Mechanism of Energy Transduction and Substrate Activation in Biological Nitrogen Fixation

项目摘要/摘要 这项建议旨在阐明细菌金属酶固氮酶是如何催化 在化学上困难地将大气中的氮素转化为生物可利用的形式氨， 以及它为什么/如何利用ATP水解来推动这一反应。是唯一负责的酶 对于还原固氮，固氮酶维持着约40%的农业/营养需求 人类人口。固氮酶除了具有全球重要性外，还是一种独特的模式系统 与生物氧化还原催化以及ATP/GTP依赖的能量广泛相关 转导过程，这两个过程都是正常细胞功能的中心，因此直接 与人类健康相关。 尽管近50年来广泛的生化、生物物理和结构 关于固氮酶机理的两个最重要的问题 详细回答：a)为什么以及如何最终利用ATP水解物进行还原 是氮气还是替代底物？B)氮素还原的密切机制是什么？ 固氮酶活性中心金属簇，FeMoco？实验中的主要挑战是 固氮酶的研究源于固氮酶的催化活性 依赖于持续的ATP周转，这导致了氧化还原和 很难相互区分的固氮酶的核苷酸结合状态。至 绕过这一挑战，我们启动了一个研究低温电子的项目 原子级固氮酶动态结构表征的显微(低温电子显微镜) 酶促转换条件下的拆分。初步实验不仅 确定了此方法的可行性，但也揭示了 固氮酶引发了新的机械论假说。在建议的项目中，我们的目标是 在这些初步发现的基础上，通过a)绘制ATP驱动的构象 在催化周转条件下，固氮酶的景观以前所未有的细节和b) 原子中FeMoco结构动力学及FeMoco与小分子相互作用的研究 决议，同时也有助于开发尖端的低温EM方法 用于高度复杂/动态的蛋白质组件的结构询问和 金属辅因子。

PROJECT SUMMARY/ABSTRACT This proposal aims to elucidate how the bacterial metalloenzyme nitrogenase catalyzes the chemically difficult transformation of atmospheric dinitrogen into a bioavailable form, ammonia, and why/how it utilizes ATP hydrolysis to drive this reaction. Being the only enzyme responsible for reductive nitrogen fixation, nitrogenase sustains the agricultural/nutritional needs of ~40% of the human population. Aside from its global importance, nitrogenase is a unique model system with broad relevance to biological redox catalysis as well as ATP/GTP-dependent energy transduction processes, which are both central to proper cellular functioning and thus directly relevant to human health. Despite nearly five decades of extensive biochemical, biophysical, and structural characterization, the two most important questions about nitrogenase mechanism have not been answered in detail: a) Why and how ATP hydrolysis is ultimately utilized for the reduction of N2 or alternative substrates? b) What is the intimate mechanism of dinitrogen reduction on the nitrogenase active site metal cluster, FeMoco? The major experimental challenge in the investigations of nitrogenase arises from the fact that the catalytic activity of nitrogenase depends on continuous ATP turnover, which leads to a heterogeneous mixture of redox and nucleotide-bound states of nitrogenase that are difficult to distinguish from one another. To circumvent this challenge, we have initiated a research program in cryogenic electron microscopy (cryoEM) to structurally characterize dynamic states of nitrogenase at atomic resolution under enzymatic turnover conditions. Preliminary experiments have not only established the feasibility of this approach but also revealed unexpected structural features of nitrogenase which have fueled new mechanistic hypotheses. In the proposed project, we aim to build upon on these preliminary findings by a) mapping the ATP-driven conformational landscape of nitrogenase in unprecedented detail under catalytic turnover conditions and b) elucidating FeMoco structural dynamics and FeMoco-small molecule interactions in atomic resolution, while also c) contributing to the development of cutting-edge cryoEM methodologies for the structural interrogation of highly complex/dynamic protein assemblies and metallocofactors.