Contribution of proton release to the B2 photocurrent of bacteriorhodopsin

Biophysical Journal
S Misra

Abstract

The contribution of proton release from the so-called proton release group to the microsecond B2 photocurrent from bacteriorhodopsin (bR) oriented in polyacrylamide gels was determined. The fraction of the B2 current due to proton release was resolved by titration of the proton release group in M. At pH values below the pKa of the proton release group in M, the proton release group cannot release its proton during the first half of the bacteriorhodopsin photocycle. At these pH values, the B2 photocurrent is due primarily to translocation of the Schiff base proton to Asp85. The B2 photocurrent was measured in wild-type bR gels at pH 4.5-7.5, in 100 mM KCl/50 mM phosphate. The B2 photocurrent area (proportional to the amount of charge moved) exhibits a pH dependence with a pKa of 6.1. This is suggested to be the pKa of the proton release group in M; the value obtained is in good agreement with previous results obtained by examining photocycle kinetics and pH-sensitive dye signals. In the mutant Glu204Gln, the B2 photocurrent of the mutant membranes was pH independent between pH 4 and 7. Because the proton release group is incapacitated, and early proton release is eliminated in the Glu204Gln mutant, this supports the idea that th...Continue Reading

References

Jan 1, 1975·Preparative Biochemistry·B M Becher, J Y Cassim
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Citations

Aug 31, 1999·Biophysical Journal·J M Sullivan, P Shukla
Sep 14, 2000·Biochimica Et Biophysica Acta·S P Balashov

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