GTP Cyclohydrolase I as a Potential Drug Target: New Insights into Its Allosteric Modulation via Normal Mode Analysis

Guanosine triphosphate (GTP) cyclohydrolase I (GCH1) catalyzes the conversion of GTP into dihydroneopterin triphosphate (DHNP). DHNP is the first intermediate of the folate de novo biosynthesis pathway in prokaryotic and lower eukaryotic microorganisms and the tetrahydrobiopterin (BH4) biosynthesis pathway in higher eukaryotes. The de novo folate biosynthesis provides essential cofactors for DNA replication, cell division, and synthesis of key amino acids in rapidly replicating pathogen cells, such as Plasmodium falciparum (P. falciparum), a causative agent of malaria. In eukaryotes, the product of the BH4 biosynthesis pathway is essential for the production of nitric oxide and several neurotransmitter precursors. An increased copy number of the malaria parasite P. falciparum GCH1 gene has been reported to influence antimalarial antifolate drug resistance evolution, whereas mutations in the human GCH1 are associated with neuropathic and inflammatory pain disorders. Thus, GCH1 stands as an important and attractive drug target for developing therapeutics. The GCH1 intrinsic dynamics that modulate its activity remains unclear, and key sites that exert allosteric effects across the structure are yet to be elucidated. This study employed the anisotropic network model to analyze the intrinsic motions of the GCH1 structure alone and in complex with its regulatory partner protein. We showed that the GCH1 tunnel-gating mechanism is regulated by a global shear motion and an outward expansion of the central five-helix bundle. We further identified hotspot residues within sites of structural significance for the GCH1 intrinsic allosteric modulation. The obtained results can provide a solid starting point to design novel antineuropathic treatments for humans and novel antimalarial drugs against the malaria parasite P. falciparum GCH1 enzyme.

三磷酸鸟苷（Guanosine triphosphate, GTP）环水解酶I（GCH1）可催化将GTP转化为三磷酸二氢新蝶呤（dihydroneopterin triphosphate, DHNP）。DHNP是原核生物与低等真核微生物叶酸从头合成途径的首个中间产物，同时也是高等真核生物四氢生物蝶呤（tetrahydrobiopterin, BH4）生物合成途径的首个中间产物。叶酸从头合成途径可为快速增殖的病原细胞（例如引发疟疾的恶性疟原虫Plasmodium falciparum, P. falciparum）提供DNA复制、细胞分裂以及关键氨基酸合成所需的必需辅因子。在真核生物中，BH4生物合成途径的产物是一氧化氮合成与多种神经递质前体生成的必需辅因子。已有研究表明，疟原虫P. falciparum的GCH1基因拷贝数增加会影响抗疟疾抗叶酸类药物的耐药性演化；而人类GCH1基因的突变则与神经性疼痛及炎症性疼痛病症相关。因此，GCH1是开发治疗性药物的重要且极具吸引力的靶点。然而，调控GCH1活性的内在动态机制尚未明确，其结构中发挥变构调控作用的关键位点仍有待阐明。本研究采用各向异性网络模型（anisotropic network model），分别分析了游离状态GCH1蛋白结构以及其与调控伴侣蛋白结合状态下的内在运动模式。研究发现，GCH1的通道门控机制受整体剪切运动以及中央五螺旋束的向外扩张所调控。本研究进一步在与GCH1内在变构调控相关的结构位点中，鉴定出了热点残基。本研究结果可为开发针对人类的新型神经性疼痛治疗药物，以及靶向恶性疟原虫GCH1酶的抗疟疾新药提供坚实的研究基础。