Tiede D M, Vashishta A C, Gunner M R
Chemistry Division, Argonne National Laboratory, Illinois 60439.
Biochemistry. 1993 May 4;32(17):4515-31. doi: 10.1021/bi00068a006.
The kinetics of electron transfer between the Rhodobacter sphaeroides R-26 reaction center and nine soluble c-cytochromes have been analyzed and compared to the patterns of the surface electrostatic potentials for each of the proteins. Characteristic first-order electron-transfer rates for 1:1 complexes formed at low ionic strength between the reaction center and the different c-cytochromes were identified and found to vary by a factor of almost 100, while second-order rates were found to differ by greater than 10(6). A correlation was found between the location of likely electrostatic interaction domains on each cytochrome and its characteristic rate of electron transfer. The interaction domains were identified by mapping electrostatic potentials, calculated from the Poisson-Boltzmann equation, onto simulated "encounter surfaces" for each of the cytochromes and the reaction center. For the reaction center, the c-cytochrome binding domain was found to have almost exclusively net negative potential (< -3 kT) and to be shifted slightly toward the M-subunit side of the reaction center. The location of interaction domains of complementary, positive potential (> 3 kT) differed for each cytochrome. The correspondence between electrostatic, structural, and kinetic properties of 1:1 reaction center-cytochrome complexes leads to a proposed mechanism for formation of reaction center-cytochrome electron-transfer complexes that is primarily driven by the juxtaposition of regions of delocalized complementary potential. In this mechanism the clustering of charged residues is of primary importance and not the location of specific residues. A consequence of this mechanism is that many different sets of charge distributions are predicted to be capable of stabilizing a specific configuration for a reaction center-cytochrome complex. This mechanism for reaction center association with water-soluble c-cytochromes fits molecular recognition mechanisms proposed for c-cytochromes in nonphotosynthetic systems. In general, the kinetic scheme for reaction center driven cytochrome oxidation was found to vary between a simple two-state model, involving cytochrome in free and reaction center bound states, and a three-state model, that includes cytochrome binding in kinetically competent ("proximal") and incompetent ("distal") modes. The kinetically incompetent mode of cytochrome binding is suggested not to be an intrinsic feature of the reaction center-cytochrome association but is likely to be due to variation in the physical state of the reaction center.
分析了球形红杆菌R-26反应中心与9种可溶性c型细胞色素之间的电子转移动力学，并将其与每种蛋白质的表面静电势模式进行了比较。确定了在低离子强度下反应中心与不同c型细胞色素形成的1:1复合物的特征一级电子转移速率，发现其变化幅度近100倍，而二级速率差异大于10^6。发现每种细胞色素上可能的静电相互作用域的位置与其特征电子转移速率之间存在相关性。通过将根据泊松-玻尔兹曼方程计算的静电势映射到每种细胞色素和反应中心的模拟“相遇表面”上，确定了相互作用域。对于反应中心，发现c型细胞色素结合域几乎完全具有净负电位（<-3kT），并且略微向反应中心的M亚基侧偏移。每种细胞色素的互补正电位（>3kT）相互作用域的位置不同。1:1反应中心-细胞色素复合物的静电、结构和动力学性质之间的对应关系导致了一种反应中心-细胞色素电子转移复合物形成机制的提出，该机制主要由离域互补电位区域的并置驱动。在这种机制中，带电残基的聚集至关重要，而不是特定残基的位置。这种机制的一个结果是，预计许多不同的电荷分布集能够稳定反应中心-细胞色素复合物的特定构型。反应中心与水溶性c型细胞色素的这种结合机制符合非光合系统中c型细胞色素提出的分子识别机制。一般来说，发现反应中心驱动的细胞色素氧化的动力学方案在简单的双态模型（涉及游离状态和与反应中心结合状态的细胞色素）和三态模型（包括以动力学上有效的（“近端”）和无效的（“远端”）模式结合的细胞色素）之间变化。细胞色素结合的动力学无效模式被认为不是反应中心-细胞色素结合的固有特征，而可能是由于反应中心物理状态的变化。
