Characterization of metal ion-binding sites in bacteriorhodopsin.

We have investigated the effects of the binding of various metal ions to cation-free bacteriorhodopsin ("blue membrane"). The following have been measured: shift of the absorption maximum from 603 to 558 nm (blue to purple transition), binding isotherms, the release of H+ upon binding, and the decay of the deprotonated intermediate of the photocycle, M412. We find that all cations of the lanthanide series, as well as the alkali and alkali earth metals earlier investigated, are able to bring about the absorption shift, whereas Hg2+ and Pt4+ are not. Sigmoidal spectroscopic titration curves and nonsigmoidal binding curves suggest that there are two high affinity sites for cations in bacteriorhodopsin. Binding to the site with the second highest affinity is responsible for the absorption shift. Divalent cation binding to blue membrane causes release of about six protons, whereas higher numbers of protons are released by trivalent cations, suggesting that the shift of absorption maximum involves proton release from carboxyl group(s). The metal ion bound to this site must be surrounded by carboxyl oxygen atoms acting together as a multidentate ligand with a specific geometry because multivalent ions are effective only when capable of octahedral coordination. Lanthanide ions dramatically inhibit M412 decay at pH above 6.3, an effect probably due to binding to lipid phosphoryl groups.