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. 1996 Feb;70(2):939–947. doi: 10.1016/S0006-3495(96)79637-6

A large photolysis-induced pKa increase of the chromophore counterion in bacteriorhodopsin: implications for ion transport mechanisms of retinal proteins.

M S Braiman 1, A K Dioumaev 1, J R Lewis 1
PMCID: PMC1224994  PMID: 8789111

Abstract

The proton-pumping mechanism of bacteriorhodopsin is dependent on a photolysis-induced transfer of a proton from the retinylidene Schiff base chromophore to the aspartate-85 counterion. Up until now, this transfer was ascribed to a > 7-unit decrease in the pKa of the protonated Schiff base caused by photoisomerization of the retinal. However, a comparably large increase in the pKa of the Asp-85 acceptor also plays a role, as we show here with infrared measurements. Furthermore, the shifted vibrational frequency of the Asp-85 COOH group indicates a transient drop in the effective dielectric constant around Asp-85 to approximately 2 in the M photointermediate. This dielectric decrease would cause a > 40 kJ-mol-1 increase in free energy of the anionic form of Asp-85, fully explaining the observed pK alpha increase. An analogous photolysis-induced destabilization of the Schiff base counterion could initiate anion transport in the related protein, halorhodopsin, in which aspartate-85 is replaced by Cl- and the Schiff base proton is consequently never transferred.

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Selected References

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