Structural requirements for transformation of substrates by the S-adenosyl-L-methionine:delta 24(25)-sterol methyltransferase. Inhibition by analogs of the transition state coordinate.

摘要: Microsomes from sunflower seedlings were used to investigate the transition state coordinate for C-24 methylation reaction that mediates phytosterol biosynthesis. They then study structurally related cationic and uncharged compounds of natural sterol substrate, which designed interfere with progress. The hypothetical course is described proceed through an Sn2 formation activated complex involving initial production a covalent structure dative bond (methyl AdoMet attacks si-face 24,25-double sterol) secondary series high energy intermediates, stabilization determines final methylated product. Derivatives lanosterol cholesterol methyl, hydrogen, oxygen, or bromine atom introduced into side chain and/or at C-3 in place nucleophile studied as inhibitors tertiary isopropylcarbinyl cation intermediate conversion cycloartenal 24(28)-methylene cycloartanol. data indicate most potent inhibitor aziridine group attached C-24(25), mimics bridged C-24(25) carbenium ion generated state, methyltransferase possesses two strategic sites: one recognizes proximal end acting proton donor other distal acts acceptor. best fit (binding/catalysis) involves flat (including substrate inhibitor) intact unsubstituted nucleophilic centers freely rotating can assume pseudocyclic conformation.