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Electrostatic dominoes: long distance propagation of mutational effects in photosynthetic reaction centers of Rhodobacter capsulatus

Sebban, P., Maroti, P., Schiffer, M., Hanson, D.K.
Biochemistry 1995 v.34 no.26 pp. 8390-8397
Rhodobacter capsulatus, absorbance, bacteria, binding sites, energy, mutants, mutation, pH, phenotype, photosynthetic reaction centers, protons, spectroscopy
Two point mutants from the purple bacterium Rhodobacter capsulatus, both modified in the M protein of the photosynthetic reaction center, have been studied by flash-induced absorbance spectroscopy. These strains carry either the M231Arg leads to Leu or M43Asn leads to Asp mutations, which are located 9 and 15 angstroms, respectively, from the terminal electron acceptor QB. In the wild-type Rb. sphaeroides structure, M231Arg is involved in a conserved salt bridge with H125Glu and H232Glu and M43Asn is located among several polar residues that form or surround the QB binding site. These substitutions were originally uncovered in phenotypic revertants isolated from the photosynthetically incompetent L212Glu-L213Asp leads to Ala-Ala site-specific double mutant. As second-site suppressor mutations, they have been shown to restore the proton transfer function that is interrupted in the L212Ala-L213Ala double mutant. The electrostatic effects that are induced in reaction centers by the M231Arg leads to Leu and M43Asn leads to Asp substitutions are roughly the same in either the double-mutant or wild-type backgrounds. In a reaction center that is otherwise wild type in sequence, they decrease the free energy gap between the QA- and QB- states by 24 +/- 5 and 45 +/- 5 meV, respectively. The pH dependences of K2, the QA-QB in equalibirum with QAQB- equilibrium constant, are altered in reaction centers that carry either of these substitutions, revealing differences in the pKas of titratable groups compared to the wild type. These results confirm that interactions among distant residues influence the electrostatic potential in the immediate vicinity of QB to ensure the efficient conduction of protons through the protein matrix and their delivery to the reduced quinone. It is possible that these influences are propagated over such large distances by mutation-induced realignments of salt bridges within a network of acidic and basic residues that is located in this region of the reaction center, which could serve as a relay mechanism to partially relocate the new negative charge much closer to the quinone.