Schulz-Gasch Tanja, Stahl Martin
Molecular Design, Pharmaceutical Division, F. Hoffmann-La Roche AG, Pharmaceuticals Division, Discovery Technologies, Bldg. 092/2.10D, CH-4070 Basel, Switzerland.
J Comput Chem. 2003 Apr 30;24(6):741-53. doi: 10.1002/jcc.10147.
2,3-Oxidosqualene cyclases (OSC) are key enzymes in sterol biosynthesis. They catalyze the stereoselective cyclization and skeletal rearrangement of (3S)-2,3-oxidosqualene to lanosterol in mammals and fungi and to cycloartenol in algae and higher plants. Sequence information and proposed mechanism of 2,3-oxidosqualene cyclases are closely related to those of squalene-hopene cyclases (SHC), which represent functional analogs of OSCs in bacteria. SHCs catalyze the cationic cyclization cascade converting the linear triterpene squalene to fused ring compounds called hopanoids. High stereoselectivity and precision of the skeletal rearrangements has aroused the interest of researchers for nearly half a century, and valuable data on studying mechanistic details in the complex enzyme-catalyzed cyclization cascade has been collected. Today, interest in cyclases is still unbroken, because OSCs became targets for the development of antifungal and hypocholesterolemic drugs. However, due to the large size and membrane-bound nature of OSCs, three-dimensional structural information is still not available, thus preventing a complete understanding of the atomic details of the catalytic mechanism. In this work, we discuss results gained from homology modeling of human OSC based on structural information of SHC from Alicyclobacillus acidocaldarius and propose a structural model of human OSC. The model is in accordance with previously performed experimental studies with mechanism-based suicide inhibitors and mutagenesis experiments with altered activity and product specificity. Structural insight should strongly stimulate structure-based design of antifungal or cholesterol-lowering drugs.
2,3-氧化角鲨烯环化酶（OSC）是甾醇生物合成中的关键酶。它们催化（3S）-2,3-氧化角鲨烯在哺乳动物和真菌中立体选择性环化并骨架重排生成羊毛甾醇，在藻类和高等植物中生成环阿屯醇。2,3-氧化角鲨烯环化酶的序列信息和推测机制与角鲨烯-藿烯环化酶（SHC）密切相关，SHC是细菌中OSC的功能类似物。SHC催化阳离子环化级联反应，将线性三萜角鲨烯转化为称为类藿烷的稠环化合物。骨架重排的高立体选择性和精确性在近半个世纪以来一直引起研究人员的兴趣，并且已经收集了关于研究复杂酶催化环化级联反应机制细节的有价值数据。如今，对环化酶的兴趣依然不减，因为OSC成为了抗真菌和降胆固醇药物开发的靶点。然而，由于OSC的大尺寸和膜结合性质，三维结构信息仍然不可得，因此阻碍了对催化机制原子细节的全面理解。在这项工作中，我们基于嗜酸 Alicyclobacillus acidocaldarius 的 SHC 的结构信息讨论了人类 OSC 同源建模的结果，并提出了人类 OSC 的结构模型。该模型与先前基于机制的自杀抑制剂实验以及活性和产物特异性改变的诱变实验结果一致。结构上的深入了解应该会有力地促进抗真菌或降胆固醇药物的基于结构的设计。
