Misra S, Ebrey TG, Crouch RK, Menick DR

Charge movements in the 13-cis photocycles of the bacteriorhodopsin mutants R82K and R82Q AB - We have examined light-induced currents in oriented membranes of the bacteriorhodopsin mutants R82K and R82Q. Our results suggest that two photocurrent components found in R82K, with 30 and 300 microsecond lifetimes, are due to the photocycle of the 13-cis rather than the all- trans form of the pigment. We investigated the pH dependence of these components and their correspondence to absorbance changes at 660 nm characteristic of photointermediates of the 13-cis cycle. The presence of a D2O effect suggests that the charge motions producing these photocurrents are related to proton or protonated amino acid movement within the molecule. The current amplitudes depend on the protonation states of at least two residues, D85 and (probably) E204. In R82Q, a 10 microsecond photocurrent is observed that also depends on the protonation state of D85 and is similar to the 30 microseconds current in R82K. We attempt to explain these currents in terms of a model for interacting residues in the extracellular half of the bacteriorhodopsin channel.

Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, USA

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