Optical characterization of the immediate electron donor to chlorophylla+IIin O2‐evolving photosystem II complexes Tyrosine as possible electron carrier between chlorophyllaIIand the water‐oxidizing manganese complex

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Abstract
The number and chemical nature of the electron carrier(s) between ChlaIIand 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 Chla+IIhas been characterized, this donor is oxidized in the nanosecond time range in exact accordance with the reduction kinetics of Chla+II. The donor is in turn re‐reduced witht> 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 Chla+IIand S. (ii) The UV‐diference spectrum due to the electron transfer from the immediate donor to Chla+IIin 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 Chla+IIin 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 IIfof 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 Chla+IIin 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.