The Protein Environment of the Bacteriopheophytin
Anion Modulates Charge Separation and Charge Recombination in Bacterial
Reaction Centers
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Abstract
The
kinetics and pathway of electron transfer has been explored
in a series of reaction center mutants from <i>Rhodobacter sphaeroides</i>, in which the leucine residue at M214 near the bacteriopheophytin
cofactor in the A-branch has been replaced with methionine, cysteine,
alanine, and glycine. These amino acids have substantially different
volumes, both from each other and, except for methionine, from the
native leucine. Though the mutation site of M214 is close to the bacteriopheophytin
cofactor, which is involved in the electron transfer, none of the
mutations alter the cofactor composition of the reaction center and
the primary charge separation reaction is essentially undisturbed.
However, the kinetics of electron transfer from H<sub>A</sub><sup>–</sup> → Q<sub>A</sub> becomes both slower and substantially
heterogeneous in three of the four mutants. The decreased H<sub>A</sub><sup>–</sup> → Q<sub>A</sub> electron transfer rate
allows charge recombination between P<sup>+</sup> and H<sub>A</sub><sup>–</sup> to compete with the forward reaction, resulting
in a drop in the overall yield of charge separation. Both the yield
change and the variation in kinetics correlate well with the volume
of the mutant amino acid side chains. Analysis of the kinetics suggests
that the introduction of a smaller side chain at M214 results in greater
protein structural heterogeneity and dynamics on multiple time scales,
resulting in perturbation of the electronic environment and its evolution
in the vicinity of the early charge-separated radical pair, P<sup>+</sup>H<sub>A</sub><sup>–</sup>, and the subsequent acceptor
Q<sub>A</sub>, affecting both the extent and time scale of dielectric
relaxation. It appears that the reaction center has been optimized
not only in terms of its static structure–function relationships,
but also finely tuned to favor particular reaction pathways on particular
time scales by adjusting protein dynamics