Czarnecki K, Cua A, Kirmaier C, Holten D, Bocian D F
Department of Chemistry, University of California, Riverside 92521-0403, USA.
Biospectroscopy. 1999;5(6):346-57. doi: 10.1002/(SICI)1520-6343(1999)5:6<346::AID-BSPY4>3.0.CO;2-9.
Qy-excitation resonance Raman (RR) spectra are reported for two mutant reaction centers (RCs) from Rhodobacter capsulatus in which the photoactive bacteriopheophytin (BPhL) is replaced by a bacteriochlorophyll (BChl) molecule, designated beta. The pigment change in both mutants is induced via introduction of a histidine residue near the photoactive cofactor. In one mutant, L(M212)H, the histidine is positioned over the core of the cofactor and serves as an axial ligand to the Mg+2 ion. In the other mutant, F(L121)H/F(L97)V, the histidine is positioned over ring V of the cofactor, which is nominally too distant to permit bonding to the Mg+2 ion. The salient observations are as follows: (1) The beta cofactor in F(L121)H/F(L97)V RCs is a five-coordinate BChl molecule. However, there is no evidence for the formation of a Mg-His bond. This bond is either much weaker than in the L(M212)H RCs or completely absent, the latter implying coordination by an alternative ligand. The different axial ligation for beta in the F(L121)H/F(L97)V versus L(M212)H RCs in turn leads to different conformations of the BChl macrocycles. (2) The C9-keto group of beta in F(L121)H/F(L97)V RCs is free of hydrogen bonding interactions, unlike the L(M212)H RCs in which the C9-keto of beta is hydrogen bonded to Glu L104. The interactions between other peripheral substituents of beta and the protein are also different in the F(L121)H/F(L97)V RCs versus L(M212)H RCs. Accordingly, the position and orientation of beta in the protein is different in the two beta-containing RCs. Nonetheless, previous studies have shown that the primary electron transfer reactions are very similar in the two mutants but differ in significant respects compared to wild-type RCs. Collectively, these observations indicate that changes in the conformation of a photoactive tetrapyrrole macrocycle or its interactions with the protein do not necessarily lead to significantly perturbed photochemistry and do not underlie the altered primary events in beta-type RCs.
报道了来自荚膜红细菌的两个突变反应中心（RCs）的Qy激发共振拉曼（RR）光谱，其中光活性细菌脱镁叶绿素（BPhL）被一个细菌叶绿素（BChl）分子取代，命名为β。两个突变体中的色素变化是通过在光活性辅因子附近引入一个组氨酸残基诱导的。在一个突变体L(M212)H中，组氨酸位于辅因子的核心上方，并作为Mg+2离子的轴向配体。在另一个突变体F(L121)H/F(L97)V中，组氨酸位于辅因子的环V上方，理论上距离太远，无法与Mg+2离子形成键合。主要观察结果如下：（1）F(L121)H/F(L97)V反应中心的β辅因子是一个五配位的BChl分子。然而，没有证据表明形成了Mg-组氨酸键。该键要么比L(M212)H反应中心的键弱得多，要么完全不存在，后者意味着由另一种配体配位。F(L121)H/F(L97)V与L(M212)H反应中心中β的不同轴向配位反过来导致BChl大环的不同构象。（2）F(L121)H/F(L97)V反应中心中β的C9-酮基没有氢键相互作用，这与L(M212)H反应中心不同，在L(M212)H反应中心中β的C9-酮基与Glu L104形成氢键。β的其他外围取代基与蛋白质之间的相互作用在F(L121)H/F(L97)V反应中心与L(M212)H反应中心中也不同。因此，在两个含β的反应中心中，β在蛋白质中的位置和取向是不同的。尽管如此，先前的研究表明，两个突变体中的初级电子转移反应非常相似，但与野生型反应中心相比，在重要方面有所不同。总的来说，这些观察结果表明，光活性四吡咯大环构象的变化或其与蛋白质的相互作用不一定会导致光化学受到显著干扰，也不是β型反应中心中初级事件改变的基础。
