Enyzmatic cyclization of squalene analogs*

Abstract: Enzymatic cyclizations of squalene and oxidosqualene lead to sterols and other triterpenoids in bacteria, fungi, plants, and animals. The cyclases for these reactions catalyze formation and stabilization of polycyclic carbocations and direct the enzyme-speci®c, templated formation of new carbon-carbon bonds in regio- and stereochemically de®ned contexts. The development of mechanism-based irreversible inhibitors, photoactivatable inhibitors, and numer-ous substrate analogs have helped to unravel the stepwise events occurring in the catalytic sites of these enzymes by covalent modi®cation of speci®c amino acid residues. MECHANISM-BASED INHIBITORS Af®nity labeling of OSLC and SHC by 29-MOS and analogs The ®rst mechanism-based irreversible inactivator of mammalian oxidosqualene:lanosterol cyclase (OSLC) was 29-methylidene-2,3-oxidosqualene (29-MOS), which showed a Ki value of 4.4 mM and kinact 221 minÿ1 for pig OSLC [1]. The proposed mechanism of inhibition involves initial cyclization to the 21-methylidene-protosterol cation as proposed for lanosterol formation. However, this allylic cation can be trapped by an active-site nucleophile, resulting in covalent bond formation and concomitant irreversible inactivation (Fig. 1). We demonstrated that other 29-functionalized 2,3-oxidosqualen