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Two tyrosines that changed the world: Interfacing the oxidizing power of photochemistry to water splitting in photosystem II
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
2012 (English)In: Biochimica et Biophysica Acta - Bioenergetics, ISSN 0005-2728, E-ISSN 1879-2650, Vol. 1817, no 1, 76-87 p.Article, review/survey (Refereed) Published
Abstract [en]

Photosystem II (PSII), the thylakoid membrane enzyme which uses sunlight to oxidize water to molecular oxygen, holds many organic and inorganic redox cofactors participating in the electron transfer reactions. Among them, two tyrosine residues, Tyr-Z and Tyr-D are found on the oxidizing side of PSII. Both tyrosines demonstrate similar spectroscopic features while their kinetic characteristics are quite different. Tyr-Z, which is bound to the D1 core protein, acts as an intermediate in electron transfer between the primary donor, P(680) and the CaMn(4) cluster. In contrast, Tyr-D, which is bound to the D2 core protein, does not participate in linear electron transfer in PSII and stays fully oxidized during PSII function. The phenolic oxygens on both tyrosines form well-defined hydrogen bonds to nearby histidine residues, His(Z) and His(D) respectively. These hydrogen bonds allow swift and almost activation less movement of the proton between respective tyrosine and histidine. This proton movement is critical and the phenolic proton from the tyrosine is thought to toggle between the tyrosine and the histidine in the hydrogen bond. It is found towards the tyrosine when this is reduced and towards the histidine when the tyrosine is oxidized. The proton movement occurs at both room temperature and ultra low temperature and is sensitive to the pH. Essentially it has been found that when the pH is below the pK(a) for respective histidine the function of the tyrosine is slowed down or, at ultra low temperature, halted. This has important consequences for the function also of the CaMn(4) complex and the protonation reactions as the critical Tyr-His hydrogen bond also steer a multitude of reactions at the CaMn(4) cluster. This review deals with the discovery and functional assignments of the two tyrosines. The pH dependent phenomena involved in oxidation and reduction of respective tyrosine is covered in detail. This article is part of a Special Issue entitled: Photosystem II.

Place, publisher, year, edition, pages
2012. Vol. 1817, no 1, 76-87 p.
Keyword [en]
Photosystem II, Water oxidation, Tyrosine Z, Tyrosine D
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:uu:diva-168512DOI: 10.1016/j.bbabio.2011.03.016ISI: 000298823000007OAI: oai:DiVA.org:uu-168512DiVA: diva2:499991
Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Trapping Tyrosine Z: Exploring the Relay between Photochemistry and Water Oxidation in Photosystem II
Open this publication in new window or tab >>Trapping Tyrosine Z: Exploring the Relay between Photochemistry and Water Oxidation in Photosystem II
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photosystem II is unique! It remains the only enzyme that can oxidize water using light as energy input. Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P680+ via the tyrosine residue D1-Tyr161 (YZ). Favorable oxidation of YZ is coupled to a proton transfer along a hydrogen bond to the nearby D1-His190 residue, resulting in the neutral radical YZ. By illuminating photosystem II at cryogenic temperatures, YZ can be trapped in a stable state. Magnetic interaction between this radical and the CaMn4 cluster gives rise to a split electron paramagnetic resonance (EPR) signal with characteristics that depend on the oxidation state (S state) of the cluster.

The mechanism by which the split EPR signals are formed is different depending on the S state. In the S0 and S1 states, split signal induction proceeds via a P680+-centered mechanism, whereas in the S2 and S3 states, our results show that split induction stems from a Mn-centered mechanism. This S state-dependent pattern of split EPR signal induction can be correlated to the charge of the CaMn4 cluster in the S state in question and has prompted us to propose a general model for the induction mechanism across the different S states. At the heart of this model is the stability or otherwise of the YZ–(D1-His190)+ pair during cryogenic illumination. The model is closely related to the sequence of electron and proton transfers from the cluster during the S cycle.

Furthermore, the important hydrogen bond between YZ and D1-His190 has been investigated by following the split EPR signal formation in the different S states as a function of pH. All split EPR signals investigated decrease in intensity with a pKa of ~4-5. This pKa can be correlated to a titration event that disrupts the essential hydrogen bond, possibly by a direct protonation of D1-His190.  This has important consequences for the function of the CaMn4 cluster as this critical YZ–D1-His190 hydrogen bond steers a multitude of reactions at the cluster.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 943
Keyword
Photosystem II, Tyrosine Z, EPR, proton-coupled electron transfer, hydrogen bond, pH
National Category
Biochemistry and Molecular Biology Biophysics
Identifiers
urn:nbn:se:uu:diva-173575 (URN)978-91-554-8390-6 (ISBN)
Public defence
2012-06-15, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-25 Created: 2012-04-27 Last updated: 2012-08-01Bibliographically approved

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Styring, StenbjörnSjöholm, JohannesMamedov, Fikret

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