Selective perturbation of the second electron transfer step in mutant bacterial reaction centers.
In order to specifically perturb the primary electron acceptor B(A) -- a monomeric bacteriochlorophyll (BChl) a -- involved in bacterial photosynthetic charge separation (CS), the protein environment of B(A) in the reaction center (RC) of Rhodobacter sphaeroides was modified by site-directed mutagenesis. Isolated RCs were characterized by redox titrations, low temperature optical spectroscopy, ENDOR/TRIPLE resonance spectroscopy and femtosecond time-resolved spectroscopy. Two mutations were studied: In the GS(M203) mutant a serine is introduced near the ring E keto group of B(A), while in FY(L146) a phenylalanine near the ring A acetyl group of B(A) is replaced by tyrosine. In all mutations the oxidation potential of the primary electron donor P as well as the electronic structure of both the P(*+) radical cation and the radical anion of the secondary electron acceptor, H(A)(*-), are not significantly altered compared to the wild type (WT), while changes of the optical absorption spectra at 77 K in the BChl Q(X) and Q(Y) regions are observed. The GS(M203) mutation only leads to a minor retardation of the CS reactions at room temperature, whereas for FY(L146) significant deviations from the native electron transfer (ET) rates could be detected: In addition to a faster first (2.9 ps) and a slower second (1 ps) ET step, a new 8-ps time constant was found in the FY(L146) mutant, which can be ascribed to a fraction of RCs with slowed down secondary ET. The results allow us to address the functional role of the acetyl group of B(A) and question the role of the free energy changes as the main determining factor of ET rates in RCs. It is concluded that structural rearrangements alter the electronic coupling between the pigments and thereby influence the rate of fast CS.
Schenkl
Selma
S
Sektion Physik, Ludwig-Maximilians-Universität München, Oettingenstr. 67, 80538 Munich, Germany.
Spörlein
Sebastian
S
Müh
Frank
F
Witt
Heike
H
Lubitz
Wolfgang
W
Zinth
Wolfgang
W
Wachtveitl
Josef
J
D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D045342:Light-Harvesting Protein Complexes; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D012242:Rhodobacter sphaeroides; D013054:Spectrophotometry, Atomic
D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins