Temperature Dependence of the PS II Reaction Pattern Detected by Flash Induced Fluorescence Changes
Photosynthetic water cleavage into dioxygen and hydrogen bound as plastoquinol takes place in a polypeptide complex referred to as photosystem II that is anisotropically incorporated into the thylakoid membrane. With QA − as reductant plastoquinol is formed in a specific site (referred to as QB −binding-site in polypeptide D-1) by a sequence of two univalent electron transfer steps via the semiquinone form of a tightly bound special plastoquinone QB − as intermediate. The plastoquinone QB is stably associated with the QB −site only in the semiquinone redox state QB − white QB is loosely bound and QBH2 rapidly exchanges with the plastoquinone of the thylakoid membrane thereby renewing the substrate for the transfer of the next two electrons. A large number of chemicals acting as PS II herbicides interfere with the QB −site in preventing binding and reduction of PQ by QA − (for review see Ref. 1). The mechanism of plastoquinol formation suggests that structural modifications at the QB−site could affect the electron transfer rate from QA − to QB/ QB − Therefore temperature induced dynamical changes of structural properties of the protein matrix are expected to modify the reactivity at the QB −site. In this communication the temperature dependence of the QA −-reoxidation is analyzed by measuring the relaxation kinetics of the flash induced enhancement of fluorescence quantum yield in thylakoids, PS II membrane fragments and PS II core particles with high oxygen evolution rate.
KeywordsThylakoid Membrane Fluorescence Quantum Yield Core Complex Core Particle Reaction Pattern
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