A quantitative XANES analysis of the calcium high-affinity binding site of the purple membrane.
In this article we report x-ray absorption measurements of Ca(2+)-substituted bacteriorhodopsin. We present a detailed study of the absorption spectrum close to the absorption edge that is very sensitive to the site geometry. We combined ab initio calculations of the x-ray absorption cross section based on a full multiple scattering approach, with a best fit of the experimental data performed by changing the cluster geometry. The Ca(2+)-bacteriorhodopsin environment is composed of six oxygen atoms showing a distorted orthorhombic symmetry, whereas the Ca(2+) in water solution has a regular octahydrated first sphere of coordination. Our results are in good agreement with previous molecular models suggesting that the high-affinity cationic site could be in the proximity of the retinal pocket. Our results provide strong direct evidence of the specific binding site of the metal cation in bacteriorhodopsin.
Sepulcre
Francesc
F
Departament d'Enginyeria Agroalimentària i Biotecnologia, Escola Superior d'Agricultura, Universitat Politècnica de Catalunya, Barcelona, Spain. francesc.sepulcre@upc.es
Proietti
M Grazia
MG
Benfatto
Maurizio
M
Della Longa
Stefano
S
García
Joaquin
J
Padrós
Esteve
E
D015502:Absorptiometry, Photon; D001436:Bacteriorhodopsins / Q000737:chemistry*; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D019617:Halobacterium salinarum / Q000737:chemistry*; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D018717:Purple Membrane / Q000737:chemistry*
D001436:Bacteriorhodopsins; D010100:Oxygen; D002118:Calcium