Optical characterization of the immediate electron donor to chlorophyll a+II in O2-evolving photosystem II complexes Tyrosine as possible electron carrier between chlorophyll aII and the water-oxidizing manganese complex

The number and chemical nature of the electron carrier(s) between Chl aII and the water-oxidizing enzyme, S, were analyzed through flash-induced absorption changes in the UV with nanosecond time resolution. (i) At all wavelengths where the reaction of the donor with Chl a+II has been characterized, this donor is oxidized in the nanosecond time range in exact accordance with the reduction kinetics of Chl a+II. The donor is in turn re-reduced with t12 > 10,μs, i.e. in the range where S is oxidized. From this time course it is concluded that there exists only one electron carrier between Chl a+II and S. (ii) The UV-diference spectrum due to the electron transfer from the immediate donor to Chl a+II in the nanosecond time range in O2-evolving PS II complexes is characterized by a maximum around 260 nm and smaller minimum around 310 nm. This spectrum is identical with that observed for the reaction of the donor with Chl a+II in the microsecond time range in Tris-treated PS II. Therefore, the donors in both reactions must be of the same chemical nature. (iii) This result, together with the well-established similarity of EPR signal IIf of the oxidized donor in Tris-treated PS II to the EPR signal IIIs, recently assigned to Tyr-160 of the D2 protein of PS II [(1988) Proc. Natl. Acad. Sci. USA 85, 427–430], provides strong evidence that the immediate donor to Chl a+II in water-oxidizing PS II is also a tyrosine. (iv) It is shown that the UV-difference spectra of the oxidation of the immediate donor in O2-evolving as well as that of Tris-treated PS II complexes are similar to the in vitro difference spectrum of the oxidation of tyrosine in water. This independent result supports the conclusion that the donor is a tyrosine.