The protonation state around TyrD/TyrD•in photosystem II is reflected in its biphasic oxidation kinetics.
The tyrosine residue D2-Tyr160 (TyrD) in photosystem II (PSII) can be oxidized through charge equilibrium with the oxygen evolving complex in PSII. The kinetics of the electron transfer from TyrDhas been followed using time-resolved EPR spectroscopy after triggering the oxidation of pre-reduced TyrDby a short laser flash. After its oxidation TyrDis observed as a neutral radical (TyrD•) indicating that the oxidation is coupled to a deprotonation event. The redox state of TyrDwas reported to be determined by the two water positions identified in the crystal structure of PSII [Saito et al. (2013) Proc. Natl. Acad. Sci. USA 110, 7690]. To assess the mechanism of the proton coupled electron transfer of TyrDthe oxidation kinetics has been followed in the presence of deuterated buffers, thereby resolving the kinetic isotope effect (KIE) of TyrDoxidation at different H/D concentrations. Two kinetic phases of TyrDoxidation - the fast phase (msec-sec time range) and the slow phase (tens of seconds time range) were resolved as was previously reported [Vass and Styring (1991) Biochemistry 30, 830]. In the presence of deuterated buffers the kinetics was significantly slower compared to normal buffers. Furthermore, although the kinetics were faster at both high pH and pD values the observed KIE was found to be similar (~2.4) over the whole pL range investigated. We assign the fast and slow oxidation phases to two populations of PSII centers with different water positions, proximal and distal respectively, and discuss possible deprotonation events in the vicinity of TyrD.
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden.
Johannes
J
Sjöholm
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden. Electronic address: felix.ho@kemi.uu.se.
Felix
F
Ho
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden.
Nigar
N
Ahmadova
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden.
Katharina
K
Brinkert
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden.
Leif
L
Hammarström
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden. Electronic address: fikret.mamedov@kemi.uu.se.
Fikret
F
Mamedov
Molecular Biomimetics, Department of Chemistry - Ångström Laboratory, Uppsala University, P.O. Box 523, SE 751 20 Uppsala, Sweden.
Stenbjörn
S
Styring
D003903:Deuterium / Q000378:metabolism; D004579:Electron Transport / Q000502:physiology; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D011522:Protons; D014443:Tyrosine / Q000378:metabolism*; D014867:Water / Q000378:metabolism
D045332:Photosystem II Protein Complex; D011522:Protons; D014867:Water; D014443:Tyrosine; D003903:Deuterium
Deuterium isotope effect; Electron transfer; Photosystem II; Proton transfer; Tyrosine D