  4.2.1.129 | hopan-22-ol |
squalenehopene cyclase from prokaryotes catalyses the conversion of the acyclic molecule of squalene into the pentacyclic triterpenes of hopene and hopanol in the the ratio 5:1. The polycyclization of squalene to hopene consists of sequential ring-forming reaction steps. The polycyclization cascade is initiated by the electrophilic attack of the acidic proton, donated by the DXDD motif, to one of the two terminal double bonds. The polycyclization reaction is quenched by proton elimination from the alternative terminal methyl group of squalene. Based on the X-ray analysis of Alicyclobacilus acidocaldarius, the catalytic base responsible for the deprotonation reaction has been suggested to be a water molecule (named a front water), the polarization of which is enhanced by other waters (named back waters) that construct the hydrogen-bonding network by a combination of seven residues T41, E45, E93, R127, Q262, W133 and Y267. The front water thus polarized can store the proton generated from either Me-29 or Me-30 of hopanyl cation to form hopene, but hopanol is produced if the front water adds as hydroxyl to the C-22 cation of the A-ring instead of accepting the proton |
Alicyclobacillus acidocaldarius |
squalene + H2O |
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