Photosystem II (PSII) is a complex of proteins, pigment molecules, and other cofactors that traps sunlight to help power photosynthesis. More importantly, PSII is the only known biological catalyst capable of oxidizing water into molecular oxygen and hydrogen ions; it accomplishes this by utilizing a Mn4O5Ca cluster, a cubane-like structure known as the oxygen-evolving complex (OEC). PSII may be damaged via intense light exposure which can cause the OEC to degrade. Once PSII is repaired the OEC can reassemble via several light dependent steps, eventually forming a functional PSII complex. This process is known as OEC photoassembly. In the Kolling lab, PSII studies are conducted using in vitro PSII-enriched membranes of Spinacia oleracacea. The objective of this study is to determine the activation energy of photoassembly by using the slope-intercept form of the Arrhenius equation: ln(k) = –EA/R + ln(A). To accomplish this, PSII samples were stripped of OEC and placed in buffer containing manganese and calcium. Samples were then exposed to light at various temperatures and probed for oxygen evolution using Clark-type oximetry. From this data, the temperature dependence of OEC photoassembly was determined. The plot of the temperature dependence was used to determine the activation energy by measuring the slope of the graph. Previous experiments in the absence of 2,6-dichloro-1,4-benzoquinone (DCBQ) determined that the rate-limiting step of photoassembly was not due to the process itself, but instead was the reduction of plastoquinones in the QB site of PSII. As solving for the activation energy can only be achieved if OEC photoassembly is rate determining, 2-phenyl-1,4-benzoquinone (PPBQ), was used to supplement plastoquinone to increase the rate of quinone reduction and ensure photoassembly is rate determining. Results from preliminary studies with PPBQ will be discussed.

Determining the activation energy of the light-dependent assembly of the oxygen evolving complex

2019 Spring National Meeting

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