Hunt Andrew P, Lehnert Nicolai
Department of Chemistry , University of Michigan , 930 North University Avenue , Ann Arbor , Michigan 48109 , United States.
Inorg Chem. 2019 Sep 3;58(17):11317-11332. doi: 10.1021/acs.inorgchem.9b00091. Epub 2019 Mar 26.
Cyt P450 nitric oxide (NO) reductase (P450nor) is an important enzyme in fungal denitrification, responsible for the large-scale production of the greenhouse gas NO. In the first step of catalysis, the ferric heme-thiolate active site of P450nor binds NO to produce a ferric heme-nitrosyl or {FeNO} intermediate (in the Enemark-Feltham notation). In this paper, we present the low-temperature preparation of six new heme-thiolate {FeNO} model complexes, [Fe(TPP)(SPh*)(NO)], using a unique series of electron-poor thiophenolates (SPh*), and their detailed spectroscopic characterization. Our data show experimentally, for the first time, that a direct correlation exists between the thiolate donor strength and the Fe-NO and N-O bond strengths, evident from the corresponding stretching frequencies. This is due to a σ-trans effect of the thiolate ligand, which manifests itself in the population of an Fe-N-O σ-antibonding (σ*) orbital. Via control of the thiolate donor strength (using hydrogen bonds), nature is therefore able to exactly control the degree of activation of the FeNO unit in P450nor. Vice versa, NO can be used as a sensitive probe to quantify the donor strength of a thiolate ligand in a model system or protein, by simply measuring the Fe-NO and N-O frequencies of the ferric NO adduct and then projecting those data onto the correlation plot established here. Finally, we are able to show that the σ-trans effect of the thiolate is the electronic origin of the "push" effect, which is proposed to mediate O-O bond cleavage and Compound I formation in Cyt P450 monooxygenase catalysis.
细胞色素P450一氧化氮（NO）还原酶（P450nor）是真菌反硝化作用中的一种重要酶，负责大量产生温室气体NO。在催化的第一步中，P450nor的铁血红素硫醇盐活性位点与NO结合，生成铁血红素亚硝酰基或{FeNO}中间体（采用埃内马克 - 费尔瑟姆表示法）。在本文中，我们展示了使用一系列独特的贫电子硫酚盐（SPh*）低温制备六种新的血红素硫醇盐{FeNO}模型配合物[Fe(TPP)(SPh*)(NO)]，以及它们详细的光谱表征。我们的数据首次通过实验表明，硫醇盐供体强度与Fe - NO和N - O键强度之间存在直接相关性，这从相应的伸缩频率中可以明显看出。这是由于硫醇盐配体的σ - 反式效应，它表现为Fe - N - O σ反键（σ*）轨道的占据。因此，通过控制硫醇盐供体强度（利用氢键），自然界能够精确控制P450nor中FeNO单元的活化程度。反之，通过简单测量铁亚硝酰加合物的Fe - NO和N - O频率，然后将这些数据投射到此处建立的相关图上，NO可以用作敏感探针来量化模型系统或蛋白质中硫醇盐配体的供体强度。最后，我们能够表明硫醇盐的σ - 反式效应是“推动”效应的电子起源，该效应被认为在细胞色素P450单加氧酶催化中介导O - O键裂解和化合物I的形成。
