Understanding the Selectivity of Oxygen Activation by Model Iron-Porphyrin Catalysts

项目总结 含血红素的蛋白质是自然界中含量最丰富的金属蛋白之一。一个重要的子集 这些催化剂利用氧气(O2)进行反应，包括细胞色素C氧化酶(CcO)，它能还原 氧到水，和细胞色素P450(细胞色素P450)，激活氧气以氧化有机 底物。CYP和CcO的功能依赖于它们将O2还原为水的能力。在一种“非耦合”中 过程中，一些当量的还原剂被浪费，活性氧物种(ROS)，如过氧化氢 释放了。众所周知，ROS会导致人体内的氧化应激和多种疾病，因此 发展对这种解耦的理解是很重要的。据推测，一种催化性的Fe-OH 中间体是H_2O和H_2O_2生成的分叉点。这种氢过氧基中间体是 广泛存在于含血红素的酶中，包括细胞色素c过氧化物酶、血红素加氧酶和 前列腺素H合成酶。在所有情况下，适当的质子输送对于O-O键的断裂都是必要的。 P450cam的诱变研究表明，质子和氢键的作用是重要的 在理解选择性方面，但关于保守残基如何防止解偶联存在争议。 此外，正则机制包括在近端或远端氧原子上加成一个质子。 分别产生过氧化氢或水，但没有直接证据表明这一点 化学计量学。我们的初步数据表明，所需的远端质子化可能涉及更高的 对质子的依赖。因此，我们的目标是研究是什么控制了H2O形成之间的选择性 或从这一关键的Fe-OOH中间体中释放出过氧化氢。 我们在这项提案中的工作是为了了解从血红素中生成H2O和过氧化氢的选择性 利用一个简单的模型系统。合成类似物提供的优势是允许我们 系统地改变和控制结构实体，在新的地方独立进入催化循环 合成中间体。因此，我们建议研究铁卟啉对O2活化的选择性 催化剂通过相同的Fe-OOH中间体。首先，我们将探讨各种反应是如何 条件(浓度、pKa和酸的结构)影响催化氧还原的选择性。 反应(ORR)。其次，我们将研究具有不同氢键基序的各种催化剂，以更好地了解 活性中心中的残基如何影响选择性。此外，我们还将探索非催化 Fe-OOH中间体的反应性以获得水和H_2O_2的相对速率的独立测量 在不同的条件下形成。最终，我们的目标是了解反应条件和H- 在模型体系中，成键网络影响H_2O和H_2O_2的选择性。这种理解将提供洞察力 酶如何控制关键的铁-过氧化氢中间体的反应活性以减少ROS 在各种含亚铁血红素的活性部位形成。

PROJECT SUMMARY Heme-containing proteins are among the most abundant metalloproteins in nature. A significant subset of these catalysts perform reactions utilizing dioxygen (O2), including cytochrome c oxidase (CcO) which reduces dioxygen to water, and cytochrome P450 (CYP) which activates dioxygen in order to oxidize organic substrates. The functions of CYP and CcO rely on their abilities to reduce O2 to water. In an “uncoupled” process, some equivalents of reductant are wasted and reactive oxygen species (ROS) such as H2O2 are released. ROS are known to lead to oxidative stress and a variety of diseases in the human body, so developing an understanding of this uncoupling is important. It is hypothesized that a catalytic Fe–OOH intermediate is the site of bifurcation between H2O and H2O2 formation. This hydroperoxy intermediate is ubiquitous in heme-containing enzymes including cytochrome c peroxidase, heme oxygenase, and prostaglandin H synthase. In all cases, proper proton delivery is necessary for O–O bond cleavage. Mutagenesis studies of P450cam have made it clear that the role of protons and H-bonding are important in understanding selectivity, but there is debate as to how the conserved residues prevent uncoupling. Additionally, the canonical mechanism involves one proton addition to either the proximal or distal oxygen atom of the Fe–OOH intermediate to yield H2O2 or H2O, respectively, but there is no direct evidence of this stoichiometry. Our preliminary data suggests that the desired distal protonation may involve a higher dependence on protons. Therefore our goal is to study what governs the selectivity between formation of H2O or release of H2O2 from this critical Fe–OOH intermediate. Our work in this proposal seeks to understand the selectivity of H2O and H2O2 formation from heme enzymes utilizing a simple model system. Synthetic analogues provide the advantage of allowing us to systematically vary and control structural entities, enter the catalytic cycle in new places, and independently synthesize intermediates. Therefore, we propose to study the selectivity of O2 activation by Fe-porphyrin catalysts proceeding through the same Fe–OOH intermediate. First, we will explore how various reaction conditions (concentration, pKa, and structure of the acid) affect selectivity in the catalytic oxygen reduction reaction (ORR). Secondly, we will study a variety of catalysts with varied H-bonding motifs to better understand how the residues in an active site may influence selectivity. Additionally, we will explore the non-catalytic reactivity of the Fe–OOH intermediate to gain independent measures of the relative rates of H2O and H2O2 formation under varied conditions. Ultimately, our goal is to understand how the reaction conditions and H- bonding networks affect H2O versus H2O2 selectivity in a model system. This understanding will provide insight into how enzymes can control the reactivity of the critical Fe-hydroperoxy intermediate to minimize ROS formation in a variety of heme-containing active sites.