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Title:Observation and characterization of inactive photosystem II reaction centers
Author(s):Chylla, Roger Alan
Doctoral Committee Chair(s):Govindjee
Department / Program:Biophysics and Computational Biology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Biochemistry
Biophysics, General
Abstract:Photosystem II (PS II) is a polypeptide complex which operates in series with the cytochrome $b/f$ complex and photosystem I to perform light-driven transport of electrons from H$\sb2$O to NADP and of protons from the outside to the inside of the thylakoid membrane. Studies of the turnover rate of the PS II reaction center from higher plants reveals that about one-third of photosystem II reaction centers inactive in energy transduction. Steady-state turnover rates of O$\sb2$ evolution indicates that active PS II reaction centers turn over at rates of about 250 e-/s but that inactive centers appear to turn over at rates 1000 fold slower, about 2 e-/s.
Inactive centers can be observed from measurements of the flash-induced electrochromic shift and the fluorescence yield in thylakoid membranes (in vitro) and intact leaves (in vivo) from spinach (Spinacia oleracea L.). Additional evidence for inactive PS II complexes in spinach leaves is provided by chlorophyll a fluorescence induction and fluorescence decay measurements which can be used to monitor the oxidation kinetics of Q$\sb{\rm A}$, the primary quinone acceptor of photosystem II.
The temperature and pH insensitivity of the kinetic activity of the inactive complexes is consistent with the notion that PS II inactive centers are not in dynamic equilibrium with active centers but are, in fact, a biochemically distinct population of reaction centers existing in parallel with active PS II centers.
Measurements of the electrochromic shift and chlorophyll a fluorescence at various dark times after light adaptation suggest that inactive centers are converted in the light into a population of PS II reaction centers, PS II(y), which do not give rise to an observable electrochromic shift or to chlorophyll a fluorescence but instead dissipate absorbed excitation energy non-radiatively. One possibility for the role of PS II(y) centers in photosynthesis is that they dissipate excess excitation energy from the PS II antenna system as a mechanism to prevent photoinhibition. Though this idea is currently speculative, it is noteworthy that in all species of higher plants surveyed thus far (spinach, pea, corn, sorghum, blackeyed pea, cotton, sunflower, soybean, and pea) evidence has been found for the existence of inactive centers. (Abstract shortened with permission of author.)
Issue Date:1990
Rights Information:Copyright 1990 Chylla, Roger Alan
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9026162
OCLC Identifier:(UMI)AAI9026162

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