Tyrozine D oxidation and redox equilibrium in photosystem II

Abstract

Tyrosine D (Tyr) is an auxiliary redox active tyrosine residue in photosystem II (PSII). The mechanism of Tyr oxidation was investigated by EPR spectroscopy, flash-induced fluorescence decay and thermoluminescence measurements in PSII enriched membranes from spinach. PSII membranes were chemically treated with 3mM ascorbate and 1mM diaminodurene and subsequent washing, leading to the complete reduction of Tyr. Tyr oxidation kinetics and competing recombination reactions were measured after a single saturating flash in the absence and presence of DCMU (inhibitor of the Q-site) in the pH range of 4.7-8.5. Two kinetic phases of Tyr oxidation were observed by the time resolved EPR spectroscopy - the fast phase (msec-sec time range) and the pH dependent slow phase (tens of seconds time range). In the presence of DCMU, Tyr oxidation kinetics was monophasic in the entire pH range, i.e. only the fast kinetics was observed. The results obtained from the fluorescence and thermoluminescence analysis show that when forward electron transport is blocked in the presence of DCMU, the SQ recombination outcompetes the slow phase of Tyr oxidation by the S state. Modelling of the whole complex of these electron transfer events associated with Tyr oxidation fitted very well with our experimental data. Based on these data, structural information and theoretical considerations we confirm our assignment of the fast and slow oxidation kinetics to two populations of PSII centers with different water positions (proximal and distal) in the Tyr vicinity.

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

“…We were not able to observe any fast decaying Tyr Z • and the signal was never higher than 100% at all pH values measured. Our conclusion from these measurements is that DPC is an effective donor to Tyr Z • at all three pH values and the addition of ferricyanide only eliminates the recombination reaction from the acceptor side of PS II which takes place at low pH values (Ahmadova et al 2017 ).…”

“…Tyr D • is much less stable in the Mn-depleted PS II than in the active PS II samples with the full O 2 -evolving activity, where reduction of Tyr D • requires application of the reducing agents (Vass and Styring 1991 ; Sjöholm et al 2016 ; Ahmadova et al 2017 ). The dark incubation of our Tris-washed PS II membranes for 10 h at room temperature resulted in the reduction of 90–95% of Tyr D • (Fig.…”

“…This oxidation pathway occurs at low and middle pH values. However, at high pH (above pK a of Tyr D (Vass 1991; Faller et al 2001 ; Ahmadova et al 2017 )), direct Tyr D oxidation by P 680 + was reported in the Mn-depleted preparation in at least half of the PS II centers (Faller et al 2001 ). As a result, partial localization of the electron hole on P D2 Chl was suggested (Faller et al 2001 ).…”

“…On the contrary, Tyr Z is in more hydrophilic surrounding with a cluster of water molecules nearby (Ferreira et al 2004 ; Umena et al 2011 ; Suga et al 2015 ). Interestingly, Tyr D has only a single water molecule nearby which can take two positions (2.6–3.1 and 4.3–4.5 Å) (Umena et al 2011 ; Saito et al 2013 ; Suga et al 2015 ; Sjöholm et al 2016 ; Ahmadova et al 2017 ).…”

The wavelength of the incident light determines the primary charge separation pathway in Photosystem II

“…PS II membranes were dark adapted for 5 min and measurements were performed in the presence or absence of 20 µM DCMU. The variable fluorescence decay traces were recorded with a FL3000 double modulated fluorometer (PSI Photon Systems Instruments, Czech Republic) according to 59 . The first data point was taken 100 μs after the actinic flash.…”