@article {1270,
	title = {Calcium and Magnesium Binding in Native and Structurally Perturbed Purple Membrane},
	journal = {The Journal of Physical Chemistry},
	volume = {100},
	year = {1996},
	note = {doi: 10.1021/jp952951i},
	month = {1996},
	pages = {929 - 933},
	publisher = {American Chemical Society},
	abstract = {The number and identity of the metal cations bound to wild-type bacteriorhodopsin (bR) are determined by using inductively coupled plasma mass spectrometry (ICP-MS) and ICP emission techniques. The results indicate that there at ≈2 total Ca2+ and Mg2+ per bR molecule with a ratio of ≈3:1 Ca2+ to Mg2+. This observed ratio is found to agree with the calculated ratio using previously determined binding constants for the two high affinity sites of Ca2+ to deionized bR (Zhang; et al. Biophys. J. 1992, 61, 1201). This suggests that the high-affinity binding sites in deionized bR are similar to those in native bR. Structural perturbation of the native membrane by cleavage of the C-terminus decreases the number of ions per bR to 1.4. The observed ratio of total ions in this sample to total ions in bR is found to agree with that calculated using known binding constants for each. The results on the number of metal cations/bR and their ratio in bacterio-opsin agrees with the calculated number using previously observed binding constants in deionized bO (Yang; et al. Biophys J., in press) only if one assumes that the second high-affinity site (not the first) is removed by retinal removal. Removal of 75\% of the lipids from the purple membrane is found to greatly reduce the number of metal cations from 2 to 0.16. This suggest that if metal cations are in the two high-affinity sites (which are the only type of binding sites evident in our native bR sample), the removal of lipids, known to change the protein tertiary structure, changes also the metal ion binding sites.The number and identity of the metal cations bound to wild-type bacteriorhodopsin (bR) are determined by using inductively coupled plasma mass spectrometry (ICP-MS) and ICP emission techniques. The results indicate that there at ≈2 total Ca2+ and Mg2+ per bR molecule with a ratio of ≈3:1 Ca2+ to Mg2+. This observed ratio is found to agree with the calculated ratio using previously determined binding constants for the two high affinity sites of Ca2+ to deionized bR (Zhang; et al. Biophys. J. 1992, 61, 1201). This suggests that the high-affinity binding sites in deionized bR are similar to those in native bR. Structural perturbation of the native membrane by cleavage of the C-terminus decreases the number of ions per bR to 1.4. The observed ratio of total ions in this sample to total ions in bR is found to agree with that calculated using known binding constants for each. The results on the number of metal cations/bR and their ratio in bacterio-opsin agrees with the calculated number using previously observed binding constants in deionized bO (Yang; et al. Biophys J., in press) only if one assumes that the second high-affinity site (not the first) is removed by retinal removal. Removal of 75\% of the lipids from the purple membrane is found to greatly reduce the number of metal cations from 2 to 0.16. This suggest that if metal cations are in the two high-affinity sites (which are the only type of binding sites evident in our native bR sample), the removal of lipids, known to change the protein tertiary structure, changes also the metal ion binding sites.},
	isbn = {0022-3654},
	doi = {doi: 10.1021/jp952951i},
	url = {http://dx.doi.org/10.1021/jp952951i},
	author = {Griffiths, Jennifer A. and King, John and Yang, Difei and Browner, Richard and El-Sayed, Mostafa A}
}
@article {860,
	title = {The Ca2+ binding to deionized monomerized and to retinal removed bacteriorhodopsin.},
	journal = {Biophysical journal},
	volume = {69},
	year = {1995},
	month = {1995 Nov},
	pages = {2056-9},
	abstract = {In our continuing effort to characterize the metal cation binding in bacteriorhodopsin (bR) using Ca(2+)-specific electrodes, potentiometric titration was carried out on deionized solubilized bR (containing monomeric units) and deionized bacterioopsin (bR with its retinal removed). Scatchard plots were analyzed. The monomer was found to have plots similar to those of the trimer, suggesting that the binding sites in bR are localized within the protein monomer unit and not between the molecules within the trimer structure. This also supports the previous assumption that the curvature in the Scatchard plot of regenerated bR is not due to cooperativity of metal cation within the trimer, but rather due to multiple sites. Recent studies further support the finding that the curved Scatchard plot is not due to the cooperativity between the metal ions in the two high affinity sites, wherever they are. The results of the analysis of the Scatchard plot for deionized bacterioopsin have shown a change in the binding characteristics of the high affinity but not the low affinity sites from that observed in bR. This result supports previous conclusions that metal cations in the high affinity sites are not far from the retinal cavity.},
	keywords = {Bacteriorhodopsins, Binding Sites, Biophysical Phenomena, Biophysics, Calcium, Electrochemistry, Halobacterium, Ions, Kinetics, Protein Conformation, Retinaldehyde},
	issn = {0006-3495},
	doi = {10.1016/S0006-3495(95)80075-5},
	author = {Yang, Difei and El-Sayed, Mostafa A}
}
