Photoelectric signals of membrane proteins
If we flash on the suspension of oriented purple membranes with a short laser flash, the molecules start to work in a
synchronised manner, and since it is an oriented system all charges move in the same direction, a macroscopic charge
displacement current can be measured. And the sample is large, it fills a cuvette, absorption kinetic measurements
can easily be performed simultaneously with the electric measurements. The photocycle has been described and defined
by absorption kinetic experiments (the intermediates in the photocycle have different colors), and now the steps in
the photocycle can be correlated with the steps of charge motion.
L. Keszthelyi, P. Ormos, Electric signals associated with the photocycle of bacteriorhodopsin,
FEBS Lett. 109 189 (1980)
The orientation of the purple membranes can be fixed in a gel: this is a most practical system where optical and
electrical kinetic experiments can be performed in great detail.
A. Der, P. Hargittai, J. Simon, Time resolved photoelectric and absorption signals from oriented purple membranes
immobilized in gel, J. Biochem. Biophys. Methods 10 295 (1985)
On this system enormous amount of information was gained about the details of proton pumping
(rates, thermodynamic parameters, study of different site directed mutants, etc.).
It has been shown that, in addition to correlate charge motions with the photocycle reactions in
bacteriorhodopsin, the method can be applied also to general problems, like the effect of water, electric field,
and different ions and buffers on the function of photoexcitable membrane proteins.
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A. Der,R. Toth-Boconadi, Z. Tokaji, L. Keszthelyi, and W. Stoeckenius,
Alternative translocation of proptons and halide ions by bacteriorhodopsin, PNAS 88 4751-4755 (1991)
G. Varo, Analogies between halorhodopsin and bacteriorhodopsin BBA Bioenergetics 1460 220-229 (2000)
A. Der, and L. Keszthelyi, Charge motion during the photocycle of bacteriorhodopsin (Review), Biochemistry Moscow
66 1234-1248 (2001) View PDF