Proton coupled electron transfer and the mechanism of MAO catalysis

在化学系生命过程化学(CLP)计划的支持下，弗吉尼亚理工学院和州立大学的James M.Tanko和Pablo Sobrado教授正在研究单胺氧化酶-A和-B(MAO-A和MAO-B)的作用机制。这些是哺乳动物体内存在的重要酶，在多巴胺、去甲肾上腺素、肾上腺素和5-羟色胺等神经递质的新陈代谢中发挥关键作用。对于这些天然(生物)胺，这种氧化的机制似乎是众所周知的。MAO还与抑郁症和帕金森氏症等中枢神经系统疾病有关，MAO抑制剂的开发也付出了相当大的努力。然而，其中一些MAO抑制剂，如L-丙戊烯或帕吉林，不能通过与生物胺相同的机制起作用。其他具有类似结构特征的化合物已被证明是有效的神经毒素。Tanko和Sobrado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，该结构特征激活了另一种氧化机制(质子耦合电子转移或PCET)，该机制与大多数天然底物的酶通常遵循的机制根本不同。这个合作项目将生物、有机、物理和量子化学联系在一起，预计将导致更好地了解这些过程是如何工作(或分解)的，并在疾病的情况下，根据基本化学原理提供治疗策略。自由基和自由基离子是化学和生物学中重要的化学物种，对它们的研究将是本文的重点。作为这项提议的教育/宣传部分的一部分，将制作一系列题为“有机化学：缺失的章节”的视频，以解决许多入门有机化学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)。和有机化学(机理和合成)。MAO催化途径将底物初始净两电子氧化为其亚胺代谢物与氧化黄素辅因子的两电子还原相耦合。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，但只有在产生的自由基阳离子极酸性时才变得重要；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耦合。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抑制剂中，以及一种名为MPTP的化合物中--MPTP是一种强烈的神经毒素，会导致人类出现帕金森氏症的症状。这些实验将涉及一种仿生方法，使用基于黄素的化学模型化合物来模拟酶的化学，以发展对反应机理的基本理解。从这些实验中获得的洞察力将为在生物相关条件下进行的实验提供依据，使用重组MAO-A和MAO-B。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.