Point-Mutations Affecting the Properties of TyrosineD in Photosystem II. Characterization by Isotopic Labeling and Spectral Simulation

Abstract

The reaction center of photosystem II (PSII) contains two redox-active tyrosines, TyrD and TyrZ, which are Tyr160 and Tyr161 of the D2 and D1 proteins, respectively. We have introduced five site-directed mutations in the vicinity of TyrD to analyze the consequences of the mutations on spectral and functional properties of TyrD(ox). Characterization of three mutants, P161A and P161L (Pro161 changed to Ala and Leu, respectively) and Q164L (Gln164 mutated to Leu), is emphasized. Of these three mutants, only P161L is an obligate photoheterotroph; it is capable of oxygen evolution, but is photoinactivated rapidly. The D2 protein of this mutant migrates slower on a SDS-polyacrylamide gel. The EPR spectrum of TyrD(ox) is modified in the three mutants. The EPR spectra of TyrD(ox) in wild type and the mutants were characterized in detail by comparison of EPR spectra of thylakoids from cells grown in the presence and absence of tyrosine that was deuterated in specific positions. The experimentally obtained EPR spectra of wild type, P161A, and Q164L could be simulated satisfactorily using current theoretical models. The angle between one of the hydrogens on the beta-methylene carbon and the 2pz orbital at C1 of the tyrosine ring was found to change slightly but significantly as a function of the mutations (52 degrees in wild type, 50 degrees in P161A, and 48 degrees in Q164L). The overall electronic structure of TyrDox is quite unaffected; only minor redistribution of the unpaired electron spin is observed between the wild type and the mutated systems.(ABSTRACT TRUNCATED AT 250 WORDS)