Probing the sterol binding site of soybean sterol methyltransferase by site-directed mutagenesis: Functional analysis of conserved aromatic amino acids in Region 1

Soybean sterol methyltransferase (SMT) in the presence of AdoMet catalyzes the transmethylation of the Δ²⁴-bond of the sterol side chain to produce phytosterols with a methyl(lene) or ethyl(idene) group at C-24. The function of six aromatic amino acids associated with the putative active center of the SMT, i.e., Region 1 that extends from Phe82 to Phe93 in soybean SMT, was studied by site-directed mutagenesis and heterologous expression in BL21(DE3) bacterial cells. The enzyme-generated products were characterized kinetically and by GC-MS analysis. Substitution of the aromatic amino acids at positions 82, 83, 85, 87, 91, and 93 with a leucine residue produced mutant SMTs with varying activities. The mutants converted cycloartenol to 24(28)-methylene cycloartanol [C₁-activity] from a few percent to as much as 95% of the control activity. In contrast, none of the leucine mutants were found to catalyze 24(28)-methylene lophenol [C₂-activity], suggesting a loss of function associated with the second C₁-transfer activity. In contrast to the loss of the second C₁-transfer activity of the Phe82Leu, replacement of the Phe82 residue to isoleucine had minimal effect on the first or second C₁-transfer activities, suggesting that the increased bulk (branching) in the leucine side chain contributes to significant perturbations in the active site that generate inaccurate positioning of the substrate side chain disfavoring the second C₁-transfer activity. Replacement of Tyr83 to phenylalanine resulted in an increase of the specificity constant (k_cat/K_m) for the substrate of the second C₁-transfer activity by a factor of 5 compared to control and an increase of Δ²⁴⁽²⁸⁾Z-ethylidene sterol formation in the 24-ethyl sterol product set, suggesting that loss of steric bulk from the phenolic hydroxyl group on tyrosine generates a less precise fit of the Δ²⁴⁽²⁸⁾ sterol side chain into the active site favoring the second C₁-transfer activity and prompting reaction channeling during catalysis. Circular dichroism spectra, equilibrium dialysis studies of AdoMet, and chromatographic information of the wild-type and Tyr83 mutants confirmed retention of the overall conformation of the enzyme during the experiments. Together, these findings suggest that the amino acids of Region 1 provide a tight substrate orientation imposed by hydrophobic interactions between the sterol side chain and the SMT active site contacts and control the production and processing of the transmethylation pathways governed by the first and second C₁-transfer activities.