One step closer to O2 R. David Britt and Paul H. Oyala Science 345, 736 (2014); DOI: 10.1126/science.1258008
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of August 14, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6198/736.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/345/6198/736.full.html#related This article cites 12 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/345/6198/736.full.html#ref-list-1 This article appears in the following subject collections: Biochemistry http://www.sciencemag.org/cgi/collection/biochem
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on August 15, 2014
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INSIGHTS | P E R S P E C T I V E S
BIOCHEMISTRY
Mn(V)-oxo intermediate
One step closer to O2
Yz
A crucial step in photosynthesis is becoming clearer
S4
Yz•
IV
P
Department of Chemistry, University of California, Davis, CA 95616, USA. E-mail: [email protected]
736
O2
IV
IV
Direct production of O2
S3Yz•
Yz
V
By R. David Britt and Paul H. Oyala
hotosynthetic organisms such as plants use the protein complex photosystem II (PS II) to split water using the energy of photons, generating molecular oxygen as a by-product. This photon-powered chemistry is a paradigm for developing catalysts for solar-generated fuels. Recent x-ray structures (1) have provided a detailed picture of the water-splitting 4 Mn−1 Ca cluster of PS II (see the figure); this cluster goes through a number of states during photosynthesis. On page 804 of this issue, Cox et al. (2) present electron paramagnetic resonance (EPR) results providing details of the state that is only one photon away from producing O2. The results get us closer to understanding this key reaction. Oxidation of two bound water molecules in PS II requires transfer of four electrons. This transfer occurs one electron at a time, starting with chlorophyll-based photooxidation of a tyrosine. The resulting tyrosine radical, YZ•, in turn oxidizes the Mn cluster in a series of intermediates, S0 to S4. O2 is released 1 ms after the highly oxidized S4 state is formed. All but the short-lived S4 state have been trapped at low temperatures, allowing detailed study of these states (3, 4). EPR spectroscopy is most commonly performed on chemical systems with an odd number of unpaired electrons, as in the S0 and S2 states. Species with an even number of electrons—such as the S3 state—are more difficult to study with EPR, because they may be either diamagnetic (total spin S = 0) and hence invisible to EPR, or have a nonzero integer spin (S = 1, 2, etc.). In the latter case, the interactions of multiple electron spins (zero field splitting, ZFS) may broaden or even eliminate transitions. The effects of ZFS may be mitigated by using higher microwave resonance frequencies. To study the integer-spin (S = 3) S3 state, Cox et al. use a frequency 10 times that previously used. Instead of the very broad EPR signal
H2 O
S0
III
IV
IV
IV IV
Yz
IV
S4 Oxygen Hydrogen Manganese
III
III
H2 O O•
IV
IV
Calcium
IV
Water
IV
Mn(IV)-oxyl radical intermediate
Three possible paths. Starting from the S3 state characterized by Cox et al. and the tyrosine radical, YZ•, there are three possible mechanisms for the final step of O2 evolution. Dashed lines denote the location of the electron hole.
