionic strength, salt composition, light intensity, and
wavelength. A maximum acidification was found between pH 9 and 10, with
delta h less at neutral or more alkaline pH, consistent with a previously
proposed three-state model. The light intensity and wavelength dependence
also support this model. The temperature dependence of delta h, interpreted
in terms of the three-state model, is anomalous. The apparent enthalpy of
proton dissociation (delta H0) is -6 kcal/mol, a value of opposite sign to
the expected delta H0 for a group of pK = 10. The apparent activation
energy (Ea) for proton uptake is 14 kcal/mol in 15 mM NaCl and 18 kcal/mol
in 3 M KCl, 5 to 10 times too large for a diffusion-limited proton transfer
reaction from water. However, both delta H0 and Ea are consistent with
conformational changes linked to light-independent proton dissociation and
pump- dependent proton uptake. An increase in ionic strength increases
delta h. This effect is shown to be quantitatively explained by a high
negative electrostatic surface potential, which accumulates protons in a
diffuse electrical double layer.
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