PROXIMITY OF THE MANGANESE CLUSTER OF PHOTOSYSTEM-II TO THE REDOX-ACTIVE TYROSINE Y-Z

Electron spin echo electron-nuclear double resonance (ESE-ENDOR) exper iments performed on a broad radical electron paramagnetic resonance (E PR) signal observed in photosystem II particles depleted of Ca2+ indic ate that this signal arises from the redox-active tyrosine Y-Z. The ty rosine EPR signal width is increased relative to that observed in a ma nganese-depleted preparation due to a magnetic interaction between the photosystem II manganese cluster and the tyrosine radical. The mangan ese cluster is located asymmetrically with respect to the symmetry-rel ated tyrosines Y-Z and Y-D. The distance between the Y-Z tyrosine and the manganese cluster is estimated to be approximately 4.5 Angstrom. D ue to this close proximity of the Mn cluster and the redox-active tyro sine Y-Z, we propose that this tyrosine abstracts protons from substra te water bound to the Mn cluster.