previously assigned to the S3 state (5), they obtain a very well resolved signal centered nearer the free electron resonance. Analysis of the line shape of the S3 signal reveals a small ZFS, comparable to that of a synthetic Mn(IV)3CaO4 complex (6) and less than would be observed if S3 contained one or more Mn(III) ions. High-resolution pulse EPR experiments confirm a six-coordinate Mn(IV) assignment for all four Mn ions. Computational modeling supports an “open cubane” structure in the S3 state (see the figure). This structure closely matches that proposed by Siegbahn (7). The all-Mn(IV) assignment of the S3 state has important implications for the mechanism of O2 formation (see the figure). S3 and the tyrosine radical YZ• may be the immediate precursors to O2, as described in the H-atom abstraction model (8). This simple model fell out of favor when highresolution x-ray structures showed that YZ• forms hydrogen bonds with water molecules of the redox-inert Ca2+ rather than a redox-active Mn ion. Alternatively, further oxidation of one Mn atom would give an S4 state, generating a single Mn(V) species that could form an O-O bond with a Ca2+bound water or hydroxide (9). Such a highspin Mn(V)-O species is unknown in other
biological contexts but has been synthesized (10). Finally, in the transient S4 state, the electron hole may be localized on a substrate O atom, as in the water-splitting mechanism in (7). It may be possible to discriminate between these mechanisms with time-resolved x-ray spectroscopy and diffraction (11, 12). With Cox et al.’s study giving a new description of the structure of the S3 state, only one step removed from O2 formation, we are rapidly closing on a detailed picture of this crucial water-splitting reaction. ■ REFERENCES
1. Y. Umena, K. Kawakami, J. R. Shen, N. Kamiya, Nature 473, 55 (2011). 2. N. Cox et al., Science 345, 804 (2014). 3. J. Yano, V. Yachandra, Chem. Rev. 114, 4175 (2014). 4. N. Cox, D. A. Pantazis, F. Neese, W. Lubitz, Acc. Chem. Res. 46, 1588 (2013). 5. A. Boussac, M. Sugiura, A. W. Rutherford, P. Dorlet, J. Am. Chem. Soc. 131, 5050 (2009). 6. S. Mukherjee et al., Proc. Natl. Acad. Sci. U.S.A. 109, 2257 (2012). 7. P. E. M. Siegbahn, Biochim. Biophys. Acta 1827, 1003 (2013). 8. C. W. Hoganson, G. T. Babcock, Science 277, 1953 (1997). 9. J. Limburg, V. A. Szalai, G. W. Brudvig, J. Chem. Soc., Dalton Trans. 1999, 1353 (1999). 10. T. Taguchi et al., J. Am. Chem. Soc. 134, 1996 (2012). 11. J. Kern et al., Science 340, 491 (2013). 12. C. Kupitz et al., Nature 10.1038/nature13453 (2014). 10.1126/science.1258008
sciencemag.org SCIENCE
15 AUGUST 2014 • VOL 345 ISSUE 6198
Published by AAAS
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of August 14, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6198/736.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/345/6198/736.full.html#related This article cites 12 articles, 4 of which can be accessed free: http://www.sciencemag.org/content/345/6198/736.full.html#ref-list-1 This article appears in the following subject collections: Biochemistry http://www.sciencemag.org/cgi/collection/biochem
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on August 15, 2014
This copy is for your personal, non-commercial use only.
INSIGHTS | P E R S P E C T I V E S
BIOCHEMISTRY
Mn(V)-oxo intermediate
One step closer to O2
Yz
A crucial step in photosynthesis is becoming clearer
S4
Yz•
IV
P
Department of Chemistry, University of California, Davis, CA 95616, USA. E-mail: [email protected]
736
O2
IV
IV
Direct production of O2
S3Yz•
Yz
V
By R. David Britt and Paul H. Oyala
hotosynthetic organisms such as plants use the protein complex photosystem II (PS II) to split water using the energy of photons, generating molecular oxygen as a by-product. This photon-powered chemistry is a paradigm for developing catalysts for solar-generated fuels. Recent x-ray structures (1) have provided a detailed picture of the water-splitting 4 Mn−1 Ca cluster of PS II (see the figure); this cluster goes through a number of states during photosynthesis. On page 804 of this issue, Cox et al. (2) present electron paramagnetic resonance (EPR) results providing details of the state that is only one photon away from producing O2. The results get us closer to understanding this key reaction. Oxidation of two bound water molecules in PS II requires transfer of four electrons. This transfer occurs one electron at a time, starting with chlorophyll-based photooxidation of a tyrosine. The resulting tyrosine radical, YZ•, in turn oxidizes the Mn cluster in a series of intermediates, S0 to S4. O2 is released 1 ms after the highly oxidized S4 state is formed. All but the short-lived S4 state have been trapped at low temperatures, allowing detailed study of these states (3, 4). EPR spectroscopy is most commonly performed on chemical systems with an odd number of unpaired electrons, as in the S0 and S2 states. Species with an even number of electrons—such as the S3 state—are more difficult to study with EPR, because they may be either diamagnetic (total spin S = 0) and hence invisible to EPR, or have a nonzero integer spin (S = 1, 2, etc.). In the latter case, the interactions of multiple electron spins (zero field splitting, ZFS) may broaden or even eliminate transitions. The effects of ZFS may be mitigated by using higher microwave resonance frequencies. To study the integer-spin (S = 3) S3 state, Cox et al. use a frequency 10 times that previously used. Instead of the very broad EPR signal
H2 O
S0
III
IV
IV
IV IV
Yz
IV
S4 Oxygen Hydrogen Manganese
III
III
H2 O O•
IV
IV
Calcium
IV
Water
IV
Mn(IV)-oxyl radical intermediate
Three possible paths. Starting from the S3 state characterized by Cox et al. and the tyrosine radical, YZ•, there are three possible mechanisms for the final step of O2 evolution. Dashed lines denote the location of the electron hole.
