Axelrod Herbert L, Abresch Edward C, Okamura Melvin Y, Yeh Andrew P, Rees Douglas C, Feher George
Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0319, USA.
J Mol Biol. 2002 May 31;319(2):501-15. doi: 10.1016/S0022-2836(02)00168-7.
In the photosynthetic bacterium Rhodobacter sphaeroides, a water soluble cytochrome c2 (cyt c2) is the electron donor to the reaction center (RC), the membrane-bound pigment-protein complex that is the site of the primary light-induced electron transfer. To determine the interactions important for docking and electron transfer within the transiently bound complex of the two proteins, RC and cyt c2 were co-crystallized in two monoclinic crystal forms. Cyt c2 reduces the photo-oxidized RC donor (D+), a bacteriochlorophyll dimer, in the co-crystals in approximately 0.9 micros, which is the same time as measured in solution. This provides strong evidence that the structure of the complex in the region of electron transfer is the same in the crystal and in solution. X-ray diffraction data were collected from co-crystals to a maximum resolution of 2.40 A and refined to an R-factor of 22% (R(free)=26%). The structure shows the cyt c2 to be positioned at the center of the periplasmic surface of the RC, with the heme edge located above the bacteriochlorophyll dimer. The distance between the closest atoms of the two cofactors is 8.4 A. The side-chain of Tyr L162 makes van der Waals contacts with both cofactors along the shortest intermolecular electron transfer pathway. The binding interface can be divided into two domains: (i) A short-range interaction domain that includes Tyr L162, and groups exhibiting non-polar interactions, hydrogen bonding, and a cation-pi interaction. This domain contributes to the strength and specificity of cyt c2 binding. (ii) A long-range, electrostatic interaction domain that contains solvated complementary charges on the RC and cyt c2. This domain, in addition to contributing to the binding, may help steer the unbound proteins toward the right conformation.
在光合细菌球形红杆菌中，水溶性细胞色素c2（细胞色素c2）是反应中心（RC）的电子供体，反应中心是膜结合色素蛋白复合物，是光诱导初级电子转移的场所。为了确定两种蛋白质（RC和细胞色素c2）的瞬时结合复合物中对对接和电子转移重要的相互作用，将RC和细胞色素c2以两种单斜晶型共结晶。在共晶体中，细胞色素c2在约0.9微秒内还原光氧化的RC供体（D+），一种细菌叶绿素二聚体，这与在溶液中测量的时间相同。这提供了强有力的证据，表明电子转移区域内复合物的结构在晶体和溶液中是相同的。从共晶体收集X射线衍射数据，最大分辨率为2.40埃，并精修至R因子为22%（R（自由）=26%）。结构显示细胞色素c2位于RC周质表面的中心，血红素边缘位于细菌叶绿素二聚体上方。两个辅因子最接近原子之间的距离为8.4埃。Tyr L162的侧链沿着最短的分子间电子转移途径与两个辅因子形成范德华接触。结合界面可分为两个结构域：（i）一个短程相互作用结构域，包括Tyr L162以及表现出非极性相互作用、氢键和阳离子-π相互作用的基团。该结构域有助于细胞色素c2结合的强度和特异性。（ii）一个长程静电相互作用结构域，在RC和细胞色素c2上含有溶剂化的互补电荷。该结构域除了有助于结合外，还可能有助于引导未结合的蛋白质形成正确的构象。
