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

Tn oxygen-evolving photosystem II (PSII), a tyrosine residue, D1Tyr161 (Y-Z), serves as the intermediate electron carrier between the catalytic Mn cluster and the photochemically active chlorophyll moiety P-680 A more direct catalytic role of Y-Z, as a hydrogen abstractor from bound water, has been postulated. That Y-Z(ox) appears as a neutral (i.e. deprotonated) radical, Y-Z(.), 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 Y-Z(ox). In Mn-depleted PSII the electron transfer between Y-Z and P-680(+) 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 Y-Z --> Y-Z(ox) 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 Y-Z resembles a hydrogen-bonded tyrosinate, Y-Z((-))... 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 Y-Z(.). It may be favorable both for the very rapid reduction by Y-Z of P-680(+) and for electron (not hydrogen) abstraction by Y-Z(.) from the Mn-water cluster.