Key cofactors of Photosystem II

• Sindra Peterson Årsköld

Photosystem II: The engine of life

Photosystem II (PSII) might be called the key to life on earth. It is a pigment-containing redox enzyme found in plants, cyanobacteria and some algae. PSII couples light-induced excitation of pigments to electron movement across the thylakoid membrane, ultimately resulting in chemical energy storage. Thus, PSII traps solar energy in a form that can be used by all forms of life in our ecosystem. As the source of electrons for PSII is water, the by-product of this process is molecular oxygen.

The key to this conversion of solar energy into redox chemistry is an enigmatic chlorophyll (chl) complex called P680. P680 is excited by energy funneled into the reaction center from the so-called antenna chl’s, which have been excited by light. The excited P680* is then autoxidized, and a nearby pheophytin (pheo) accepts the electron. This primary charge separated state is prevented from recombining by a chain of cofactors that rapidly separates the positive and negative charges from each other. The result is oxidation of H₂O at one side of the membrane and reduction of a plastoquinone on the other side.

P680: The enigmatic heart of PSII

The capability of P680 to eject an electron upon excitation is very unlike the chl’s of the antenna, which pass on excitation energy without chemical change. Furthermore, the oxidized P680⁺ is the most potent electron abstractor in nature, with a potential of 1.12 V or more. This allows it to drive the oxidation of water, catalyzed by the Mn cluster in PSII. These unique characteristics may be caused by chl-chl interactions within the reaction center, or by chl-protein interactions, modifying the properties of the pigment.

The nature of P680 is under intense debate. Six chl’s and two pheo’s are present in the PSII reaction center. Hypotheses of the identity of P680 range from a single, monomeric chl to an excitonically coupled multimer model involving four chl’s and both pheo’s.

Due to the spectral congestion in the Q_y absorption region of P680, intact PSII complexes can not be used to study P680 optically. Instead, a minimal “D1/D2” preparation was developed, containing only the innermost proteins, 6 chl’s and 2 pheo’s. Most studies on P680 have been conducted on such material. Unfortunately, the D1/D2 preparation has lost most resemblance to the native reaction center, casting doubt on the relevance of data derived from this material. I contributed to the development of a PSII core preparation which contains the minimal amount of subunits and pigments, while retaining full enzymatic activity and spectral structure (1). We used this material to study the active pheo in its intact form (2).

Homing in on P680

With this particularly intact and pure PSII preparation, and our potent set of quantifiable spectroscopic fingerprints, including low-temperature absorption, CD, and MCD signatures and characteristic electrochrimic shifts, we set out to characterize the key pigments of the active PSII reaction center. An array of different PSII samples was investigated and compared to our fully active cores:

a) solubilized membrane fragments, containing the full PSII system (2).

b) isolated subunits CP43, CP47, D1/D2/cyt b559, and D1/D2/cyt b559/CP47.

c) PSII cores from spinach, pea, the cyanobacterium Syn. 6803 and the thermophillic cyanobacterium Vulcanus (3).

In order to gain further spectral detail, we aim to perform sigle-crystal spectroscopic studies of crystallized Vulcanus PSII. This will also allow us to analyze our data in an exact structural framework, as these crystals have yielded an X-ray crystallographic structure (4).

References

1.  Smith, P. J., Peterson, S., Masters, V. M., Wydrzynski, T., Styring, S., Krausz, E., and Pace, R. J. (2002) Biochemistry 41, 1981-1989.

2.  Peterson Årsköld, S., Masters, V. M., Prince, B. J., Smith, P. J., Pace, R. J., and Krausz, E. (2003) Journal of the American Chemical Society 125, 13063-13074.

3.  Peterson Årsköld, Smith, Shen, Pace & Krausz (2005) Photosyn Res 84, 309-316.

4.  Kamiya, N., and Shen, J.-R. (2003) Proceedings of the National Academy of Science USA 100, 98-103.