Conversion of squalene to the pentacarbocyclic hopene

The membrane protein squalene-hopene cyclase was cocrystallized with 2-azasqualene and analyzed by X-ray diffraction to 2.13 Angstrom resolution. The conformation of this close analog was clearly established, and it agreed with the common textbook presentation. The bound squalene undergoes only small conformational changes during the formation of rings A through D, thus requiring no intermediate. However, ring E formation is hindered by an entropic barrier, which may explain its absence in the steroids. The structure analysis revealed a mobile region between the active center cavity and the membrane, which may melt, opening a passage for squalene and hopene.