Proton coupled electron transfer and the mechanism of MAO catalysis

在化学系生命过程化学（CLP）项目的支持下，James M.弗吉尼亚理工学院和州立大学的Tanko和巴勃罗索布拉多正在研究单胺氧化酶-A和-B（MAO-A和MAO-B）的作用机制。这些是哺乳动物中存在的重要酶，在多巴胺、去甲肾上腺素、肾上腺素和血清素等神经递质的代谢中起关键作用。对于这些天然（生物）胺，这种氧化的机制似乎是很好理解的。单胺氧化酶也与中枢神经系统疾病如抑郁症和帕金森氏病有关，因此人们在开发单胺氧化酶抑制剂方面付出了相当大的努力。然而，这些MAO抑制剂中的一些如L-丙炔苯丙胺或帕吉林不能通过与生物胺相同的机制反应。具有类似结构特征的其他化合物已被证明是有效的神经毒素。Tanko和索布拉多教授将测试并进一步发展一个新的假设，即某些胺存在特定的结构特征，激活另一种氧化机制（质子耦合电子转移或PCET），这种机制与大多数天然底物的酶通常遵循的机制根本不同。这个合作项目，连接生物，有机，物理和量子化学，预计将导致更好地了解这些过程如何工作（或分解），并在疾病的情况下，告知基于基本化学原理的治疗策略。自由基和自由基离子是化学和生物学中重要的化学物种，它们的研究将是本文的重点。作为该提案的教育/外联部分，将制作一系列题为“有机化学：缺失的章节”的录像带，以解决许多有机化学入门教科书中对这些物种的有限讨论。课程内容包括生物化学背景下的自由基和自由基离子（酶催化过程，自氧化等）。MAO催化途径将底物的初始净双电子氧化与其亚胺基代谢物与氧化的黄素辅因子的双电子还原偶联。这项工作的核心假设是，底物和黄素辅因子之间的电子转移总是以不利的平衡存在，但只有当所得的自由基阳离子是极端酸性时才变得重要;本质上，这种不利的电子转移发生是因为它与极其有利的质子转移相结合。导致自由基阳离子呈酸性的结构特征存在于许多常见的MAO抑制剂和一种称为MPTP的化合物中，MPTP是一种导致人类帕金森病症状的强效神经毒素。这些实验将涉及仿生方法，使用模拟酶化学的黄素基化学模型化合物，以发展对反应机制的基本理解。从这些实验中获得的洞察力将为在生物相关条件下使用重组MAO-A和MAO-B进行的实验提供信息。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

With the support of the Chemistry of Life Processes (CLP) Program in the Division of Chemistry, Professors James M. Tanko and Pablo Sobrado of Virginia Polytechnic Institute and State University are studying the mechanism of action for monoamine oxidase-A and -B (MAO-A and MAO-B). These are important enzymes present in mammals, which play a key role in the metabolism of neurotransmitters such as dopamine, norepinephrine, epinephrine and serotonin. For these natural (biogenic) amines, the mechanism of this oxidation appears to be well understood. MAOs have also been linked to central nervous system disorders such as depression and Parkinson’s disease, and considerable effort has gone into the development of MAO inhibitors. However, some of these MAO inhibitors such as L-deprenyl or pargyline cannot react by the same mechanism as the biogenic amines. Other compounds, which possess similar structural features have proven to be potent neurotoxins. Professors Tanko and Sobrado will test and further develop a new hypothesis that with certain amines, specific structural features are present that activate an alternative mechanism of oxidation (proton coupled electron transfer or PCET) that is fundamentally different from the mechanism typically followed by the enzyme with most natural substrates. This collaborative project, linking biological, organic, physical, and quantum chemistry, is expected to lead to a better understanding of how these processes work (or break down) and, in the case of disease, inform strategies for treatment based upon fundamental chemical principles. Radicals and radical ions are important chemical species in chemistry and biology and their study will be a focus here. As part of the education/outreach component of this proposal, a series of videos entitled "Organic Chemistry: The Missing Chapters" will be developed to address the limited discussion of these species in many introductory organic chemistry textbooks. Topics to be covered include radicals and radical ions in the context of biological chemistry (enzyme catalyzed processes, autoxidation, etc.) and organic chemistry (mechanisms and synthesis).The MAO catalytic pathway couples the initial net two-electron oxidation of the substrate to its iminyl metabolite with a two-electron reduction of the oxidized flavin cofactor. The core hypothesis of this work is that electron transfer between the substrate and flavin cofactor is always present as an unfavorable equilibrium, but only becomes important when the resulting radical cation is extremely acidic; in essence, this unfavorable electron transfer occurs because it is coupled to an extremely favorable proton transfer. The structural features that cause the radical cation to be acidic are present in many common MAO inhibitors, and in a compound known as MPTP-a potent neurotoxin that leads to symptoms of Parkinson's disease in humans. These experiments will involve a biomimetic approach, using flavin-based chemical model compounds that mimic the chemistry of the enzyme, to develop the fundamental understanding of the reaction mechanism. The insight gained from these experiments will then inform experiments conducted under biologically relevant conditions, using recombinant MAO-A and MAO-B.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.