Tyrosine-Z in oxygen-evolving photosystem II: a hydrogen-bonded tyrosinate

In oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (YZ), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P680. A more direct catalytic role of YZ, as a hydrogen abstractor from bound water, has been postulated. That YZox appears as a neutral (i.e. deprotonated) radical, YZ*, in EPR studies is compatible with this notion. Data based on electrochromic absorption transients, however, are conflicting because they indicate that the phenolic proton remains on or near to YZox. In Mn-depleted PSII the electron transfer between YZ and P680+ can be almost as fast as in oxygen-evolving material, however, only at alkaline pH. With an apparent pK of about 7 the fast reaction is suppressed and converted into an about 100-fold slower one which dominates at acid pH. In the present work we investigated the optical difference spectra attributable to the transition YZ --> YZox as function of the pH. We scanned the UV and VIS range and used Mn-depleted PSII core particles and also oxygen-evolving ones. Comparing these spectra with published in vitro and in vivo spectra of phenolic compounds, we arrived at the following conclusions: In oxygen-evolving PSII YZ resembles a hydrogen-bonded tyrosinate, YZ(-).H(+).B. The phenolic proton is shifted toward a base B already in the reduced state and even more so in the oxidized state. The retention of the phenolic proton in a hydrogen-bonded network gives rise to a positive net charge in the immediate vicinity of the neutral radical YZ*. It may be favorable both for the very rapid reduction by YZ of P680+ and for electron (not hydrogen) abstraction by YZ* from the Mn-water cluster.