PubMed1997
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 delta(COH) modes of TyrD at 1275 and 1250 cm-1, respectively. It is shown that these modes and in particular the delta(COH) IR mode are very sensitive to the formation of hydrogen-bonded complexes with amide C=O 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 delta(COH) mode at approximately 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) Biochemistry 32, 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(C=O) 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.