previously assigned to the S3 state (5), they obtain a very well resolved signal centered nearer the free electron resonance. Analysis of the line shape of the S3 signal reveals a small ZFS, comparable to that of a synthetic Mn(IV)3CaO4 complex (6) and less than would be observed if S3 contained one or more Mn(III) ions. High-resolution pulse EPR experiments confirm a six-coordinate Mn(IV) assignment for all four Mn ions. Computational modeling supports an “open cubane” structure in the S3 state (see the figure). This structure closely matches that proposed by Siegbahn (7). The all-Mn(IV) assignment of the S3 state has important implications for the mechanism of O2 formation (see the figure). S3 and the tyrosine radical YZ• may be the immediate precursors to O2, as described in the H-atom abstraction model (8). This simple model fell out of favor when highresolution x-ray structures showed that YZ• forms hydrogen bonds with water molecules of the redox-inert Ca2+ rather than a redox-active Mn ion. Alternatively, further oxidation of one Mn atom would give an S4 state, generating a single Mn(V) species that could form an O-O bond with a Ca2+bound water or hydroxide (9). Such a highspin Mn(V)-O species is unknown in other
biological contexts but has been synthesized (10). Finally, in the transient S4 state, the electron hole may be localized on a substrate O atom, as in the water-splitting mechanism in (7). It may be possible to discriminate between these mechanisms with time-resolved x-ray spectroscopy and diffraction (11, 12). With Cox et al.’s study giving a new description of the structure of the S3 state, only one step removed from O2 formation, we are rapidly closing on a detailed picture of this crucial water-splitting reaction. ■ REFERENCES
1. Y. Umena, K. Kawakami, J. R. Shen, N. Kamiya, Nature 473, 55 (2011). 2. N. Cox et al., Science 345, 804 (2014). 3. J. Yano, V. Yachandra, Chem. Rev. 114, 4175 (2014). 4. N. Cox, D. A. Pantazis, F. Neese, W. Lubitz, Acc. Chem. Res. 46, 1588 (2013). 5. A. Boussac, M. Sugiura, A. W. Rutherford, P. Dorlet, J. Am. Chem. Soc. 131, 5050 (2009). 6. S. Mukherjee et al., Proc. Natl. Acad. Sci. U.S.A. 109, 2257 (2012). 7. P. E. M. Siegbahn, Biochim. Biophys. Acta 1827, 1003 (2013). 8. C. W. Hoganson, G. T. Babcock, Science 277, 1953 (1997). 9. J. Limburg, V. A. Szalai, G. W. Brudvig, J. Chem. Soc., Dalton Trans. 1999, 1353 (1999). 10. T. Taguchi et al., J. Am. Chem. Soc. 134, 1996 (2012). 11. J. Kern et al., Science 340, 491 (2013). 12. C. Kupitz et al., Nature 10.1038/nature13453 (2014). 10.1126/science.1258008
sciencemag.org SCIENCE
15 AUGUST 2014 • VOL 345 ISSUE 6198
Published by AAAS
