Kinetic studies of mechanism of ubiquinol:cytochrome c(2) oxidoreductase of Rhodobacter sphaeroides: A steady state approach
Chen, Yue
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Permalink
https://hdl.handle.net/2142/22947
Description
Title
Kinetic studies of mechanism of ubiquinol:cytochrome c(2) oxidoreductase of Rhodobacter sphaeroides: A steady state approach
Author(s)
Chen, Yue
Issue Date
1990
Doctoral Committee Chair(s)
Crofts, Antony R.
Department of Study
Biophysics and Computational Biology
Discipline
Biophysics and Computational Biology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biophysics, General
Language
eng
Abstract
The work presented in this thesis is among the first efforts to study the mechanism of electron transfer through b/c$\sb1$ complex of R. sphaeroides under the coupled conditions. It aims at (1) establishing a kinetic model for b/c$\sb1$ complex in the presence of a substantial proton gradient and thereby extending kinetic studies of b/c$\sb1$ complex from an uncoupled system to a coupled system; and (2) understanding the interrelations among the redox poise of the electron transfer chain, electron transfer rate and the proton gradient. (1) A self-consistent steady state approach has been developed by which the redox poise of the electron transfer chain, the electron transfer flux and the proton gradient could be simultaneously monitored and the interplay among these parameters could be studied. (2) In steady state close to a static head, we find: (2a) The maximal proton gradient established is close to 200 mV. (2b) The free energy drop associated with each reaction step within b/c$\sb1$ complex is close to zero. (2c) The measured steady state electron transfer rate and the redox poise of the electron transfer chain are consistent with the kinetic and thermodynamic parameters established in uncoupled system and defined by the modified Q-cycle model. (2d) The electrogenic step between two b-cytochromes accounts for 65%-70% of the total electric span across the membrane. (3) A complete dynamic thermodynamic profile has been established for the electron transfer chain as it approaches a steady state in response to imposed light illumination. (3a) The free energy drop associated with each electron transfer step suggested by the modified Q-cycle model approaches zero as the system approaches a static head. (3b) The electron transfer pathways suggested by the dynamic thermodynamic profile are consistent with those suggested by the modified Q-cycle model. (4) The coupling degrees for b/c$\sb1$ complex and the electron transfer chain as a whole have been estimated to be larger than 0.92. (5) Results discussed above suggest a natural extension of the modified Q-cycle model for the coupled b/c$\sb1$ complex. (6) In the presence of antimycin, Q$\sb{\rm Z}$-site could still turn over at a rate less than 5 s$\sp{-1}$ without concomitant turnover of Q$\sb{\rm C}$-site, suggesting an intrinsic decoupling mechanism for b/c$\sb1$ complex. (7) An antimycin-sensitive but myxothiazol-insensitive reduction of two b-cytochromes has been observed in the presence of a large $\Delta\Psi$, indicating that the reaction associated with turnover of Q$\sb{\rm C}$-site could be reversed when it is thermodynamically feasible.
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