Toward a Comprehensive Model of Photosystem II Oxygen Evolving Complex Photoassembly

Elucidating the mechanism of photoassembly of the oxygen-evolving complex (OEC) will lead to a better understanding of how nature catalyzes water oxidation. Although the temperature dependence of oxygen evolution has been described in the literature [1,2], we have performed a comprehensive study that covers Photosystem II (PSII) activity oxygen evolution in the presence and absence of 2,6-dichloro-1,4benzoquinone (DCBQ) from 5–45°C. In addition, we performed similar measurements for OEC photoassembly in the absence of DCBQ. PSIIenriched particles used for our measurements were prepared according to Berthold, Babcock, and Yocum[3], with modifications from Kolling, et al.[4] The oxygen-evolution of intact BBY particles at a temperature range of 5–45°C was measured and found to have a maximum at 28°C, which is consistent with literature values[4]. We then generated OEC-depleted BBY (apo-BBY) particles by using an established aminosulfonic buffer (20 mM CHES/NaOH, pH 9.4) and a divalent cation (200 mM MgCl2) treatment with minor modifications. [1] These apo-BBY particles were combined with a mixture of Mn2+, Ca2+, HCO3-, and K3[Fe(CN)6] and then subjected to light pulses within a temperature range of 5–45 °C followed by oxygen-evolution measurements at 28°C. The apo-BBYs were then subjected to different K3[Fe(CN)6] concentrations and a range of pulses with varying flash number and frequency in order probe the rate limiting step of photoassembly. The shared temperature profiles between oxygen evolution and photoassembly, as well as the frequency titrations and K3[Fe(CN)6] concentration experiments, have contributed to a more comprehensive model of photoassembly of the PSII protein complex. Experiments with DCBQ during PSII water oxidation were found to flatten the temperature-dependence profile in the range of 10–32°C; and protease activity within the BBY preps also showed a temperature dependence and was responsible for some loss of activity over 28°C. These new data will lead to a better understanding of how PSII functions