AN EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDY OF THE HIGH-AFFINITY CATION-BINDING SITE IN THE PURPLE MEMBRANE

The structure of the high-affinity cation-binding site of bacteriorhod opsin was studied using extended x-ray absorption fine structure techn iques. The results obtained for Mn2+ in aqueous solution and for the c omplex BR-Mn2+ (1:1 molar ratio) show great similarities, suggesting t hat Mn2+, when bound to this site, is coordinated with six atoms of ox ygen, forming an octahedral disposition, The interatomic distance betw een the atoms of oxygen and the Mn2+ was found to be 2.17 Angstrom for the complex BR-Mn2+, similar to Mn2+ in solution (2.15 Angstrom). In addition, the absence of any other peak at greater distances in the Fo urier-transformed spectrum indicates that neither phosphorus nor sulph ur atoms are present in the second coordination shell. This suggests t hat this binding site is located in the protein, discarding the proxim ity of lipid polar headgroups.