Taking snapshots of different redox states of the water oxidation catalyst in Photosystem II

9 Jul 2014

BISfuel, July 9, 2014 –  Deciphering the puzzles of the natural photosynthetic water oxidation mechanism empowers designers of artificial photosynthesis with knowledge to construct better water oxidation catalysts for solar fuel production. A group of Bisfuel researchers working with collaborators at the DOE free electron laser has achieved an important milestone in understanding the photosynthetic water oxidation catalytic process. Using femtosecond X-ray crystallography combined with laser pulse excitation the team obtained a structure of a short-lived intermediate state in the process. This report has been published in Nature magazine (online publication on July 9, 2014).

Oxygen clock: More than 2.5 billion year ago photosynthetic organisms acquired ability to use water as a source of electrons to feed the natural machinery of solar energy conversion. Nature kept the secrets of water splitting until the second half of the XX century when it became evident that water oxidation occurs through a clock mechanism that depends on the presence of manganese whose oxidation state transforms from 0 to +4 via a step-by-step light-driven loss of four electrons and accumulation of four oxidizing equivalents. Each step in the transformation corresponds to a distinct redox state of the water oxidation catalyst, termed S1, S2, S3 and S4. Transformation from S1 to S0 (dark neutral state) completes the cycle and results in the evolution of one molecule of oxygen:

2H2O + [Mn4O5Ca]reduced  → [Mn4O5Ca]oxidized  + 4H+ + 4e + O2

The driving force for the oxidation of water is provided by the Photosystem II reaction center which is coupled to the water oxidation complex. By absorbing a quantum of light the molecule of the primary donor, called P680, in the reaction center launches a transfer of an electron along a series of redox active molecules. As a result the photooxidized primary donor P680+ acquires a strong oxidation potential and removes one electron from the water oxidation catalyst. After four turnovers of the reaction center the cycle of oxidation of two molecules of water and replenishing of the Mn cluster is complete by releasing one molecule of oxygen. This takes about 1 millisecond.

 

Taking snapshots of different states of the water oxidation catalyst

Petra Fromme and colleagues have pioneered the development of a new method for obtaining X-ray crystal structures from nanocrystals of photosynthetic proteins and similar materials (Nature, 470, 73-77, 2011). The technique allows collecting X-ray diffraction “snapshots” from a stream of nanocrystals irradiated by femtosecond X-ray pulses from the DOE Linac coherent light source at SLAC.

In the newly published Nature paper the Fromme group, in collaboration with the Moore group of BISfuel and others, report on advancement of this technique towards characterization of different oxidation states of the natural water splitting catalyst. Since the time for the transition between different S-states in the cycle is known it is possible to prepare the states by exciting the Photosystem II microcrystals by laser pulses at fixed times before they are interrogated by the X-ray pulse.

The serial femtosecond X-ray crystal structure of the putative S3-state of the water oxidation complex that is produced after two laser excitations shows for the first time the structural changes that take place in the metal cluster of the water oxidation complex and its ligand environment. These include shrinking of the metal cluster due to shorter O-Mn distances in the S3 state where all Mn are in the oxidation state +IV, and extension of the distance of the dangler Mn to the Mn3O5Ca cubane. Furthermore, electron density changes of the protein environment are indicative of significant changes in the ligand sphere of the metal cluster, including D170 which may not serve as a ligand in the higher S-states, and D61, which is only a second coordination sphere ligand in the dark state but may coordinate the dangler Mn in the S3 state.

 

Sources:

  1. Kupitz, Christopher; Basu, Shibom; Grotjohann, Ingo; Fromme, Raimund; Zatsepin, Nadia A.; Rendek, Kimberly N.; Hunter, Mark; Shoeman, Robert L.; White, Thomas A.; Wang, Dingjie; James, Daniel; Yang, Jay-How; Cobb, Danielle E.; Brenda, Reeder; Raymond, G. Sierra; Liu, Haiguang; Barty, Anton; Aquila, Andrew L.; Deponte, Daniel; Kirian, Richard A.; Bari, Sadia; Bergkamp, Jesse J.; Beyerlein, Kenneth R.; Bogan, Michael J.; Caleman, Carl; Chao, Tzu-Chiao; Conrad, Chelsie E.; Davis, Katherine M.; Fleckenstein, Holger; Galli, Lorenzo; Hau-Riege, Stefan P.; Kassemeyer, Stephan; Laksmono, Hartawan; Liang, Mengning; Lomb, Lukas; Marchesini, Stefano; Martin, Andrew V.; Messerschmidt, Marc; Milathianaki, Despina; Nass, Karol; Ros, Alexandra; Roy-Chowdhury, Shatabdi; Schmidt, Kevin; Seibert, Marvin; Steinbrener, Jan; Stellato, Francesco; Yan, Lifen; Yoon, Chunhong; Moore, Thomas A.; Moore, Ana L.; Pushkar, Yulia; Williams, Garth J.; Boutet, Sébastien; Doak, R. Bruce; Weierstall, Uwe; Frank, Matthias; Chapman, Henry N.; Spence, John C.H., and Fromme, Petra Serial Time-resolved crystallography of Photosystem II using a femtosecond X-ray laser, Nature, published online July 9, 2014, doi:10.1038/nature13453 (2014)
  1. Chapman, H.N., Fromme, P., Barty, A., White, T.A., Kirian, R.A., Aquila, A., Hunter, M.S., Schulz, J., DePonte, D.P., Weierstall, U., Doak, R.B., Maia, F.R.N.C., Martin, A.V., Schlichting, I., Lomb, L., Coppola, N., Shoeman, R.L., Epp, S.W., Hartmann, R., Rolles, D., Rudenko, A., Foucar, L., Kimmel, N., Weidenspointner, G., Holl, P., Liang, M., Barthelmess, M., Caleman, C., Boutet, S., Bogan, M.J., Krzywinski, J., Bostedt, C., Bajt, S., Gumprecht, L., Rudek, B., Erk, B., Schmidt, C., Homke, A., Reich, C., Pietschner, D., Struder, L., Hauser, G., Gorke, H., Ullrich, J., Herrmann, S., Schaller, G., Schopper, F., Soltau, H., Kuhnel, K.-U., Messerschmidt, M., Bozek, J.D., Hau-Riege, S.P., Frank, M., Hampton, C.Y., Sierra, R. G., Starodub, D., Williams, G.J., Hajdu, J., Timneanu, N., Seibert, M.M., Andreasson, J., Rocker, A., Jonsson, O., Svenda, M., Stern, S., Nass, K., Andritschke, R., Schroter, C.-D., Krasniqi, F., Bott, M., Schmidt, K.E., Wang, X., Grotjohann, I., Holton, J.M., Barends, T.R.M., Neutze, R., Marchesini, S., Fromme, R., Schorb, S., Rupp, D., Adolph, M., Gorkhover, T., Andersson, I., Hirsemann, H., Potdevin, G., Graafsma, H., Nilsson, B., and Spence, J.C.H. (2011) Femtosecond X-ray protein nanocrystallography, Nature, 470, 73-77
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