Calcium and chloride as cofactors and their binding in photosystem II

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

Abstract: Photosystem II (PSII) is the membrane-bound protein complex which participates in the photosynthetic conversion of sunlight energy into chemical energy and produces molecular oxygen. Calcium and chloride are important cofactors that PSII requires in the process of oxygen evolution. They may be thought of as involved in a bisubstrate mechanism, where each is required for the activation of evolution of oxygen. The binding site of calcium has been identified as part of the catalytic Mn4Ca cluster, and two chloride binding sites have been identified near the catalytic cluster. In this research, the dependence of oxygen evolution activity by PSII as a function of chloride and calcium concentrations was measured and analyzed according to enzyme kinetics methods to determine a series of KM values for Cl- activation. The data were next analyzed in terms of bisubstrate enzyme kinetics models, leading to kinetic constants for both Ca2+ and Cl- activation. While the order of binding could not be determined from the data, the results are consistent with a random sequential binding model for the two ions. Further experiments were performed to reveal details about the calcium and chloride binding sites. The possibility of using isothermal titration calorimetry (ITC) to analyze Cl- binding to PSII was explored. Finally, the response of the Tyr Z radical, which is involved in electron transfer from the Mn4Ca cluster, was analyzed using electron paramagnetic resonance (EPR) spectroscopy, to determine the effects of the presence of Ca2+ and Cl-. The results of these studies help to clarify the relationship between the critically required activators calcium and chloride during oxygen evolution by PSII.

Additional Information

Language: English
Date: 2018
Calcium and Chloride Ions, Manganese Cluster, Oxygen Evolving Complex, Photosystem II
Photosynthetic oxygen evolution
Binding sites (Biochemistry)

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