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GE & Science Preis für Dr. Bernhard Loll

Dr. Bernhard Loll, der am 10.2.2005 in der Arbeitsgruppe von Prof. Wolfram Saenger promoviert hat mit dem Thema:

Photosystem II aus dem Cyanobacterium Thermosynechococcus elongatus bei 3.2 Å Auflösung

hat im Dezember 2006 in Stockholm den "GE & Science Prize for Young Life Scientists" erhalten.

GE Healthcare, formerly Amersham Biosciences, and Science/AAAS have joined forces in creating the GE & Science Prize for Young Life Scientists. Since 1995, the aim of the prize has been to recognize outstanding Ph.D. graduate students from around the world and reward their research in the field of molecular biology. Both Science/AAAS and GE Healthcare believe that support of promising scientists at the beginning of their careers is critical for continued scientific progress. Each year, the grand prize winner receives a prize of US-$ 25,000, while runners-up receive prizes of US-$ 5,000. The staffs of both GE Healthcare and Science/AAAS salute the efforts of past winners and look forward to research findings from future entrants.

Für die Preisverleihung mußte Dr. Loll einen Essay schreiben:

Photosystem II, a bioenergetic nanomachine

Bernhard Loll

Photosynthesis is one of the most fundamental bioenergetic processes on our planet. At the heart of photosynthesis is photosystem II (PSII), which catalyses the thermodynamically most demanding reaction in biological systems, the splitting of water into oxygen and reducing equivalents. This process is driven by solar energy, which is captured by pigments (chlorophyll a and carotenoids) embedded within PSII. Photosynthesis by cyanobacteria the ancestors of higher plants is thought to have been the reason why the oxygen level in the primeval earth's reducing atmosphere rose to the point at which higher forms of life could develop about 2 billion years ago. Additionally, the reducing equivalents are necessary to fix carbon dioxide to organic molecules that lie at the basis of nearly all food chains and the electrochemical gradient generated across the thylakoid membrane powers the production of the energy-storing molecule ATP (1).

A prerequisite for understanding how PSII functions is knowledge of its precise three-dimensional structure. Despite intensive research in this field, fundamental functional aspects, such as protein-cofactor interactions and the structure of the catalytic site, have remained elusive.

At the start of my graduate research, a low resolution structure of dimeric PSII had been published which revealed merely the overall arrangement of the protein scaffold and a limited number of cofactors, but could not assign the amino acid sequence nor show the exact location and orientation of cofactors (2). In order to obtain crystals diffracting to higher resolution, I attempted to perfect both, the biochemical preparation of the light-sensitive membrane protein and the quality of its crystals. The sensitivity of the crystals to X-ray radiation necessitated the development of an X-ray diffraction data collection strategy, which was the key to collect data to high resolution and simultaneously to lower the radiation dose. Out of the about 3000 crystals analysed, two diffracted to high resolution (3.0 Å), which allowed me to elucidate the architecture of cyanobacterial PSII (3). The monomer of this bewilderingly complex machinery consists of 20 different protein subunits that bind 77 organic cofactors and 7 metal ions. In the reaction centre and light-harvesting antenna proteins, 35 chlorophyll a, 11 carotenoid, 2 pheophytin a, 2 plastoquinone, 2 haem, and 1 bicarbonate molecule(s) as well as 4 manganese, 2 calcium ions and an iron ion are held at precise distances and relative orientations that are required for optimal absorption and conversion of light energy to perform efficient electron transfer. The detection of electron density from the heretofore unidentified 14 lipids and 3 detergent molecules located within the protein-subunits led to a new and interesting discussion of their function. Similarly, I proposed a lipophilic pathway for diffusion of secondary plastoquinone that transfers redox equivalents from PSII to the photosynthetic chain and answers the long-standing question of how the hydrophobic plastoquinone diffuses through the entire complex. Based on my observation that lipids are located in the vicinity of quinone binding niches in several other non-homologous membrane proteins (photosystem I, cytochrome b6f- and cytochrome bc1-complex), I proposed this to be a general structural feature. The assignment of 11 carotenoids gives a clear picture of the energy transfer and photo-protection mechanisms in PSII, which could generate harmful oxidizing intermediates.

