Electrostatic coupling between retinal isomerization and the ionization state of Glu-204: a general mechanism for proton release in bacteriorhodopsin.
AUTOR(ES)
Sampogna, R V
RESUMO
The pKa values of ionizable groups that lie between the active site region of bacteriorhodopsin (bR) and the extracellular surface of the protein are reported. Glu-204 is found to have an elevated pKa in the resting state of bR, suggesting that it corresponds to the proton-releasing group in bR. Its elevated pKa is predicted to be due in part to strong repulsive interactions with Glu-9. Following trans-cis isomerization of the retinal chromophore and the transfer of a proton to Asp-85, polar groups on the protein are able to interact more strongly with the ionized state of Glu-204, leading to a substantial reduction of its pKa. This suggests a general mechanism for proton release in which isomerization and subsequent charge separation initially produce a new electrostatic balance in the active site of bR. Here it is proposed that those events in turn drives a conformational change in the protein in which the ionized state of Glu-204 can be stabilized through interactions with groups that were previously inaccessible. Whether these groups should be identified with polar moieties in the protein, bound waters, or Arg-82 is an important mechanistic question whose elucidation will require further study.
ACESSO AO ARTIGO
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1233583Documentos Relacionados
- Molecular mechanism of protein-retinal coupling in bacteriorhodopsin.
- Decoupling of photo- and proton cycle in the Asp85-->Glu mutant of bacteriorhodopsin.
- Mechanism of light-dependent proton translocation by bacteriorhodopsin.
- Arginine-82 regulates the pKa of the group responsible for the light-driven proton release in bacteriorhodopsin.
- Absolute quantum yields and proof of proton and nonproton transient release and uptake in photoexcited bacteriorhodopsin.