Tracewell C A, Cua A, Stewart D H, Bocian D F, Brudvig G W
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
Biochemistry. 2001 Jan 9;40(1):193-203. doi: 10.1021/bi001992o.
Photosystem II (PSII) contains two accessory chlorophylls (Chl(Z), ligated to D1-His118, and Chl(D), ligated to D2-His117), carotenoid (Car), and heme (cytochrome b(559)) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O(2)-evolving complex to P680(+) is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl(+) near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl(Z)(+) based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040-10046]; (2) two Chl(+) near-IR bands are observed at 817 and 850 nm in spinach PSII membranes which are formed with variable relative yields depending on the illumination temperature and are assigned to Chl(Z)(+), and Chl(D)(+), respectively; (3) the Chl and Car cation radicals have significantly different stabilities at reduced temperatures with Car(+) decaying much faster; (4) in Synechocystis PSII core complexes, Car(+) decays by recombination with Q(A)(-) and not by Chl(Z)/Chl(D) oxidation, with multiphasic kinetics that are attributed to an ensemble of protein conformers that are trapped as the protein is frozen; and (5) in spinach PSII membranes, Car(+) decays mainly by recombination with Q(A)(-), but also partly by formation of the 850 nm Chl cation radical. The greater stability of Chl(Z)(+) at low temperatures enabled us to confirm that resonance Raman bands previously assigned to Chl(Z)(+) are correctly assigned. In addition, the formation and decay of these cations provide insight into the alternate electron-donation pathways to P680(+).
光系统II（PSII）包含两个辅助叶绿素（与D1-His118相连的Chl(Z)和与D2-His117相连的Chl(D)）、类胡萝卜素（Car）和血红素（细胞色素b(559)）辅因子，在从放氧复合体到P680(+)的初级电子供体途径受到抑制的条件下，这些辅因子可作为替代电子供体发挥作用。在集胞藻PSII核心复合体和菠菜PSII膜中，在6至120 K的一系列低温下，通过近红外（near-IR）吸光度、位移激发拉曼差光谱（SERDS）和电子顺磁共振（EPR）光谱对氧化还原活性辅助叶绿素和Car的光氧化进行了表征。得到了以下关键观察结果：（1）在集胞藻PSII核心复合体中，仅在814 nm处观察到一个Chl(+)近红外波段，根据之前对D1-H118Q和D2-H117Q突变体的光谱研究[Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040 - 10046]，该波段被归为Chl(Z)(+)；（2）在菠菜PSII膜中，在817和850 nm处观察到两个Chl(+)近红外波段，它们的相对产率随光照温度变化，分别被归为Chl(Z)(+)和Chl(D)(+)；（3）Chl和Car阳离子自由基在低温下具有显著不同的稳定性，Car(+)衰减得快得多；（4）在集胞藻PSII核心复合体中，Car(+)通过与Q(A)(-)复合而衰减，而非通过Chl(Z)/Chl(D)氧化，其多相动力学归因于蛋白质冷冻时捕获的一系列蛋白质构象体；（5）在菠菜PSII膜中，Car(+)主要通过与Q(A)(-)复合而衰减，但也部分通过形成850 nm的Chl阳离子自由基而衰减。Chl(Z)(+)在低温下更高的稳定性使我们能够确认先前归为Chl(Z)(+)的共振拉曼波段归属正确。此外，这些阳离子的形成和衰减为通往P680(+)的替代电子供体途径提供了深入了解。
