Redox Control and Hydrogen Bonding Networks: Proton-Coupled Electron Transfer Reactions and Tyrosine Z in the Photosynthetic Oxygen-Evolving Complex

Abstract

In photosynthetic oxygen evolution, redox active tyrosine Z (YZ) plays an essential role in proton-coupled electron transfer (PCET) reactions. Four sequential photooxidation reactions are necessary to produce oxygen at a Mn(4)CaO(5) cluster. The sequentially oxidized states of this oxygen-evolving cluster (OEC) are called the S(n) states, where n refers to the number of oxidizing equivalents stored. The neutral radical, YZ•, is generated and then acts as an electron transfer intermediate during each S state transition. In the X-ray structure, YZ, Tyr161 of the D1 subunit, is involved in an extensive hydrogen bonding network, which includes calcium-bound water. In electron paramagnetic resonance experiments, we measured the YZ• recombination rate, in the presence of an intact Mn(4)CaO(5) cluster. We compared the S(0) and S(2) states, which differ in Mn oxidation state, and found a significant difference in the YZ• decay rate (t(1/2) = 3.3 ± 0.3 s in S(0); t(1/2) = 2.1 ± 0.3 s in S(2)) and in the solvent isotope effect (SIE) on the reaction (1.3 ± 0.3 in S(0); 2.1 ± 0.3 in S(2)). Although the YZ site is known to be solvent accessible, the recombination rate and SIE were pH independent in both S states. To define the origin of these effects, we measured the YZ• recombination rate in the presence of ammonia, which inhibits oxygen evolution and disrupts the hydrogen bond network. We report that ammonia dramatically slowed the YZ• recombination rate in the S(2) state but had a smaller effect in the S(0) state. In contrast, ammonia had no significant effect on YD•, the stable tyrosyl radical. Therefore, the alterations in YZ• decay, observed with S state advancement, are attributed to alterations in OEC hydrogen bonding and consequent differences in the YZ midpoint potential/pK(a). These changes may be caused by activation of metal-bound water molecules, which hydrogen bond to YZ. These observations document the importance of redox control in proton-coupled electron transfer reactions.

Detection of an intermediary, protonated water cluster in photosynthetic oxygen evolution

“…A negative band was also observed on the third flash, corresponding to S 0 -minus-S 3 (Fig. 3C, black), and on the fourth flash, corresponding mainly to S 1 -minus-S 0 , but with mixing of other S states, due to misses (22) (Fig. 3D, black).…”

“…In another approach, the recombination kinetics of YZ• were used as a probe of electrostatic changes in the network in the S 2 and S 0 states (21, 22). In both studies, ammonia, a substrate-based inhibitor (23-25), was used to perturb hydrogen bonding in the OEC and was shown to have significant effects on the spectroscopic signals (18,20,22).Proton-coupled electron-transfer reactions occur during the S-state cycle (11,12). Although the S 1 -to-S 2 transition is not accompanied by a net proton release to sucrose-containing buffers, proton release accompanies the other S-state transitions (26).…”

“…In another approach, the recombination kinetics of YZ• were used as a probe of electrostatic changes in the network in the S 2 and S 0 states (21, 22). In both studies, ammonia, a substrate-based inhibitor (23-25), was used to perturb hydrogen bonding in the OEC and was shown to have significant effects on the spectroscopic signals (18,20,22).…”

“…Recent reaction-induced FTIR studies of the S 1 -to-S 2 transition showed that the frequencies of hydrogen-bonded amide C=O groups were markers of hydrogenbonding changes in the network. In another approach, the recombination kinetics of YZ• were used as a probe of electrostatic changes in the network in the S 2 and S 0 states (21,22). In both studies, ammonia, a substrate-based inhibitor (23)(24)(25), was used to perturb hydrogen bonding in the OEC and was shown to have significant effects on the spectroscopic signals (18,20,22).…”

Could tyrosine and tryptophan serve multiple roles in biological redox processes?

“…It is well established that multi-step tunnelling, called hopping, is required for functional charge transport in many redox enzymes (examples include ribonucleotide reductase [1][2][3][4][5][6][7][8][9], photosystem II [10][11][12], DNA photolyase [13][14][15][16][17][18][19][20][21], MauG [22][23][24][25] and cytochrome c peroxidase [26,27]). Here, we advance the hypothesis that many such enzymes, most especially those that generate high-potential intermediates during turnover, could be irreversibly damaged if the intermediates are not inactivated in some way.…”