The catalytic site of PSII is the oxygen evolving centre which harbours a unique redox-active metal cluster composed of four mixed-valence manganese ions, one calcium ion and probably a closely associated chloride ion (1). Without doubt this metal cluster is the holy grail for many scientists engaged in photosynthesis research and has been investigated by a wide array of spectroscopic and biochemical methods. Oxidation of water at the Mn4Ca-cluster occurs in four steps (S-states) described by the Kok-cycle (4), at each of which the Mn4Ca complex is oxidized to a higher oxidation state and, after the fourth step, molecular oxygen is released.

To identify and confirm the position of the metal ions, I collected anomalous X-ray diffraction data. Due to the physical limits of resolution, the Mn-Mn and Mn-Ca distances are not resolved. Consequently I fitted distances derived from Extended X-ray Absorption Fine Structure experiments. The arrangement of the Mn4Ca-cluster can be best approximated by the shape of a "hook", corresponding to "3+1" models as suggested earlier for the Mn-cluster (5), that shows considerable differences in the architecture compared to the postulated cubane-like model (6). Most of the cations in the Mn4Ca-cluster are -oxo bridged and stabilized in bidentate mode by carboxylate groups of amino acids. Even though some other potential ligands are not close enough to directly coordinate the Mn4Ca-cluster in the oxidation state (S1-state) present in the crystals, they might be suitable for interaction through water molecules or may provide ligation in other S-states during the photosynthetic cycle. An additional difficulty in elucidating the structure of the oxygen evolving complex is the reduction of manganese ions possibly associated with structural changes due to disruption of -oxo bridges. Stimulated by this observation, we further investigated this question and could prove that Mn(II) is indeed accumulated under the conditions used to collect X-ray crystallographic data (7). Consequently, reduction of metal ions during X-ray data collection might be of general significance for PSII and other metallo-enzymes. The described details in the structure and amino acid ligands of the Mn4Ca-cluster, highlight the limitations and discrepancies in the earlier low resolution structures (2, 6, 8, 9) due to the positional uncertainties of metal ions and difficulties in the assignment of the primary sequence.

Combining results of dichroism in Polarized Extended X-ray Absorption Fine Structure experiments and our X-ray diffraction data, we were able to derive the architecture of the water splitting site for the S1-state, including -oxo bridges and amino acid ligands (10). The X-ray structure, supplemented with recent spectroscopic results (10-12), is a solid foundation for a more comprehensive understanding of the catalytic mechanism of water oxidation. This is certainly at the top of the wish list of many molecular enzymologists and bioinorganic chemists engaged in photosynthesis research. In addition, the PSII structure will stimulate investigations on artificial photo-catalysts for water splitting.

References

  1. T. Wydrzynski, K. Satoh, Photosystem II: The light-driven water:plastoquinone oxidoreductase, Advances in Photosynthesis and Respiration Series, vol. 22 (Springer, Dordrecht, Netherlands 2005).
  2. A. Zouni et al., Nature 409, 739 (2001).
  3. B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature 438, 1040 (2005).
  4. B. Kok, B. Forbush, M. McGloin, Photochem Photobiol 11, 457 (1970).
  5. T. G. Carrell, A. M. Tyryshkin, G. C. Dismukes, J Biol Inorg Chem 7, 2 (2002).
  6. K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science 303, 1831 (2004).
  7. J. Yano et al., Proc Natl Acad Sci U S A 102, 12047 (2005).
  8. N. Kamiya, J. R. Shen, Proc Natl Acad Sci U S A 100, 98 (2003).
  9. J. Biesiadka, B. Loll, J. Kern, K.-D. Irrgang, A. Zouni, Phys Chem Chem Phys 6, 4733 (2004).
  10. J. Yano et al., Science 314, 821 (2006).
  11. M. Haumann et al., Science 310, 1019 (2005).
  12. B. A. Barry et al., Proc Natl Acad Sci U S A 103, 7288 (2006).

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