Structural and functional relationship of photosynthetic bacterial reaction centers
Gao, Jiliang
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Permalink
https://hdl.handle.net/2142/22147
Description
Title
Structural and functional relationship of photosynthetic bacterial reaction centers
Author(s)
Gao, Jiliang
Issue Date
1991
Doctoral Committee Chair(s)
Wraight, Colin A.
Department of Study
Biophysics and Computational Biology
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Molecular
Chemistry, Biochemistry
Biophysics, General
Language
eng
Abstract
The structure of reaction centers (RCs) was modified with various sulfhydryl reagents. It was found that mercurial reagents, especially p-chloromercuribenzenesulfate (pCMBS), inhibit Q$\sb{\rm B}$ function in RCs from Rhodobacter sphaeroides and Rhodobacter capsulatus, but not in Rhodopseudomonas viridis. By comparison of the protein sequences and structures of RCs of three species, it was suggested that Cys$\sp{\rm L108}$ in Rb. sphaeroides (Cys$\sp{\rm L109}$ in Rb. capsulatus) is the best candidate for the mercurial reagents to inhibit Q$\sb{\rm B}$ function. And a mechanism was suggested. By construction of a mutant of Rb. sphaeroides, in which Cys$\sp{\rm L108}$ was changed to serine using site-directed mutagenesis, this suggestion was strongly supported by the absence of any mercurial reagent effect on this mutant.
Charge recombination kinetics and thermodynamics equilibrium involving the high potential c-type cytochromes (C$\sb{\rm H1}$ and C$\sb{\rm H2}$) and quinone Q$\sb{\rm A}$ in RCs from Rp. viridis were investigated. It was suggested that the charge recombinations have two pathways, a direct pathway involving long distance electron transfer between Q$\sb{\rm A}\sp-$ and either C$\sb{\rm H1}\sp+$ and C$\sb{\rm H2}\sp+$, and indirect pathway, via thermal repopulation of P$\sp+$ Q$\sb{\rm A}\sp-$ followed by recombination of that state. From the schemes, we calculated the decay rates of C$\rm\sb{H2}\sp+C\sb{H1}Q\sb{A}\sp-$ and C$\rm\sb{H2}\sp+C\sb{H1}\sp+Q\sb{A}\sp-$ charge recombinations. In a wide pH and temperature range, the calculated rates fit the experimental data very well. This result indicates that there are no significant electrostatic interactions between the relevant redox centers in isolated RCs.
Heterogeneity was observed in the $\rm P\sp+Q\sb{B}\sp-$ recombination of RCs from Rp. viridis. The kinetics of the $\rm P\sp+Q\sb{B}\sp-$ recombination were well fitted by two exponential components. The two components were interconvertible, and pH, temperature and salt dependent. The presence of two long-lived, kinetically distinct conformations of the RCs was suggested. Calculation of the equilibrium constant of electron transfer between Q$\sb{\rm A}$ and Q$\sb{\rm B}$, K$\sb2$, for both components, showed that the two values were almost the same over a wide range of pH. This result suggests that the source of heterogeneity is in the energetic level of intermediate state $\rm P\sp+I\sp-$ through which both $\rm P\sp+Q\sb{A}\sp-$ and $\rm P\sp+Q\sb{B}\sp-$ recombine. Based on the pH dependences of K$\sb2$ and the conformational equilibrium constant, K$\sb{\rm cs}$, a mechanism of the heterogeneity was proposed.
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