Rapid formation of the stable tyrosyl radical in photosystem II

Abstract

Two symmetrically positioned redox active tyrosine residues are present in the photosystem II (PSII) reaction center. One of them, TyrZ, is oxidized in the ns-s time scale by P680 ؉ and reduced rapidly (s to ms) by electrons from the Mn complex. The other one, TyrD, is stable in its oxidized form and seems to play no direct role in enzyme function. Here, we have studied electron donation from these tyrosines to the chlorophyll cation (P680 ؉ ) in Mndepleted PSII from plants and cyanobacteria. In particular, a mutant lacking TyrZ was used to investigate electron donation from TyrD. By using EPR and time-resolved absorption spectroscopy, we show that reduced TyrD is capable of donating an electron to P680 ؉ with t 1/2 Ϸ 190 ns at pH 8.5 in approximately half of the centers. This rate is Ϸ10 5 times faster than was previously thought and similar to the TyrZ donation rate in Mn-depleted wild-type PSII (pH 8.5). Some earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD (compared with that from TyrZ) can be reassessed. At pH 6.5, TyrZ (t 1/2 ‫؍‬ 2-10 s) donates much faster to P680 ؉ than does TyrD (t1/2 > 150 s). These different rates may reflect the different fates of the proton released from the respective tyrosines upon oxidation. The rapid rate of electron donation from TyrD requires at least partial localization of P680 ؉ on the chlorophyll (P D2) that is located on the D2 side of the reaction center.T yrosyl radicals play key roles in the mechanisms of a wide range of enzymes. Understanding these roles and how proteins are able to control these reactive species to carry out quite specific chemical reactions has been an important aim of researchers in this area (for review, see ref. 1). One of the earliest demonstrations of redox active tyrosines was in photosystem II (PSII), the water oxidizing enzyme, in which a tyrosyl radical, designated tyrosine Z ⅐ (TyrZ ⅐ ), is thought to play a key role in the active site, abstracting electrons and possibly protons from the substrate water that is bound to the highly oxidized Mn cluster (2-5).PSII contains a second tyrosyl radical, TyrD ⅐ , which is stable during enzyme function and which is located in a 2-fold rotationally symmetrical position to TyrZ on a subunit (D2) adjacent to that (D1) in which water oxidation takes place. TyrZ and TyrD are equally situated relative to the central chlorophyll (Chl) pair (P D1 and P D2 ) with the center-to-center distances for TyrZ-P D1 and TyrD-P D2 being Ϸ12.4 Å (6). It is this pair of Chls that bears the photogenerated cation (P680 ϩ ) that is considered to be the oxidant for the tyrosines (refs. 6-12; for recent reviews see refs. 2 and 3). The existing enzyme may have evolved from an ancestor in which the core was a homodimer with the two tyrosines having identical redox functions (13).Despite homologous positions in subunits D1 and D2, TyrZ and TyrD exhibit extremely different kinetics, different redox potentials, and play completely different functional roles in the enzyme. Thus, they con...

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