Formation of tyrosine radicals in photosystem II under far-red illumination

Nigar Ahmadova,Fikret Mamedov

doi:10.1007/s11120-017-0442-3

Abstract

Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P680. TyrZ, part of the water-oxidizing complex, is a preferential fast electron donor while TyrD is a slow auxiliary donor to P680+. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of TyrD occurs via equilibrium with TyrZ• at pH 4.7 and 6.3. At pH 8.5 direct TyrD oxidation by P680+ occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P680+) on the ChlD1 in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested.

Highlights

Solar energy is successfully utilized by plants, algae, and bacteria in the process called photosynthesis
Solar energy is converted to chemical energy in the form of carbohydrates and O 2 is released as a byproduct (Kern and Renger 2007; Renger and Renger 2008; Vinyard et al 2013)
The active photosystem II (PS II) complex is made from 25 protein subunits and host a chain of the redoxactive cofactors involved in the key water oxidation reaction and subsequent electron transfer (Umena et al 2011; Wei et al 2016)

Summary

Introduction

Solar energy is successfully utilized by plants, algae, and bacteria in the process called photosynthesis. The active PS II complex is made from 25 protein subunits and host a chain of the redoxactive cofactors involved in the key water oxidation reaction and subsequent electron transfer (Umena et al 2011; Wei et al 2016). These cofactors are bound by the PS II central core which is composed of the D 1 and D 2 proteins, the inner pigment–protein antenna complexes CP43 and CP47, Cyt b559, and several low molecular weight essential subunits (Danielsson et al 2006; Umena et al 2011; Suga et al 2015).

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Conclusion