Hydrogen bonding, solvent exchange, and coupled proton and electron transfer in the oxidation and reduction of redox-active tyrosine Y-z in Mn-depleted core complexes of Photosystem II

摘要

The redox-active tyrosines, Y-Z and Y-D, of Photosystem Il are oxidized by P680(+) to the neutral tyrosyl radical. This oxidation thus involves the transfer of the phenolic proton as well as an electron. It has recently been proposed that tyrosine Y-Z might replace the lost proton by abstraction of a hydrogen atom or a proton from a water molecule bound to the manganese cluster, thereby increasing the driving force for water oxidation, To compare and contrast with the intact system, we examine here, in a simplified Mn-depleted PSII core complex, isolated from a site-directed mutant of Synechocystis PCC 6803 lacking Y-D, the role of proton transfer in the oxidation and reduction of Y-Z, We show how the oxidation and reduction rates for Y-Z, the deuterium isotope effect on these rates, and the Y-Z(.) - Y-Z difference spectra all depend on pH (from 5.5 to 9.5). This simplified system allows examination of electron-transfer processes over a broader range of pH than is possible with the intact system and with more tractable rates. The kinetic isotope effect for the oxidation of P680+ by Y-Z is maximal at pH 7.0 (3.64). It decreases to lower pH as charge recombination, which shows no deuterium isotope, starts to become competitive with Y-Z oxidation, To higher pH, Y-Z becomes increasingly deprotonated to form the tyrosinate, the oxidation of which at pH 9.5 becomes extremely rapid (1260 ms(-1)) and no longer limited by proton transfer. These observations point to a mechanism for the oxidation of Y-Z in which the tyrosinate is the species from which the electron occurs even at lower pH. The kinetics of oxidation of Y-Z show elements of rate limitation by both proton and electron transfer, with the former dominating at low pH and the latter at high pH. The proton-transfer limitation of Y-Z oxidation at low pH is best explained by a gated mechanism in which Y-Z and the acceptor of the phenolic proton need to form an electron/proton-transfer competent complex in competition with other hydrogen-bonding interactions that each have with neighboring residues. In contrast, the reduction of Y-Z(.) appears not to be limited by proton transfer between pH 5.5 and 9.5. We also compare, in Mn-depleted Synechocystis PSII core complexes, Y-Z and Y-D with respect to solvent accessibility by detection of the deuterium isotope effect for Y-Z oxidation and by H-2 ESEEM measurement of hydrogen-bond exchange. Upon incubation of H2O-prepared PSII core complexes in D2O, the phenolic proton of Y-Z is exchanged for a deuterium in less than 2 min as opposed to a t(1/2) of about 9 h for Y-D. In addition, we show that Y-D(.) is coordinated by two hydrogen bonds. Y-Z(.) shows more disordered hydrogen bonding, reflecting inhomogeneity at the site. With H-2 ESEEM modulation comparable to that of Y-D(.), Y-Z(.) would appear to be coordinated by two hydrogen bonds in a significant fraction of the centers. [References: 90]