Separated Concerted Proton-Electron Transfer with Biomimetic Models of Peroxidase

描述(申请人提供)：非常广泛的生化过程涉及质子和电子的运动，称为质子耦合电子转移(PCET)。这在血红素酶的作用中尤其明显，包括细胞色素P-450、过氧化物酶、过氧化氢酶和许多其他酶。底物氧化和这些酶激活O2或H2O2都需要精确控制质子和电子的运动。一个完整的生物系统的复杂性，即使在体外，也很难分析对PCET重要的特征。这项建议提出了一个实验计划，将在与这些酶的高价铁氧基中间体化合物I和化合物II相关的仿生系统中发展对PCET的详细机制理解。特别重要的是质子和电子的转移在单个动力学步骤中发生的机制，如协同质子-电子转移(CPET)。CPET机制可以避免在分离的质子转移(PT)或电子转移(ET)时产生的高能中间体。这一提议中的一个重要区别是分离的协同质子-电子转移(SCPET)的概念，在该概念中，质子和电子起源于或终止于不同的位置。这与氢原子转移(HAT)反应形成对比，在HAT反应中，PT和ET都从一个单一物种转移到另一个单一物种。在这里，我们建议使用定义良好的高价铁-血红素(卟啉)-氧代模型络合物进行电化学、光谱和化学动力学研究，并选择非血红素铁氧络合物。要得到的关键热力学信息包括使用不同的卟啉和轴向配体的铁氧基和相关中间体的酸度(PKa)和还原电位(E)。将使用跨越一定强度范围的质子和电子给体来开发质子化和铁氧基官能团还原的综合动力学轮廓，而Eyring分析将为这些转化提供自由能垒。这将使在所研究的仿生系统中区分sCPET和单独的ET-PT或PT-ET机制，并将为将sCPET的概念扩展到包含铁血红素和非血红素铁氧基中间体的复杂生物系统提供实验基础。 与公共健康相关：含铁酶执行与细胞能量存储和转换、光合作用和中和有害物质相关的显著化学变化。这里提出的工作将通过开发这些化学转化的简化实验模型来更好地理解这些酶是如何完成这些任务的。对这些化学模型系统的研究将为了解整个生物系统的功能和功能提供重要的新见解。

DESCRIPTION (provided by applicant): A very wide variety of biochemical processes involve movement of protons and electrons, called proton-coupled electron transfer (PCET). This is particularly evident in the actions of heme enzymes, including cytochromes P-450, peroxidases, catalases, and many other enzymes. Both substrate oxidation and O2 or H2O2 activation by these enzymes require precise control of the movement of protons and electrons. The complexity of a full biological system, even in vitro, makes it difficult to analyze the features important to PCET. This proposal presents an experimental program that will develop a detailed mechanistic understanding of PCET in biomimetic systems relevant to the high-valent ferryl oxo intermediates compound I and compound II of these enzymes. Of particular importance are mechanisms where the transfer of protons and electrons occurs in a single kinetic step, as concerted proton-electron transfer (CPET). The CPET mechanism can avoid high-energy intermediates that would be otherwise be generated upon separate proton transfer (PT) or electron transfer (ET). An important distinction being made in this proposal is the concept of separated concerted proton-electron transfer (sCPET), in which the proton and electron to originate from or terminate at separate sites. This contrasts with hydrogen atom transfer (HAT) reactions where both PT and ET proceed from one single species to another single species. Herein, we propose electrochemical, spectroscopic, and chemical kinetic investigations using well-defined high-valent iron-heme (porphyrin)-oxo model complexes and select non-heme iron oxo complexes. Key thermodynamic information to be derived includes the acidity (pKa) and the reduction potential (E) of ferryl oxo and related intermediates using different porphyrin and axial ligands. A comprehensive kinetic profile for protonation and reduction of the ferryl oxo functional group will be developed using proton and electron donors that span a range of strengths, and Eyring analyses will provide the free energy barriers for these transformations. This will enable discrimination between sCPET and isolated ET-PT or PT-ET mechanisms in the biomimetic systems studied, and will provide an experimental foundation for extending the concept of sCPET to complex biological systems that incorporate ferryl heme and non-heme iron oxo intermediates. PUBLIC HEALTH RELEVANCE: Iron-containing enzymes execute remarkable chemical transformations related to cellular energy storage and conversion, photosynthesis, and neutralizing harmful materials. The work proposed here will provide greater understanding of how these enzymes achieve these tasks by developing simplified experimental models of these chemical transformations. Studies of these chemical model systems will provide important new insights into the functions and functioning of the full biological systems.