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

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Botany

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Blubaugh, Danny J.

• Biology, Plant Physiology

Language dc:language

eng

Identifier

(UMI)AAI8802984

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/77664