N-acetylglutamate Synthase: Structure, Function & Defects

描述（由申请人提供）： N-乙酰谷氨酸合酶（NAGS）是产生同源辅因子N-乙酰谷氨酸（NAG）的酶，其是哺乳动物中第一和限速尿素生成酶（CPS I）的必需变构激活剂，并且是微生物中精氨酸生物合成的第一个确定底物。我们对小鼠和人类NAGS基因以及来自不同物种的许多其他NAGS基因的克隆和表达，使我们现在有可能深入了解这种有趣的蛋白质的结构/功能。我们发现在一些变形菌物种（X。campestris，M. maris、马里斯岛O. alexamii，黄毛菊X. axonopodis和X. fastidiosa狄克逊）的双功能NAGS/NAGK的基因，其类似于哺乳动物NAGS，并且我们已经获得了其蛋白质晶体。最近，我们获得了一个高质量的密度图，这将导致确定的第一个三维结构的NAGS（从N。淋病）。由于NAGS可能是尿素生成的调节剂，并且其功能受精氨酸的变构影响，因此现在可以了解精氨酸作用的机制并比较肝脏与肠道组织中NAGS的调节。遗传性NAGS缺乏症、有机酸缺乏症和丙戊酸盐治疗中的NAG缺乏症导致高氨血症，高氨血症经常导致脑损伤、发育障碍和死亡。更好地了解NAG/NAGS系统将改善这些疾病的诊断和治疗。本项目的具体目标是：1）解决NAGS的配体和非配体结构，并表征其催化机制和精氨酸的作用：2）表征跨门NAGS蛋白的生化性质，重点是精氨酸对结构和功能的影响;第三章通过表征和比较NAGS在肝脏中表达的调节，肠; 4）确定导致遗传性NAGS缺陷的自然发生的突变的功能效应。生物化学，晶体学和分子生物学的方法将被用来深入了解结构生物学，生物化学，病理生理学，基因型/表型相关的NAGS基因和蛋白质在这个系统的进化发展的背景下。在解决了NAGS的第一个三维结构之后，难治性NAGS蛋白的其他结构将变得可用。这将导致构建哺乳动物NAGS的结构模型，推导出在催化机制，确定精氨酸作用的机制，以及突变引起NAGS功能障碍和高氨血症的影响。

DESCRIPTION (provided by applicant): N-acetylglutamate synthase (NAGS) is an enzyme that produces the cognate cofactor N-acetylglutamate (NAG), an essential allosteric activator of the first and rate limiting enzyme of ureagenesis (CPS I) in mammals, and the first committed substrate for arginine biosynthesis in microorganisms. Our cloning and expression of the mouse and human NAGS genes and many other NAGS genes from various species, makes it now possible to gain structure/function insights into this interesting protein. We found in a number of proteobacteria species (X. campestris, M. maris, O. alexandrii, X. axonopodis, and X. fastidiosa Dixon) a gene for a bifunctional NAGS/NAGK that is similar to mammalian NAGS and for which we have obtained protein crystals. Recently, we obtained a high quality density map which will lead to the determination of the first three-dimensional structure of NAGS (from N. gonorrhoeae). Since NAGS is a likely regulator of ureagenesis and its function is allosterically affected by arginine, it is now possible to understand the mechanism(s) of the arginine effect and to compare the regulation of NAGS in hepatic vs. intestinal, tissues. The deficiency of NAG in inherited NAGS deficiency, organic acidemias and valproate treatment causes hyperammonemia that frequently leads to brain damage, developmental disabilities and death. Better understanding of the NAG/NAGS system will improve the diagnosis and treatment of these conditions. The specific aims of this project are 1) To solve the liganded and unliganded structures of NAGS and characterize mechanisms for catalysis and the effect of arginine; 2) To characterize the biochemical properties of NAGS proteins across phyla, focusing on the effect of arginine on structure and function; 3) To differentiate regulatory mechanisms that are specific to liver ureagenesis by characterizing and comparing the regulation of NAGS expression in liver and intestine; 4) To determine the functional effects of naturally-occurring mutations that cause inherited NAGS deficiency. Biochemical, crystallographic and molecular methods will be employed to gain an in depth understanding of the structural biology, biochemistry, pathophysiology, genotype/phenotype correlations of the NAGS genes and proteins in the context of evolutionary development of this system. After the first three- dimensional structure of NAGS has been solved, other structures of refractory NAGS proteins will become available. This will lead to constructing a structural model of mammalian NAGS, deriving at a catalytic mechanism, determining the mechanism of arginine effect, and the effects of mutations causing NAGS dysfunction and hyperammonemia.