Bicarbonate Reversible Anionic Inhibition of the Quinone Reductase in Photosystem II
Eaton-Rye, Julian John
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Permalink
https://hdl.handle.net/2142/77665
Description
Title
Bicarbonate Reversible Anionic Inhibition of the Quinone Reductase in Photosystem II
Author(s)
Eaton-Rye, Julian John
Issue Date
1987
Department of Study
Plant Biology
Discipline
Botany
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Plant Physiology
Language
eng
Abstract
The HCO$\sb3\sp-$ anion regulates photosynthetic electron transport from H$\sb2$O to a Hill oxidant. HCO$\sb3\sp-$ depletion in the presence of inhibitory anions depresses the electron transport rates by as much as ten-fold. All the inhibitory effects of HCO$\sb3\sp-$ removal were found to be reversible by the addition of 5 mM HCO$\sb3\sp-$ unless prolonged depletion procedures were used where the inhibitory anion had been omitted. This study investigated contradictory conclusions regarding the site of HCO$\sb3\sp-$ action in the electron transport chain. Using methyl viologen as the electron acceptor and various artificial electron donors to different regions in the electron transport chain, the major site of HCO$\sb3\sp-$ action was located on the acceptor side of photosystem II (PS II). The oxidation of Q$\sb{\rm A}\sp-$ (the primary quinone acceptor of PS II) by Q$\sb{\rm B}$ or Q$\sb{\rm B}\sp-$ (Q$\sb{\rm B}$ is the secondary quinone acceptor of PS II) exhibited a smaller overall half-time at pH 7.5 than at pH 6.5 in HCO$\sb3\sp-$-depleted or treated membranes. However, the slowest oxidation of Q$\sb{\rm A}\sp-$ in these membranes, as indicated by the overall half-time parameter, depends on the flash number and the flash frequency in addition to the pH. The operating redox potential for the Q$\sb{\rm B}$/Q$\sb{\rm B}\sp-$ couple was found to be pH independent in treated membranes and the equilibrium for the sharing of an electron between Q$\sb{\rm A}$ and Q$\sb{\rm B}$ was decreased by a factor of 4 at pH 6.0. Also in treated membranes the back reaction between Q$\sb{\rm A}\sp-$ and the S$\sb2$ state of the oxygen evolving complex was inhibited four-fold below pH 7.0 but was unaffected above pH 7.5 in the presence of DCMU. A HCO$\sb3\sp-$ sensitive back reaction with a half-time of $<$100 $\mu$s was observed at pH 8.0 in the presence of DCMU in approximately half of the PS II centers. The kinetic stability of Q$\sb{\rm B}\sp-$ also appeared to be reduced in HCO$\sb3\sp-$-depleted samples. It was concluded that HCO$\sb3\sp-$ is a ligand to Fe$\sp{2+}$ in the PS II reaction center, and that the rate-limiting step introduced in the linear electron flow by HCO$\sb3\sp-$ depletion was the protonation of Q$\sb{\rm B}\sp-$.
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