Calcium and chloride function in oxygen evolution by photosystem II through bisubstrate enzyme kinetics and EPR spectroscopy of fluoride inhibition

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Henry Tai (Creator)
The University of North Carolina at Greensboro (UNCG )
Web Site:
Alice Haddy

Abstract: Photosystem II (PSII) is the light-harvesting water oxidase of thylakoid membranes. The catalytic center where oxygen forms in PSII is located at the manganese cluster, Mn4O5Ca, which is highly conserved throughout nature as seen in cyanobacteria and in higher plants. PSII requires both Ca2+ and Cl- for oxygen evolution activity, but their roles in catalysis and the interactions between the two cofactors are not well understood. Two studies were done to elucidate the interaction between Ca2+ and Cl-. The first study investigated the interdependence of Ca2+ and Cl- activation of oxygen evolution in spinach PSII using enzyme kinetics analyses at the low pH of 5.5. At this pH substrate inhibition at high chloride concentrations was observed, with a decrease in inhibition when the Ca2+ concentration increased. The KI for Cl- doubled from 31 mM at 0.2 mM Ca2+ to 68 mM at 5 mM Ca2+. Bisubstrate enzyme kinetics analyses of activation showed that the Michaelis constants were 0.26 mM for Ca2+and 0.97 mM for Cl- and the dissociation constants were 0.31 mM for Ca2+ and 1.15 mM for Cl-. The second study investigated the effect of fluoride, a competitive inhibitor of Cl- activation, on the tyrosine Z (TyrZ) radical that is involved in electron transfer from the manganese cluster. Using electron paramagnetic resonance (EPR) spectroscopy, the TyrZ radical was observed at 77 K in fluoride-inhibited PSII, indicating that electron transfer as blocked. The study showed a positive correlation in TyrZ radical formation with increasing F- concentration which is believed to indicate formation of the S2YZ* state.

Additional Information

Language: English
Date: 2013
Enzyme Kinetics, EPR Spectroscopy, Oxygen Evolution, Photosystem II
Enzyme kinetics
Photosynthetic oxygen evolution

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