A fluorimetric study of Mg2]nduced structural changes in thylakoid membrane protein. |
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Authors: | J Isaakidou G Papageorgiou |
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Institution: | Nuclear Research Center “Democritus,” Department of Biology, Athens, Greece |
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Abstract: | Low concentrations of Mg2+ (concn < 10 mm) generate structural changes in delipidated spinach chloroplast lamellae, that appear as changes in the fluorescence yield of native tryptophyl residues and of the externally added polarity probe magnesium 1-anilinonaphthalene-8-sulfonate.The delipidated lamellae, consisting essentially of structural protein monomers and aggregates, bind magnesium 1-anilinonaphthalene-8-sulfonate to the extent of 126 ± 13 nmol/mg protein, and with a dissociation constant KD = 167 μM. Bound ANS fluoresces at 458 nm with a quantum yield Φ = 0.121. Tryptophyls sensitize the fluorescence of bound ANS with a maximal efficiency Tmax = 0.85. Assuming completely random orientation of the interacting chromophores, an interchromophore separation is calculated. Only two-thirds of the membrane tryptophyls have ANS-binding sites in their vicinity.Mg2+ binds to the delipidated membranes with a dissociation constant KD = 2 mM. The binding is attended by enhancement of magnesium 1-anilinonaphthalene-8-sulfonate fluorescence, and deenhancement of tryptophyl fluorescence, while the efficiency of interchromophore excitation transfer increases only slightly. These effects suggest that Mg2+ generates a structural change which lowers the polarity of the membrane region where tryptophyl and magnesium 1-anilinonaphthalene-8-sulfonate are situated, but which has a minor effect only on the interchromophore separation. |
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