Bernard M T, MacDonald G M, Nguyen A P, Debus R J, Barry B A
Department of Biochemistry, University of Minnesota, St. Paul 55108.
J Biol Chem. 1995 Jan 27;270(4):1589-94. doi: 10.1074/jbc.270.4.1589.
Photosystem II, the photosynthetic water oxidizing complex, contains two well characterized redox active tyrosines, D and Z. D forms a stable radical of unknown function. Z is an electron carrier between the primary chlorophyll donor and the manganese catalytic site. The vibrational difference spectra associated with the oxidation of tyrosines Z and D have been obtained through the use of infrared spectroscopy (MacDonald, G. M., Bixby, K.A., and Barry, B.A. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11024-11028). Here, we examine the effect of deuterium exchange on these vibrational difference spectra. While the putative C-O vibration of stable tyrosine radical D. downshifts in 2H2O, the putative C-O vibration of tyrosine radical Z. does not. This result is consistent with the existence of a hydrogen bond to the phenol oxygen of the D. radical; we conclude that a hydrogen bond is not formed to the Z. radical. In an effort to identify the amino acid residue that is the proton acceptor for Z, we have performed global 15N labeling. While significant 15N shifts are observed in the vibrational difference spectrum, substitution of a glutamine for a histidine that is predicted to lie in the environment of tyrosine Z has little or no effect on the difference infrared spectrum. There is also no significant change in the yield or lineshape of the Z. EPR signal under continuous illumination in this mutant. Our results are inconsistent with the possibility that this residue, histidine 190 of the D1 polypeptide, acts as the sole proton acceptor for tyrosine Z.
光系统II，即光合水氧化复合物，包含两个特性明确的氧化还原活性酪氨酸，D和Z。D形成一个功能未知的稳定自由基。Z是初级叶绿素供体和锰催化位点之间的电子载体。通过红外光谱法已获得与酪氨酸Z和D氧化相关的振动差异光谱（MacDonald, G. M., Bixby, K.A., and Barry, B.A. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11024 - 11028）。在此，我们研究了氘交换对这些振动差异光谱的影响。虽然稳定酪氨酸自由基D的假定C - O振动在2H₂O中发生下移，但酪氨酸自由基Z的假定C - O振动并未下移。该结果与D自由基的酚氧存在氢键一致；我们得出结论，Z自由基未形成氢键。为了确定作为Z质子受体的氨基酸残基，我们进行了全局¹⁵N标记。虽然在振动差异光谱中观察到显著的¹⁵N位移，但将预测位于酪氨酸Z环境中的组氨酸替换为谷氨酰胺对红外差异光谱几乎没有影响。在该突变体连续光照下，Z的电子顺磁共振信号的产率或线形也没有显著变化。我们的结果与D1多肽的组氨酸190作为酪氨酸Z的唯一质子受体这一可能性不一致。
