The Mechanism of Bicarbonate Activation of Plastoquinone Reduction in Photosystem II of Photosynthesis

Blubaugh, Danny J.

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Description

Title

The Mechanism of Bicarbonate Activation of Plastoquinone Reduction in Photosystem II of Photosynthesis

Author(s)

Blubaugh, Danny J.

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

Bicarbonate (HCO$\sb{3}\sp{-}$) is required for photosystem II (PS II) electron transport. Depleting thylakoids of HCO$\sb{3}\sp{-}$ slows down electron transfer from the primary quinone acceptor Q$\sb{\rm A}$ to the secondary quinone acceptor Q$\sb{\rm B}$. It also blocks electron transfer from Q$\sb{\rm B}$ to the plastoquinone (PQ) pool. This effect is reversible, and is specific for HCO$\sb{3}\sp{-}$. A variety of biochemical and biophysical methods were used to probe the mechanism of this requirement. The chemical species required is HCO$\sb{3}\sp{-}$, not CO$\sb2$, H$\sb2$CO$\sb3$ or CO$\sb{3}\sp{2-}$: over the pH range of 6.3 to 6.9, the rate of electron flow in HCO$\sb{3}\sp{-}$ depleted thylakoids increases in proportion to the equilibrium (HCO$\sb{3}\sp{-}$), but is independent of the equilibrium (CO$\sb2$), (H$\sb2$CO$\sb3$), or (CO$\sb{3}\sp{2-}$). A kinetic analysis of the Hill activity as a function of the equilibrium (HCO$\sb{3}\sp{-}$) indicates that there are at least two sites of HCO$\sb{3}\sp{-}$ binding, if it is assumed that the basal activity in the absence of added HCO$\sb{3}\sp{-}$ is due to endogenous HCO$\sb{3}\sp{-}$. In thylakoids in which all but 7% of the Hill activity was reversibly inhibited by HCO$\sb{3}\sp{-}$ depletion, the activity as a function of chlorophyll (Chl) concentration was nonlinear, indicating the presence of endogenous HCO$\sb{3}\sp{-}$. When the endogenous HCO$\sb{3}\sp{-}$ is included in the total (HCO$\sb{3}\sp{-}$), the kinetics are those of a two-site system with high cooperativity between the binding sites. An analog of PQ, containing an azido group capable of photoaffinity attachment, was used to probe whether quinone binding at the Q$\sb{\rm B}$ site is affected by HCO$\sb{3}\sp{-}$ removal. Less of the analog appears to be able to label the Q$\sb{\rm B}$ site when HCO$\sb{3}\sp{-}$ is removed, than when it is present, suggesting that the quinone binds less tightly in the absence of HCO$\sb{3}\sp{-}$. The PQ analog appeared to be able to oxidize Q$\sb{\rm A}\sp{-}$ directly, and may also impair electron flow from pheophytin (Pheo) to Q$\sb{\rm A}$. These latter effects are more pronounced when HCO$\sb{3}\sp{-}$ is removed and may be due to conformational changes induced by the removal of HCO$\sb{3}\sp{-}$. A model was developed to explain HCO$\sb{3}\sp{-}$ action, in which one HCO$\sb{3}\sp{-}$ forms a salt bridge between the non-heme Fe$\sp{2+}$ in PS II and a histadine protein residue, another HCO$\sb{3}\sp{-}$ is involved in protonating a histidine near the Q$\sb{\rm B}$ site to stabilize Q$\sb{\rm B}\sp{-}$, and a low affinity pool of HCO$\sb{3}\sp{-}$ keeps the (HCO$\sb{3}\sp{-}$) high in the vicinity of the binding sites.

Graduation Semester

1987

Type of Resource

text

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