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Biophys J, August 1998, p. 777-784, Vol. 75, No. 2

Experimental and Theoretical Characterization of the High-Affinity Cation-Binding Site of the Purple Membrane

Leonardo Pardo,* Francesc Sepulcre,# Josep Cladera,# Mireia Duñach,# Amílcar Labarta,§ Javier Tejada,§ and Esteve Padrós#

 *Laboratori de Medicina Computacional, Unitat de Bioestadística, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona;  #Unitat de Biofísica, Departament de Bioquímica i de Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona; and  §Departament de Física Fonamental, Facultat de Física, Universitat de Barcelona, 08028 Barcelona, Spain

Binding of Mn2+ or Mg2+ to the high-affinity site of the purple membrane from Halobacterium salinarium has been studied by superconducting quantum interference device magnetometry or by ab initio quantum mechanical calculations, respectively. The binding of Mn2+ cation, in a low-spin state, to the high-affinity site occurs through a major octahedral local symmetry character with a minor rhombic distortion and a coordination number of six. A molecular model of this binding site in the Schiff base vicinity is proposed. In this model, a Mg2+ cation interacts with one oxygen atom of the side chain of Asp85, with both oxygen atoms of Asp212 and with three water molecules. One of these water molecules is hydrogen bonded to both the nitrogen of the protonated Schiff base and the Asp85 oxygen. It could serve as a shuttle for the Schiff base proton to move to Asp85 in the L-M transition.

Biophys J, August 1998, p. 777-784, Vol. 75, No. 2
© 1998 by the Biophysical Society   0006-3495/98/08/777/08  $2.00


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