Mechanistic Studies of Functional Switching in the PutA Flavoprotein

描述（由申请人提供）：本研究的总体目标是为参与调节和催化脯氨酸代谢的多功能酶的氧化还原功能开关提供分子和结构上的理解。在革兰氏阴性菌中，脯氨酸转化为谷氨酸的两步过程是由脯氨酸利用酶PutA （proline utilization A）催化的，PutA是一种大型膜相关黄酶。PutA通过协调黄素依赖的脯氨酸脱氢酶（PRODH）和NAD+依赖的1-吡啶- 5-羧酸脱氢酶（P5CDH）结构域的活性，催化脯氨酸的四电子氧化生成谷氨酸。在某些原核生物如大肠杆菌中，PutA还含有一个带-螺旋-螺旋（RHH） dna结合域，并且是脯氨酸利用基因PutA和putP（编码高亲和力脯氨酸转运蛋白）的自生转录抑制因子。为了实现其作为转录抑制因子和膜结合酶的互斥功能，PutA经历了依赖脯氨酸的功能转换。因此，具有DNA结合活性的PutAs是独特的三功能黄蛋白，通过响应脯氨酸的可用性作为细胞代谢的传感器。早期的研究已经证实脯氨酸还原黄素激活PutA膜结合，从而触发PutA从转录抑制因子转变为膜结合酶。本研究的主要假设是黄素活性位点的氧化还原信号控制着PutA的构象、亚细胞定位和功能。本研究的目的是

DESCRIPTION (provided by applicant): The overall goal of this study is to provide molecular and structural understanding for the redox based functional switching of a multifunctional enzyme involved in regulating and catalyzing proline metabolism. The two-step conversion of proline to glutamate in Gram-negative bacteria is catalyzed by PutA (proline utilization A), a large membrane-associated flavoenzyme. PutA catalyzes the four-electron oxidation of proline to glutamate by coordinating the activities of separate flavin-dependent proline dehydrogenase (PRODH) and NAD+-dependent 1-pyrroline- 5-carboxylate dehydrogenase (P5CDH) domains. In certain prokaryotes such as Escherichia coli, PutA also contains a ribbon-helix-helix (RHH) DNA-binding domain and is an autogenous transcriptional repressor of the proline utilization genes putA and putP (encodes a high affinity proline transporter). To fulfill its mutually exclusiv functions as a transcriptional repressor and membrane-bound enzyme, PutA undergoes proline-dependent functional switching. Thus, PutAs with DNA binding activity are unique trifunctional flavoproteins that act as sensors of cellular metabolism by responding to proline availability. Earlier studies have established that proline reduction of the flavin activates PutA membrane- binding thereby triggering PutA switching from a transcriptional repressor to a membrane-bound enzyme. The principal hypothesis of this proposal is that redox signals in the flavin active site control the conformation, subcellular localization, and function of PutA. The goal of this study is to further examine this hypothesis by building a structural and dynamic model for how reduction of the flavin cofactor drives PutA functional switching. Several major milestones achieved in the previous funding period form the basis for the proposed studies. In particular, conformational changes in the flavin itself and surrounding active site residues were identified and shown to be critical for initiating functional switching. The thermodynamic and structural basis of the PutA repressor function was elucidated. The first crystal structure of a full-length bifunctional PutA was determined. The solution structure of a trifunctional PutA was modeled using SAXS data and crystal structures of domains. And most recently, the elusive membrane-binding domain of PutA was identified. These results provide an outstanding framework for understanding, at unprecedented detail, the molecular mechanisms whereby PutA transforms from a gene regulatory protein to a membrane-bound enzyme. A new direction integrated into this study is to understand how proline catabolism is coupled to reduction of the respiratory chain in vivo. The specific aims are the following: 1. Determine the organization and structure of trifunctional PutA. 2. Characterize the bioenergetics of proline metabolism. 3. Elucidate the mechanism of functional switching in PutA. PUBLIC HEALTH RELEVANCE: The amino acid proline has multifaceted roles that impact human health. Inborn errors in proline metabolic genes are manifested in neurological dysfunctions such as schizophrenia, increased incidence of seizures, connective tissue diseases, premature aging, and osteopenia. This project will further the understanding of proline metabolism in cancer preventing mechanisms, type I hyperprolinemia, schizophrenia susceptibility, and pathogens that rely on proline as a major fuel source, such as the causative agents of peptic ulcers (Helicobacter pylori) and African sleeping sickness (Trypanosoma brucei).