設計與合成細菌轉醣酶抑制劑之研究

Abstract

抗生素是人們對抗致病型細菌的利器，然而因抗生素長期的濫用及過度依賴，使得許多因突變而具抗藥性的菌種相繼出現，抗藥性的問題漸漸成為醫療上的難題，也使人類健康受到莫大的威脅，因此科學家希望能開發新穎的抗生素以對抗細菌，讓人類能繼續與細菌抗衡。細菌擁有哺乳類動物細胞所沒有的細胞壁，其對細菌的存活相當重要。而轉醣酶是形成細菌細胞壁所特有的酵素，不存在於哺乳類動物中，且其暴露於細菌細胞膜的表面，藥物不需要進入細胞質，這也使得藥物更易抵達活性區對酵素進行抑制，因此科學家認為以轉醣酶為新的研究目標開發新型的抗生素，相當具有潛力。 我們所設計之轉醣酶抑制劑，其設計概念主要是模擬以lipid II為單體，在轉醣化過程中所形成的過渡態之電荷及結構，並搭配moenomycin作為結構修飾的參考，設計出新穎具有潛力的轉醣酶抑制劑。我們利用moenomycin脂質區的特徵基團作為基礎骨架，以短鏈之phosphoglycerate衍生物來模擬lipid II的雙磷酸脂質鏈，接著以適當長度的連接鏈，連接模擬lipid II多醣部分骨架之芳香環結構。 合成出一系列潛在的轉醣酶抑制劑之後，我們將這些化合物進行轉醣酶活性分析及最低抑菌濃度檢測。在這幾個phosphoglycerate衍生物中，具有活性之化合物其結構為苯環上帶有適當的取代基；化合物44和47在500 μM下對轉醣酶皆具有抑制性，化合物44對於金黃色葡萄球菌的最低抑菌濃度為50 μM。 我們推測苯環上所延伸之適當取代基，在距離上剛好能與支鏈帶amide基團的天門冬醯胺殘基(Asn275)有作用力，使得此類化合物對轉醣酶具有抑制性。後續我們將以化合物44和47為基礎，嘗試在苯環上修飾能產生氫鍵作用力的取代基(如OH基)，以發展出更具有抑制效果的轉醣酶抑制劑。

Using antibiotics has a great impact on the treatment of bacterial infections. However, the emergence of multi-drug resistant bacteria requires new antibacterial targets and drugs. Bacteria are surrounded by the netlike polymer peptidoglycan, which is responsible for a defined cell shape and allows bacteria to live in a variable internal osmotic environment. Among the several enzymes involved in cell-wall biosynthesis, transglycosylase (TGase) is considered a promising antibiotic target due to its essential function and ready accessibility. TGase is located on the external surface of bacterial membrane so that inhibitors are easy to reach. We has designed and synthesized some potential transglycosylase inhibitors with the aryl moiety bearing different substituents to mimic the saccharide part of lipid II, the natural substrate of TGase. On the other hand, a long chain phosphoglycerate, which is a characteristic portion of moenomycin, a natural TGase inhibitor, was used to mimic the long chain pyrophosphate part of lipid II. These kinds of inhibitors are expected to block the process of transglycosylation by mimicking the transition state of lipid II polymerization. Some potential transglycosylase (TGase) inhibitors were synthesized and subjected to the HPLC-based TGase fluorescence assay and MIC assay. Among these derivatives of phosphoglycerate with the aryl moiety bearing different substituents, compounds 44 and 47 showed the best inhibition against TGase at 500 μM, and compound 44 has the best MIC value of 50 μM against Staphylococcus aureus. We suggested that the appropriate substituent on the benzene ring could interact with the amide side chain of Asn275 in the active site of TGase to show a good inhibition. Based on the structures of 44 and 47, further introduction of hydrogen-bonding groups (e.g. OH) on the aryl moiety might improve the inhibitory activity to act as efficient TGase inhibitors.