Fourier Transform Infrared Difference Spectroscopy of Photosystem II Tyrosine D Using Site-Directed Mutagenesis and Specific Isotope Labeling

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Abstract
Tyrosine D (TyrD), a side path electron carrier of photosystem II (PS II), has been studied by light-induced Fourier transform infrared (FTIR) difference spectroscopy in PS II core complexes of Synechocystis sp. PCC 6803 using the experimental conditions previously optimized to generate the pure TyrD•/TyrD FTIR difference spectrum in PS II-enriched membranes of spinach [Hienerwadel, R., Boussac, A., Breton, J., and Berthomieu, C. (1996) Biochemistry 35, 115447−115460]. IR modes of TyrD and TyrD• have been identified by specific 2H- or 13C-labeling of the tyrosine side chains. The v8a(CC) and v19(CC) IR modes of TyrD are identified at 1615 and 1513−1510 cm-1, respectively. These frequencies show that TyrD is protonated. Comparison of isotope-sensitive signals in situ with those of the model compound p-methylphenol dissolved in different solvents leads to the assignment of the v7‘a(CO) and δ(COH) modes of TyrD at 1275 and 1250 cm-1, respectively. It is shown that these modes and in particular the δ(COH) IR mode are very sensitive to the formation of hydrogen-bonded complexes with amide CO or with imidazole nitrogen atoms. The frequencies observed in situ show that TyrD is hydrogen-bonded to the imidazole ring of a neutral histidine. For the radical TyrD•, isotope-sensitive IR modes are identified at 1532 and 1503 cm-1. The signal at 1503 cm-1 is assigned to the v(CO) mode of TyrD• since it is sensitive to 13C-labeling at the ring carbon involved in the C4−O bond. The perturbation of TyrD and TyrD• IR modes upon site-directed replacement of D2-His189 by Gln confirms that a hydrogen bond exists between both TyrD and TyrD• and D2-His189. In the D2-His189Gln mutant, the v7‘a(CO) mode of TyrD at 1267 cm-1 and the δ(COH) mode at ≈1228 cm-1 show that a hydrogen bond is formed between TyrD and an amide carbonyl, probably that of the D2-Gln189 side chain. Electron nuclear double resonance (ENDOR) measurements have shown that TyrD• is hydrogen-bonded in the wild type but not in the mutant [Tang, X.-S., Chrisholm, D. A., Dismukes, G. C., Brudwig, G. W., and Diner, B. A. (1993) Biochemistry32, 13742−13748]. The v(CO) mode of TyrD• at 1497 cm-1 is downshifted by 6 cm-1 compared to WT PS II, indicating that hydrogen bonding induces a frequency upshift of the v(CO) IR mode of Tyr•. IR signals from the Gln side chain v(CO) mode are proposed to contribute at 1659 and 1692 cm-1 in the TyrD and TyrD• states, respectively. These frequencies are consistent with the rupture of a hydrogen bond upon TyrD• formation in the mutant. The frequency of the v(CO) mode of TyrD•, observed at 1503 cm-1 for WT PS II, is intermediate between that observed at 1497 cm-1 in the D2-His189Gln mutant and at 1513 cm-1 for Tyr• formed by UV irradiation in borate buffer, suggesting weaker or fewer hydrogen bonds for TyrD• in PS II than in solution. The role of D2-His189 in proton uptake upon TyrD• formation is also investigated.