Abstract
The active site for water oxidation in Photosystem II (PSII) consists of a Mn4CaO5 cluster close to a redox-active tyrosine residue (TyrZ). The enzyme cycles through five sequential oxidation states, from S0 to S4, in the water splitting process. O2 evolution occurs in the final S3Tyrz• to S0TyrZ transition. Chloride is also involved in this mechanism. By using PSII from Thermosynechococcus elongatus in which both Ca and Cl have been substituted for Sr and Br, in order to slow down the S3TyrZ• to S0TyrZ + O2 transition, with a t1/2 ∼ 5 ms at room temperature, it is shown that the kinetics of the recovery of a functional S0 has a t1/2 also close to 5 ms. It is suggested that, similarly, the reformation of a functional S0 state follows the S3TyrZ• to S0TyrZ + O2 transition in CaCl-PSII and that the insertion of a new substrate molecule of water (O5) and protons does not require further delay.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
alain.boussac{at}laposte.net
julien.selles{at}ibpc.fr
miwa.sugiura{at}ehime-u.ac.jp
Abbreviations
- PSII
- Photosystem II
- Chl
- chlorophyll
- ChlD1/ChlD2,
- monomeric Chl on the D1 or D2 side, respectively;
- MES
- 2-(N-morpholino) ethanesulfonic acid
- P680
- primary electron donor
- PD1 and PD2
- individual Chl on the D1 or D2 side, respectively, which constitute a pair of Chl with partially overlapping aromatic rings
- PheD1 and PheD2
- pheophytin on the D1 or D2 side, respectively
- PPBQ
- phenyl p–benzoquinone
- QA
- primary quinone acceptor
- QB
- secondary quinone acceptor
- TyrZ
- the tyrosine 161 of D1 acting as the electron donor to P680
- WT*3
- T. elongatus mutant strain deleted psbA1 and psbA2 genes and with a His-tag on the carboxy terminus of CP43. DCMU, 3-(3,4-dichlorophényl)-1,1-diméthyl-urée
- XFEL
- serial femtosecond X-ray free electron laser crystallography.
