A large photolysis-induced pKa increase of the chromophore counterion in bacteriorhodopsin: implications for ion transport mechanisms of retinal proteins.
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.
Braiman
M S
MS
Biochemistry Department, University of Virginia Health Sciences Center, Charlottesville 22908, USA. msb7e@virginia.edu
Dioumaev
A K
AK
Lewis
J R
JR
D001224:Aspartic Acid / Q000737:chemistry; D001436:Bacteriorhodopsins / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects*; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D004563:Electrochemistry; D005136:Eye Proteins / Q000378:metabolism / Q000528:radiation effects; D019617:Halobacterium salinarum / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D010777:Photochemistry; D010782:Photolysis; D017354:Point Mutation; D017494:Proton Pumps / Q000378:metabolism / Q000528:radiation effects; D012168:Retinal Pigments / Q000378:metabolism / Q000528:radiation effects; D012545:Schiff Bases; D013816:Thermodynamics
D005136:Eye Proteins; D017494:Proton Pumps; D012168:Retinal Pigments; D012545:Schiff Bases; D001224:Aspartic Acid; D001436:Bacteriorhodopsins