Effect of pH buffer molecules on the light-induced currents from oriented purple membrane

The effect of pH buffers on the microsecond photocurrent component, B2, of oriented purple membranes has been studied. We found that under low salt conditions (less than 10 mM monovalent cationic salt) pH buffers can dramatically alter the waveform of the B2 component. The effect is induced by the p...

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Bibliographic Details
Published in:Biophysical journal 1991-07, Vol.60 (1), p.204-216
Main Authors: Liu, S.Y., Kono, M., Ebrey, T.G.
Format: Article
Language:English
Subjects:
bacteriorhodopsin
Bacteriorhodopsins - physiology
Bacteriorhodopsins - radiation effects
Biological and medical sciences
Buffers
Cell physiology
effects on
Fundamental and applied biological sciences. Psychology
Halobacterium
Halobacterium - metabolism
Hydrogen-Ion Concentration
Kinetics
Light
Molecular and cellular biology
purple membranes
Structure-Activity Relationship
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Summary:The effect of pH buffers on the microsecond photocurrent component, B2, of oriented purple membranes has been studied. We found that under low salt conditions (less than 10 mM monovalent cationic salt) pH buffers can dramatically alter the waveform of the B2 component. The effect is induced by the protonation process of the buffer molecules by protons expelled from the membrane. These effects can be classified according to the charge transition upon protonation of the buffer. Buffers that carry two positive charges in their protonated form add a negative current component (N component) to B2. Almost all of the other buffers add a positive current component (P component) to B2, which is essentially a mirror image of the N component. Buffers with a pK less than 5.5 have only a small positive buffer component. The pH dependence of the buffer effect is closely related to the pK of the buffer; it requires that the buffer be in its unprotonated form. The rise time of the buffer component increases with the concentration of the buffer molecules. All the buffer effects can be inhibited by the addition of 5 mM of a divalent cation such as Ca2+. Reducing the surface potential slows down the N component but accelerates the P component without affecting the amplitude of the buffer effect significantly. Many of the buffer effects can be explained if we assume that upon protonation of the buffer by a proton expelled from the membrane by light, the buffer molecules move toward the membrane. This backward movement of buffer molecules forms a counter current very similar to that due to cations discussed in Liu, S. Y., R. Govindjee, and T. G. Ebrey. (1990. Biophys. J. 57:951–963).
ISSN:0006-3495
1542-0086
DOI:10.1016/S0006-3495(91)82044-6