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
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).
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
analization of the quinone pocket after duroquinone replacement in R-26 and th IleL229
--> Met mutant is under progress.
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