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Structures and functions of different ubiquinones at the QB site of reaction centers László Rinyu, Nóra Méray, Júlia Tandori, Ilona Pfeiffer, Péter Maróti and László Nagy
Summary
The stability of the light induced charge pair in the photosynthetic reaction centers depends, essentially, on the in situ redoxpotential of the quinones which is determined by a) the polarization of their environment b) and the conformation and chemical identity of the quinone substituents. The polarizability of the quinone environment highly depends on the protonation state of the amino acid chains in the QB -pocket coupled very strongly (through protonation channels) with the water phase of the cytoplasm. This can be changed by mutation of specific amino acids which are either involved in the proton delivery to the quinones or make thestructural arrangement of the quinone site stable. The IleL229 (the isoleucine of the L-protein of the reaction center on the 229th position) belongs to the second group. The protonation statement of the amino acid side chains can be changed by the pH of the environment too. In the Rb. sphaeroides R-26 both QA and QB are UQ10. The methoxy oxygenes in the 2nd and 3rd position can be coupled with the quinone ring (due to mezomeric effect of the pz orbitals) resulting a decrease in the degree of freedom of the rotation of the methoxy groups. As a result of this effect mutation of amino acid which is close to the quinone methoxy group, like IleL229 --> Met, will change either the binding and/or the in situ redox potential of the quinones. We found that reconstituting the reaction centers with duroquinone, which contain methyl groups, instead of methoxy groups, can be achieved and the binding constant wasmeasured 12 uM. This value is much more smaller than that of measured for UQ0 (110 mM). If the IleL229 was mutated to Met. the binding constant for duroquinone was found 23 mM increased in some extent, but there was a larger increase in the case of UQ0 (1.2 mM). The free energy for stabilization, DGo, was less negative (smaller the stabilization) in the Rb. sphaeroides R-26, but more negative if the IleL229 was changed to Met (larger the stabilization). This less stabilization in the R-26 (considered as wild type for the IleL229 --> Met mutant) can be explained by the small movement of the quinone ring of the duroquinone. Further analization of the quinone pocket after duroquinone replacement in R-26 and th IleL229 --> Met mutant is under progress. Aknowledgements: This work was supported by grants OTKA T0 23773, MKMPFP 5189/1997, FKFP B-23/1997. Thanks are due to Gergely Nagy-Lajos, (JATE Department of Microbiology) and Eszter Molnár for technical contribution.
Any question or comment should go to: lnagy@sol.cc.u-szeged.hu