Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin

Kazimierz Czarnecki, Agnes Cua, Christine Kirmaier, Dewey Holten, David F. Bocian
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引用次数: 2

Abstract

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 β. 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 β 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 β 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 β 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 β is hydrogen bonded to Glu L104. The interactions between other peripheral substituents of β 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 β in the protein is different in the two β-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. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 346–357, 1999

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细菌叶绿素取代光活性细菌叶绿素的突变反应中心中结构改变与光化学的关系
报道了荚膜红杆菌(Rhodobacter capsulatus)的两个突变反应中心(RCs)的qy激发共振拉曼(RR)光谱,其中光活性细菌叶绿素(BPhL)被细菌叶绿素(BChl)分子β取代。两种突变体的色素变化都是通过在光活性辅助因子附近引入组氨酸残基而诱导的。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V环上,名义上距离太远,无法与Mg+2离子结合。主要观察结果如下:(1)F(L121)H/F(L97)V RCs中的β辅因子为五坐标BChl分子。然而,没有证据表明形成了Mg-His键。该键要么比L(M212)H RCs弱得多,要么完全不存在,后者意味着由替代配体配位。β在F(L121)H/F(L97)V和L(M212)H RCs中不同的轴向连接反过来导致BChl大环的不同构象。(2)在F(L121)H/F(L97)V RCs中,β的c9 -酮基团不存在氢键相互作用,而在L(M212)H RCs中,β的c9 -酮与Glu L104存在氢键相互作用。在F(L121)H/F(L97)V RCs和L(M212)H RCs中,β的其他外周取代基与蛋白质之间的相互作用也不同。因此,在两种含β的RCs中,β在蛋白质中的位置和取向不同。尽管如此,先前的研究表明,两个突变体的初级电子转移反应非常相似,但与野生型RCs相比,在许多方面存在差异。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的显著紊乱,也不是β型RCs中原发性事件改变的基础。©1999 John Wiley &儿子,Inc。生物光谱学杂志,1999
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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Transient increase of tryptophan fluorescence of enzyme caused by photoexcitation of ligand in luciferase–luciferin complex Alteration of infrared spectrum of serum transferrin by iron binding and lowered pH Vibrational spectroscopic study of glutathione complexation in aqueous solutions Certain species of the Proteobacteria possess unusual bacteriochlorophyll a environments in their light-harvesting proteins Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin
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