Fragata Mário, Dudekula Subhan
Université du Québec à Trois-Rivières, Département de Chimie-Biologie, Section de Chimie et Biochimie, Trois-Rivières, Québec G9A 5H7, Canada.
J Phys Chem B. 2005 Aug 4;109(30):14707-14. doi: 10.1021/jp052445l.
Electron transport through photosystem II, measured as oxygen evolution (OE), was investigated in isolated thylakoid membranes treated with beta-cyclodextrin (beta-CD, a cyclic oligosaccharide constituted of seven alpha-d-glucose residues linked by alpha-1,4 glycosidic bonds) and irradiated with white light of variable intensity. First, we found that the light-response curves of oxygen evolution are well fitted with a hyperbolic function, the shape of which is not affected by the beta-CD concentration. Second, we showed that under conditions of irradiation with white light of saturating intensity ( approximately 5000 mumol of photons/m(2).s) beta-CD enhances the oxygen evolution in the thylakoid membranes according to a sigmoid function displaying a sharp inflection point, or transition. Unexpectedely, this beta-CD effect is not observed at irradiances of less than approximately 300 mumol of photons/m(2).s. We attempted a theoretical analysis of the combined effect of irradiance and beta-CD concentration on oxygen evolution (OE(th)). For this purpose, the effect of irradiance (I) was modeled with a hyperbola (i) and the beta-CD concentration (C) contribution with a Hill equation, that is, a sigmoid function (ii). The mathematical simulations generated the following general expressions: (i) OE(th) = [OE(max)(0) G(1)(C)]I/[L(1/2)(0) G(2)(C) + I] and (ii) G(i)()(C) = 1 + p[C(n)()/(K(1/2)(n)() + C(n)())], where OE(max)(0) is the OE maximum (OE(max)) in the absence of beta-CD, L(1/2)(0) is the photon flux density giving OE(max)/2 in the absence of beta-CD, G(1)(C) or G(2)(C) is obtained from G(i)()(C) where i is 1 or 2, n is the Hill coefficient, p is a parameter to account for the beta-CD-mediated maximum OE increase, and K(1/2) is the beta-CD concentration giving half-maximal OE activity. The results of the calculations yielded the expression (iii) OE(th) = 151[1 + 3.3C(4.8)/(13.1(4.8) + C(4.8))]I/{97.5[1 + 5.2C(7.8)/(14.8(7.8) + C(7.8))] + I} which agrees well with the experimental data for a broad range of I and C. Note that, for C = 0, eq iii reverts to the light-response curve of oxygen evolution in the absence of beta-CD. We conclude that eq iii is a good approximation of the combined effect of irradiance and beta-CD concentration, meaning that the model has a significant value for predicting the outcome of associated photochemical and biochemical reactions.
在经β-环糊精（β-CD，一种由7个通过α-1,4糖苷键连接的α-D-葡萄糖残基构成的环状寡糖）处理并以不同强度白光照射的分离类囊体膜中，研究了通过光系统II的电子传递，以放氧量（OE）来衡量。首先，我们发现放氧的光响应曲线能很好地用双曲线函数拟合，其形状不受β-CD浓度影响。其次，我们表明在饱和强度（约5000 μmol光子/m²·s）的白光照射条件下，β-CD根据呈现急剧拐点或转变的S形函数增强类囊体膜中的放氧量。出乎意料的是，在低于约300 μmol光子/m²·s的辐照度下未观察到这种β-CD效应。我们尝试对辐照度和β-CD浓度对放氧量（OE(th)）的联合效应进行理论分析。为此，辐照度（I）的效应用双曲线（i）建模，β-CD浓度（C）的贡献用希尔方程建模，即S形函数（ii）。数学模拟得出以下一般表达式：（i）OE(th) = [OE(max)(0) G(1)(C)]I/[L(1/2)(0) G(2)(C) + I] 和（ii）G(i)()(C) = 1 + p[C(n)()/(K(1/2)(n)() + C(n)())]，其中OE(max)(0)是在不存在β-CD时的OE最大值（OE(max)），L(1/2)(0)是在不存在β-CD时给出OE(max)/2的光子通量密度，G(1)(C)或G(2)(C)从G(i)()(C)中获得，其中i为1或2，n是希尔系数，p是一个参数，用于说明β-CD介导的最大OE增加，K(1/2)是给出半最大OE活性的β-CD浓度。计算结果得出表达式（iii）OE(th) = 151[1 + 3.3C(4.8)/(13.1(4.8) + C(4.8))]I/{97.5[1 + 5.2C(7.8)/(14.8(7.8) + C(7.8))] + I}，该表达式在很宽的I和C范围内与实验数据吻合良好。注意，对于C = 0，等式iii恢复为不存在β-CD时放氧的光响应曲线。我们得出结论，等式iii很好地近似了辐照度和β-CD浓度的联合效应，这意味着该模型对于预测相关光化学反应和生化反应的结果具有重要价值。
