The EPR Spectrum of Tyrosine Z^• and Its Decay Kinetics in O₂-Evolving Photosystem II Preparations

Abstract

The O2-evolving complex of photosystem II, Mn 4Ca, cycles through five oxidation states, S0,..., S4, during its catalytic function, which involves the gradual abstraction of four electrons and four protons from two bound water molecules. The direct oxidant of the complex is the tyrosine neutral radical, YZ(*), which is transiently produced by the highly oxidizing power of the photoexcited chlorophyll species P680. EPR characterization of YZ(*) has been limited, until recently, to inhibited (non-oxygen-evolving) preparations. A number of relatively recent papers have demonstrated the trapping of YZ(*) in O2-evolving preparations at liquid helium temperatures as an intermediate of the S0 to S1, S1 to S2, and S2 to S3 transitions. The respective EPR spectra are broadened and split at g approximately 2 by the magnetic interaction with the Mn cluster, but this interaction collapses at temperatures higher than about 100K [Zahariou et al. (2007) Biochemistry 46, 14335 -14341]. We have conducted a study of the Tyr Z(*) transient in the temperature range 77-240 K by employing rapid or slow EPR scans. The results reveal for the first time high-resolution X-band spectra of Tyr Z(*) in the functional system and at temperatures close to the onset of the S-state transitions. We have simulated the S 2Y Z(*) spectrum using the simulation algorithm of Svistunenko and Cooper [(2004) Biophys. J. 87, 582 -595]. The small g(x) = 2.00689 value inferred from the analysis suggests either a H-bonding of Tyr Z (*) (presumably with His190) that is stronger than what has been assumed from studies of Tyr D(*) or Tyr Z(*) in Mn-depleted preparations or a more electropositive environment around Tyr Z(*). The study has also yielded for the first time direct information on the temperature variation of the YZ(*)/QA(-) recombination reaction in the various S states. The reaction follows biphasic kinetics with the slow phase dominating at low temperatures and the fast phase dominating at high temperatures. It is tentatively proposed that the slow phase represents the action of the YZ(*)/YZ(-) redox couple while the fast phase represents that of the YZ(*)/YZH couple; it is inferred that Tyr Z at elevated temperatures is protonated at rest. It is also proposed that YZ(*)/YZH is the couple that oxidizes the Mn cluster during the S1-S2 and S2-S3 transitions. A simple mechanism ensuring a rapid (concerted) protonation of Tyr Z upon oxidation of the Mn cluster is discussed, and also, a structure-based molecular model suggesting the participation of His190 into two hydrogen bonds is proposed.

Photosystem II: The machinery of photosynthetic water splitting

“…Y z S 2 are in line with the idea that both reactions are triggered by H ? transfer reactions, but markedly differ in the details of the pathways of the proton which is released into the bulk phase in the former transition and resides near the Mn 4 O X Ca cluster in S 2 at a site that is not precisely known (for a discussion, see Ioannidis et al 2008).…”

Role of Hydrogen Bonds in Photosynthetic Water Splitting

“…A recent analysis of the X-band EPR spectrum of state S 2 Y ox Z suggests that the hydrogen bonding of Y Z is stronger and its environment might be more electropositive in PS II complexes with an intact WOC compared with Mn-depleted samples [110]. However, the resolution of the g-values is poor, and no detailed information can be gathered from these data on the hydrogen bonding in native PS II complexes.…”

Designing photosystem II: molecular engineering of photo-catalytic proteins

“…The tyrosine/Mn distance is shorter than other electron pathway cofactors and this is also found in PSII. In PSII the tyrosine acts as an intermediary for the stepwise oxidation of the Mn center by P680 (Ioannidis et al 2008). This is an important consideration in the design of an efficient model of electron transfer with a metal center as the donor group.…”