Biophys J 1993 Aug;65(2):652-60

Study of Wild Type and Genetically Modified Reaction Centers from Rhodobacter capsulatus: Structural Comparison with Rhodopseudomonas viridis and Rhodobacter sphaeroides

Baciou L., Bylina E. J., Sebban P.

UPR 407, Bat. 24, Centre National de le Recherche Scientifique Gif/Yvette, France.

Reaction centers from the purple bacterium Rhodobacter (Rb.) capsulatus and from two mutants
ThrL226-->Ala and IleL229-->Ser, modified in the binding protein pocket of the secondary
quinone acceptor (QB), have been studied by flash-induced absorbance spectroscopy. In
ThrL226-->Ala, the binding affinities for endogenous QB (ubiquinone 10) and UQ6 are found to
be two to three times as high as the wild type. In contrast, in IleL229-->Ser, the binding affinity
for UQ6 is decreased about three times compared to the wild type. In ThrL226-->Ala, a
markedly increased sensitivity (approximately 30 times) to o-phenanthroline is observed. In
Rhodopseudomonas viridis, where Ala is naturally in position L226, the sensitivity to
o-phenanthroline is close to that observed in ThrL226-->Ala. We propose that the presence of
Ala in position L226 is responsible for the high sensitivity to that inhibitor. The pH dependencies
of the rate constants of P+QB- (kBP) charge recombination kinetics (P is a dimer of
bacteriochlorophyll, and QB is the secondary quinone electron acceptor) show destabilization of
QB- in ThrL226-->Ala and IleL229-->Ser, compared to the wild type. At low pH, similar
apparent pK values of protonation of amino acids around QB- are measured in the wild type and
the mutants. In contrast to Rb. sphaeroides, in the wild type Rb. capsulatus, kBP substantially
increases in the pH range 7-10.

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