Hervás M, Navarro J A, Díaz A, De la Rosa M A
Instituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla, Spain.
Biochemistry. 1996 Feb 27;35(8):2693-8. doi: 10.1021/bi951876z.
A comparative thermodynamic analysis of photosystem I (PSI) reduction by plastocyanin (Pc) and cytochrome c6 (Cyt) has been carried out by laser-flash absorption spectroscopy in the cyanobacteria Anabaena PCC 7119 and Synechocystis PCC 6803 as well as in spinach. These three organisms have been reported to exhibit different reaction mechanisms [Hervas, M., Navarro, J. A.. Díaz, A., Bottin, H., & De la Rosa, M. A. (1995) Biochemistry, 34, 11321-11326]. Whereas the activation free energy for the overall reaction is mainly enthalpic in nature, long-range electrostatic interactions appear to be attractive in Anabaena, but repulsive in Synechocystis and spinach. The net interaction between PSI and its two donor proteins in Anabaena is similarly affected by ionic strength (the rate constant decreases with increasing salt concentration), but the activation parameters delta H+/+ delta S+/+ show different dependencies on ionic strength. A compensation effect between entropy and enthalpy at varying ionic strength is found in all these Pc/PSI and Cyt/PSI systems, except with Cyt and PSI from Anabaena. Such a compensation effect is proposed to be mainly due to stabilization of the intermediate electrostatic complex by hydrophobic forces. The electron transfer step seems to be well optimized in the Anabaena Cyt/PSI couple, which exhibits a temperature-independent fast kinetic phase and, therefore, a low activation energy barrier. Short-distance forces appear to have gained relevancy in the reaction mechanism of PSI reduction by Cyt and Pc throughout evolution, whereas long-range interactions are prevalent in less evolved organisms.
通过激光闪光吸收光谱法，对蓝藻鱼腥藻PCC 7119、集胞藻PCC 6803以及菠菜中质体蓝素（Pc）和细胞色素c6（Cyt）还原光系统I（PSI）进行了比较热力学分析。据报道，这三种生物表现出不同的反应机制[埃尔瓦斯，M.，纳瓦罗，J. A.，迪亚斯，A.，博廷，H.，& 德拉罗萨，M. A.（1995年）《生物化学》，34，11321 - 11326]。虽然整个反应的活化自由能本质上主要是焓变，但长程静电相互作用在鱼腥藻中似乎具有吸引力，而在集胞藻和菠菜中则具有排斥性。鱼腥藻中PSI与其两种供体蛋白之间的净相互作用同样受离子强度影响（速率常数随盐浓度增加而降低），但活化参数ΔH⁺/ + ΔS⁺/ + 对离子强度表现出不同的依赖性。在所有这些Pc/PSI和Cyt/PSI系统中，除了鱼腥藻的Cyt和PSI外，在不同离子强度下都发现了熵和焓之间的补偿效应。这种补偿效应被认为主要是由于疏水力对中间静电复合物的稳定作用。电子转移步骤在鱼腥藻Cyt/PSI偶联中似乎得到了很好的优化，它表现出与温度无关的快速动力学阶段，因此具有较低的活化能垒。在整个进化过程中，短程力在Cyt和Pc还原PSI的反应机制中似乎变得更加重要，而长程相互作用在进化程度较低的生物中更为普遍。
