Rationally designed inhibitors as tools for comparing the mechanism of squalene-hopene cyclase with oxidosqualene cyclase

The inhibition of squalene-hopene cyclase (SHC) (E.C. 5.4.99.-), an enzyme of bacterial membranes catalyzing the formation of pentacyclic sterol-like triterpenes, was studied by using different classes of compounds originally developed as inhibitors of oxidosqualene cyclase (OSC) (E.C. 5.4.99.7), th e enzyme of eukaryotes responsible for the formation of tetracyclic precurs ors of sterols. The mechanism of cyclization of squalene by SHC, beginning with a protonation of the 2,3 double bond by an acidic residue of the enzym e, followed by a series of electrophilic additions of the carbocationic int ermediates to the double bonds, is similar to the mechanism of cyclization of 2,3-oxidosqualene by OSC. The inhibitors studied included: (i) analogs o f the carbocationic intermediates formed during cyclization, such as aza-an alogs of squalene and 2,3-oxidosqualene; (ii) affinity-labeling inhibitors bearing a methylidene reactive group; and (iii) vinyldioxidosqualenes and v inyl sulfide derivatives of the substrates. Comparison of the results obtai ned with the two enzymes, SHC and OSC, showed that many of the most effecti ve inhibitors of OSC were also able to inhibit SHC, while some derivatives acted as specific inhibitors. Differences could be easily explained on the basis of the different substrate specificity of the two enzymes.