Function of Redox-Active Tyrosine in Photosystem II

ABSTRACT

Water oxidation at photosystem II Mn-cluster is mediated by means of the redox-active tyrosine Y^sub Z^ We calculated the redox potential (E^sub m^) of Y^sub Z^ and its symmetrical counterpart Y^sub D^ by the agency of solving the linearized Poisson-Boltzmann equation. The calculated E^sub m^(Y^sup ^/Y^sup -^) were +926 mV/-694 mV for Y^sub Z^/Y^sub D^ with the Mn-cluster in S2 state. Together with the asymmetric position of the Mn-cluster relative to Y^sub Z/D^ differences in H-bond network between Y^sub Z^ (Y^sub Z^/D1-His^sup 190^/D1-Asn^sup 298^) and Y^sub D^ (Y^sub D^/D2-His^sup 189^/D2-Arg^sup 294^/CP47-Glu^sup 364^) are crucial for E^sub m^(Y^sub Z/D^) When D1-His^sup 190^ is protonated, corresponding to a thermally activated state, the calculated E^sub m^(Y^sub Z^) was +1216 mV which is as high as the E^sub m^ for P^sub D1/D2^ We observ deprotonation at CP43-Arg^sup 357^ on S-state transition, which may advise its involvement in the proton exit pathway. E^sub m^(Y^sub D^) was affected by dint of formation of P^sup +^^sub D2^ (but not P^sup +^^sub D1^) and sensitive to the protonation state of D2-Arg^sup 180^ This points to an electrostatic link between Y^sub D^ and P^sub D2^

INTRODUCTION

O2 upon earth is generated by water oxidation at the Mn-cluster of the photosynthetic protein-pigment manifold photosystem II (PSII). The photosynthetic reaction in PSII is initialized by dint of light absorption, resulting in electronic excitation that is ultimately changeed to chemical potential by a charge separation proces at the P680 chlorophyll a (ChIa) of the PSII reaction center (RC) Charge separation leads to formation of an oxidized positively charged radical P680^sup +^ while the released electron travels along the electron transfer (ET) chain. The ET chain is located in the D1/D2-chain harboring a quasi-dimer ChIa (Po 1/02) and pairs of accessory ChIa (Chl^sub D1/D2^) of pheophytina (Pheo^sub D1/D2^) and of plastoquinone (Q^sub A/B^). In intact PSII, P680^sup +^ is reduc by means of the redox-active tyrosine D1-Tyr^sup 161^ (Yz) which is subsequently reduc through an electron from the Mn-cluster. An overview of the location of these cofactors in the consummate PSII protein complex is shown in Fig. 1

Sequential excitations of P680 drive the redox state of the Mn-cluster from the lowest in like manner to the highest oxidized state S4 Release of O2 as a produce of water oxidation is associated with transition from S4 to in like manner that completes the redox circle of time Y7 possesses D1-His^sup 190^ while the symmetrical counterpart D2-Tyr^sup 160^ (YD) possesse D2-His^sup 189^ as H-bond partner (Fig. 2) The apparent proximity of Y^sub Z^ to the Mncluster (edge-to-edge distance between Mn-cluster and Y^sub Z^ is 5 [Angstrom] (1)) and its redox-activity supports its significant part in water oxidation at the Mn-cluster. It is a matter of debate whether the part of Y^sub Z^ in water oxidation is to function as inflammable air abstractor (2) or electrostatic promoter (3) The exit pathway of protons released on water oxidation (4) as supported by means of the recent crystal structure of PSII (156) is a of recent origin aspect to be considered. The other tyrosine Y^sub D^ is 28 [Angstrom] from the Mn-cluster (1) and can therefore not directly be involved in water oxidation occurring at the Mn-cluster. Nevertheless, it plays an important part in tuning the energy of the charge state of P680 and of the S-states of the Mn-cluster before ET from Y^sub Z^ to P680 as has been discussed (7-9)

There are, in principle, three possible mechanistic patterns for the involvement of tyrosine in the redox reaction between P680 and Mn-cluster. These are the following redox reactions with corresponding redox pairs (notation: YH protonated neutral species; Y^sup -^ deprotonated anion; YH^sup +^ protonated cationic radical; Y^sup ^ deprotonated neutral radical):

1 Y^sup -^ [arrow right] Y^sup ^ + e^sup -^ involving Y^sup ^/Y^sup -^ (E^sub m^(Y^sup ^/Y^sup -^) = +680 mV measured for N-acetyl-^sub L^-tyrosinamide).

2 YH [arrow right] Y^sup ^ + H+ + e^sup -^ involving Y^sub ^/YH with pKared of 96 referring to YH (E^sub m^(Y^sup ^/YH) = +970 mV measured for N-acetyl-^sup L^-tyrosinamide in aqueous solution at pH 50)

3 YH [arrow right] YH^sup +^ + e^sup -^ involving YH^sup +^/YH with pKa^sub ox^ of-2 referring to YH^sup +^ (E^sub m^(YH^sup +^/YH) = + 1380 mV measured for N-acetyl-^sub L^-tyrosinamide; reviewed in (10))

In the following, the pKa for tyrosine is generally used as pKa^sub red^ if not otherwise specified. In aqueous solution, the availability of these three possible redox reactions hangs predominantly on pH. In a protein, these reactions can also be controll by dint of its environment as, for instance, H-bond network, neighborhood of charged residues, and cofactors.

For efficient function of PSII, E^sub m^(Y^sub Z^) should lie between the E^sub m^ values of P680 (+ 1110-1300 mV) (1112) and water ( + 820 mV) Based upon the equilibrium constants of the redox reactions involving Y^sub Z^ and Y^sub D^ and the pH-dependence of these redox reactions, the E^sub m^(Y^sub Z^) and E^sub m^(Y^sub D^) were estimated to be ~ + 970 mV (13) and +720 to +760 mV (1314) respectively. upon the other hand, equilibrium constants measured for the redox reaction between P680 and Y^sub Z^ give an inkling of that E^sub m^(Y7) operating in the ET proces between P680 and Y^sub Z^ should be relatively high, yielding a value of + 1200 mV which is ~100 mV below E^sub m^(P680) (315) (reviewed in (10)) However, the mechanism of by what mode the Y^sub Z/D^ environment of PSII shifts E^sub m^(Y^sub Z/D^) straits further investigations to be clarified ( diocese discussion in (9)). To obtain a stable Y^sub Z^ radical, many investigations upon its function were, for technical reasons, performed upon Mn-depleted PSII, since under these circumstances spectroscopic signals originating from Y^sub Z/D^ could be exposeed under stationary conditions. However, it was adviseed that Mn-depleted PSII samples could be make subordinate to critical changes in conformation or H-bond network ( diocese for instance (16,17)). Hence, an interpretation of these experiments involves uncertainties.