Sterol methyltransferase2: Purification, properties, and inhibition

Expression of the Arabidopsis sterol methyltransferase2 (SMT2) cDNA in Escherichia coli yields a native protein, when purified to homogeneity, has the predicted molecular mass ca. 40kDa on SDS-PAGE and recognizes native sterols synthesized by Arabidopsis with a Δ²⁴⁽²⁵⁾-bond (cycloartenol; K_m 35μM and k_cat 0.001s^-1) and Δ ²⁴⁽²⁸⁾-bond (24(28)-methylenelophenol; K_m 28μM and k_cat 0.01s^-1). Cycloartenol was converted to a single olefinic product-24(28)-methylenecycloartanol whereas 24(28)-methylenelophenol was converted to a mixture of three stereochemically related products with the Δ²⁴⁽²⁸⁾Z-ethylidene, Δ²⁴⁽²⁸⁾E-ethylidene, and Δ²⁵⁽²⁷⁾-24β-ethyl side chains. Structural determinants essential to activity were the nucleophilic features at C-3 and C-24. The double bond position in the sterol substrate influenced catalytic efficiency according to the order: side chain, Δ²⁴⁽²⁴⁾<Δ ²⁴⁽²⁸⁾ and nucleus, Δ⁷<Δ ⁸<Δ⁵=9,19-cyclopropane. The 14α-methyl group was harmful to catalysis, reducing the suitability of cycloartenol as a substrate. On the basis of substrate activity and product distribution, SMT action was probed further using substrate (26,27-dehydrozymosterol: 26,27-DHZ) and intermediate (25-azacycloartenol: 25-AC) analogs of the SMT-catalyzed reactions. 26,27-DHZ was C-methylated to 26-homocholesta-8(9), 23(24)E, 26(26′)-trienol as well as 26-homocholesta-8(9),26(26′)-3β, 24β-dienol by SMT2, K_m of 15μM, k_cat of 0.001s^-1. In addition, 26,27-DHZ acted as a mechanism-based irreversible inhibitor that results in the specific covalent modification of SMT2, exhibiting K_i of 49μM, k_inact of 0.009s ^-1 and partition ratio of 0.11. Substrate protection with zymosterol, 24(28)-methylenelophenol against 26,27-DHZ and similar inhibition of the first and second C₁-transfer activities by the reversible inhibitor 25-AC of K_i 20nM suggested the analogs interacted at the same active site. [28E-²H]- and [28Z-²H]24(28)-methylenelanosterols were paired with AdoMet and differences of ²H-incorporation in the enzyme-generated 24-ethyl olefins supported an antimechanism. The results suggest plant SMT2 has a position-specific substrate specificity for Δ²⁴⁽²⁵⁾-sterols and contains a single active center to catalyze the consecutive C₁-transfer activities by substrate reaction channels similar to the fungal SMT1.