Isotopic labeling and EPR spectroscopy show that a tyrosine residue is the terminal electron donor, Z, in manganese-depleted photosystem II preparations

Abstract

Photosystem II contains two redox active species, D and Z, which in the oxidized and paramagnetic form give rise to similar EPR signals. Both signals are broadened by anisotropic interactions and show no well-resolved hyperfine splittings, even at room temperature. D+ is a dark stable radical of unknown function. Z, on the other hand, acts as the intermediate electron carrier between the primary chlorophyll donor and the manganese catalytic site. In manganese-depleted photosystem II preparations, where Z+ is the terminal electron donor, the Z+ EPR signal can be easily detected. Isotopic labeling of intact cells of Synechocystis 6803 and EPR spectroscopy were used to demonstrate that D+ is a tyrosine radical (Barry, B.A., and Babcock, G. T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 7099-7103). However, since the Z+ signal is not observable in intact cyanobacterial cells, a definitive assignment of the Z redox active species has not yet been performed. Here, we describe the use of a purified photosystem II preparation to assign the Z+ EPR signal to a neutral tyrosine radical. Further, a comparison of the Z+ and D+ EPR lineshapes in deuterated samples shows that there are slight differences between the two signals. These alterations may be caused by differences in the protein environments of the D+ and Z+ radicals.