P. Kolodner et al., ELECTRIC-FIELD-INDUCED SCHIFF-BASE DEPROTONATION IN D85N MUTANT BACTERIORHODOPSIN, Proceedings of the National Academy of Sciences of the United Statesof America, 93(21), 1996, pp. 11618-11621
The application of an external electric field to dry films of Asp-85 -
-> Asn mutant bacteriorhodopsin causes deprotonation of the Schiff bas
e, resulting in a shift of the optical absorption maximum from 600 nm
to 400 nm. This is in marked contrast to the case of wild-type bacteri
orhodopsin films, in which electric fields produce a red-shifted produ
ct whose optical properties are similar to those of the acid-blue form
of the protein. This difference is due to the much weaker binding of
the Schiff-base proton in the mutant protein, as indicated by its low
pK of approximate to 9, as compared with the value pK approximate to 1
3 in the wild type. Other bacteriorhodopsins with lowered Schiff-base
pK values should also exhibit a field-induced shift in the protonation
equilibrium of the Schiff base. We propose mechanisms to account for
these observations.