Deciphering the function of Fe/S cofactors with alternative cluster ligands: model studies using synthetic analogues

铁-硫团簇(Fe/S)是最广泛和最古老的生物辅因子之一，几乎存在于生命的所有领域。它们最突出的作用是在不同的途径中进行电子转移(ET)，并且有一系列不同核度的Fe/S团簇可用于这一任务。它们大多含有硫化物桥联的铁簇核心，该核心由蛋白质中的末端半胱氨酸(Cys)硫代化合物连接。化学合成的铁/S蛋白活性中心类似物对我们理解生物辅因子的电子结构和内在性质具有重要意义。近年来，越来越多的蛋白质被发现含有非半胱氨酸配体的Fe/S簇，称为替代簇配体。伴随着生物铁/S辅因子电子传递以外的功能迅速增加，包括它们参与NO、DNA修复、自由基反应和催化等小分子的传感。该项目继续沿着第一个SPP资助期的思路，在迄今已有成果的基础上，最终目标是通过合成类比方法用替代簇状配体破译铁/S辅因子的功能。具体目标是：(A)阐明Fe/S团簇(主要是具有类组氨酸末端配体的[2Fe-2S]团簇)质子耦合电子转移反应的热力学和动力学参数的内在趋势，以证明His-配体在Rieske中心以外的潜在的PCET作用。(B)揭示在特定设计的[2Fe-2S]团簇上的本征配体重排和交换过程，这对于理解生物Fe/S团簇成熟和转移过程中的配体交换是相关的；具体地说，将研究从固体硫代高铁酸盐(MFeS_2)的聚合结构中产生[2Fe-2S]单元和更大片段的过程。(C)表征与NO结合的[2Fe-2S]S/N混合团簇与NO反应形成的物种，以揭示团簇氧化态及其质子化状态对NO反应途径的影响，以及对NO、H_2S与其他小信号分子之间的潜在串扰的影响。(D)模拟自由基SAM酶中辅助铁/S辅助因子的自由基反应性，并阐明控制铁/S簇向底物供硫的因素，如生物素合成酶(BIOB)或硫磺基合酶(LIPA)。(E)为SPP网络中的其他基团提供合适的化学合成的Fe/S模型体系，作为新的光谱或分析方法的基准体系。特别是，将对第一个在团簇核心内含有碳化物(C)或碳氢单元(CR)的Fe/S团簇采取合成战略。

Iron-sulfur (Fe/S) clusters are among the most widespread and ancient biological cofactors, occurring in virtually all domains of life. Their most prominent role is electron transfer (ET) in diverse pathway, and a range of Fe/S clusters of different nuclearity is available for that task. They mostly contain a sulfide-bridged iron cluster core that is ligated by terminal cysteine (Cys) thiolates from the protein. Chemically synthesized analogues of the active sites of Fe/S proteins have contributed significantly to our understanding of the electronic structures and intrinsic properties of the biological cofactors. In recent years, an increasing number of proteins containing Fe/S clusters with non-cysteine ligands, called alternative cluster ligands, is being discovered. This goes along with a rapidly increasing number of functions beyond electron transfer that are recognized for biological Fe/S cofactors, including their involvement in sensing of small molecules such as NO, DNA repair, radical reactions and catalysis.Continuing along the lines of the first SPP funding period and building upon the results achieved so far, this projects ultimately aims at deciphering the function of Fe/S cofactors with alternative cluster ligands in a synthetic analogue approach. Specific objectives are: (A) Elucidating intrinsic trends for the thermodynamic and kinetic parameters of proton coupled electron transfer (PCET) reactions at Fe/S clusters, mainly [2Fe-2S] clusters with His-like terminal ligands, to demonstrate a potential PCET role of His-ligation beyond the Rieske centers. (B) Unravelling intrinsic ligand rearrangement and exchange processes at specifically designed [2Fe-2S] clusters, which is relevant for understanding ligand exchange during biological Fe/S cluster maturation and transfer; specifically, the generation of [2Fe-2S] units and larger fragments from the polymeric structure of solid thioferrates (MFeS2) will be investigated. (C) Characterizing the species that form upon reaction of mixed S/N ligated [2Fe-2S] clusters with nitric oxide in order to enlighten the effect of cluster oxidation state and His protonation state on NO reactivity pathways, and on potential crosstalk between NO, H2S and other small signaling molecules. (D) Emulating radical reactivity at auxiliary Fe/S cofactors in radical SAM enzymes and elucidating factors that control sulfur donation from the Fe/S cluster to a substrate, as seen in biotin synthase (BioB) or lipoyl synthase (LipA). (E) Providing suitable chemically synthesized Fe/S model systems, which serve as benchmark systems for new spectroscopic or analytical methods, for other groups in the SPP network. In particular, synthetic strategies will be pursued towards a first Fe/S cluster that contains a carbide (C) or carbyne unit (CR) within the cluster core.