Stereochemistry of Hydrogen Migration from C-24 to C-25 during Phytosterol Biomethylation

Characterization, mutagenesis and mechanistic analysis of an ancient algal sterol C24-methyltransferase: Implications for understanding sterol evolution in the green lineage

Sterol C24-methyltransferases (SMTs) constitute a group of sequence-related proteins that catalyze the pattern of sterol diversity across eukaryotic kingdoms. The only gene for sterol alkylation in green algae was identified and the corresponding catalyst from Chlamydomonas reinhardtii (Cr) was characterized kinetically and for product distributions. The properties of CrSMT were similar to those predicted for an ancient SMT expected to possess broad C3-anchoring requirements for substrate binding and formation of 24β-methyl/ethyl Δ(25(27))-olefin products typical of primitive organisms. Unnatural Δ(24(25))-sterol substrates, missing a C4β-angular methyl group involved with binding orientation, convert to product ratios in favor of Δ(24(28))-products. Remodeling the active site to alter the electronics of Try110 (to Leu) results in delayed timing of the hydride migration from methyl attack of the Δ(24)-bond, that thereby produces metabolic switching of product ratios in favor of Δ(25(27))-olefins or impairs the second C1-transfer activity. Incubation of [27-(13)C]lanosterol or [methyl-(2)H3]SAM as co-substrates established the CrSMT catalyzes a sterol methylation pathway by the "algal" Δ(25(27))-olefin route, where methylation proceeds by a conserved SN2 reaction and de-protonation proceeds from the pro-Z methyl group on lanosterol corresponding to C27. This previously unrecognized catalytic competence for an enzyme of sterol biosynthesis, together with phylogenomic analyses, suggest that mutational divergence of a promiscuous SMT produced substrate- and phyla-specific SMT1 (catalyzes first biomethylation) and SMT2 (catalyzes second biomethylation) isoforms in red and green algae, respectively, and in the case of SMT2 selection afforded modification in reaction channeling necessary for the switch in ergosterol (24β-methyl) biosynthesis to stigmasterol (24α-ethyl) biosynthesis during the course of land plant evolution.

Mechanism-based enzyme inactivators of phytosterol biosynthesis

Current progress on the mechanism and substrate recognition by sterol methyl transferase (SMT), the role of mechanism-based inactivators, other inhibitors of SMT action to probe catalysis and phytosterol synthesis is reported. SMT is a membrane-bound enzyme which catalyzes the coupled C-methylation-deprotonation reaction of sterol acceptor molecules generating the 24-alkyl sterol side chains of fungal ergosterol and plant sitosterol. This C-methylation step can be rate-limiting in the post-lanosterol (fungal) or post-cycloartenol (plant) pathways. A series of sterol analogs designed to impair SMT activity irreversibly have provided deep insight into the C-methylation reaction and topography of the SMT active site and as reviewed provide leads for the development of antifungal agents.

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. 40 kDa on SDS-PAGE and recognizes native sterols synthesized by Arabidopsis with a Delta(24(25))-bond (cycloartenol; K(m) 35 microM and k(cat) 0.001s(-1)) and Delta(24(28))-bond (24(28)-methylenelophenol; K(m) 28 microM and k(cat) 0.01 s(-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 Delta(24(28))Z-ethylidene, Delta(24(28))E-ethylidene, and Delta(25(27))-24 beta-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, Delta(24(24))<Delta(24(28)) and nucleus, Delta(7)<Delta(8)<Delta(5)=9,19-cyclopropane. The 14 alpha-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')-3 beta,24 beta-dienol by SMT2, K(m) of 15 microM, k(cat) of 0.001 s(-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 49 microM, k(inact) of 0.009 s(-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 C(1)-transfer activities by the reversible inhibitor 25-AC of K(i) 20 nM 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 Delta(24(25))-sterols and contains a single active center to catalyze the consecutive C(1)-transfer activities by substrate reaction channels similar to the fungal SMT1.

Biosynthesis of phytosterols. Kinetic mechanism for the enzymatic C-methylation of sterols

Cloned soybean sterol methyltransferase was purified from Escherichia coli to gel electrophoretic homogeneity. From initial velocity experiments, catalytic constants for substrates best suited for the first and second C1 transfer activities, cycloartenol and 24(28)-methylenelophenol, were 0.01 and 0.001 s-1, respectively. Two-substrate kinetic analysis using cycloartenol and S-adenosyl-l-methionine (AdoMet) generated an intersecting line pattern characteristic of a ternary complex kinetic mechanism. The high energy intermediate analog 25-azacycloartanol was a noncompetitive inhibitor versus cycloartenol and an uncompetitive inhibitor versus AdoMet. The dead end inhibitor analog cyclolaudenol was competitive versus cycloartenol and uncompetitive versus AdoMet. 24(28)-Methylenecycloartanol and AdoHcy generated competitive and noncompetitive kinetic patterns, respectively, with respect to AdoMet. Therefore, 24(28)-methylenecycloartanol combines with the same enzyme form as does cycloartenol and must be released from the enzyme before AdoHcy. 25-Azacycloartanol inhibited the first and second C1 transfer activities with about equal efficacy (Ki = 45 nm), suggesting that the successive C-methylation of the Delta 24 bond occurs at the same active center. Comparison of the initial velocity data using AdoMet versus [2H3-methyl]AdoMet as substrates tested against saturating amounts of cycloartenol indicated an isotope effect on VCH3/VCD3 close to unity. [25-2H]24(28)-Methylenecycloartanol, [28E-2H]24 (28)-methylenelanosterol, and [28Z-2H]24(28)-methylene lanosterol were prepared and paired with AdoMet or [methyl-3H3]AdoMet to examine the kinetic isotope effects attending the C-28 deprotonation in the enzymatic synthesis of 24-ethyl(idene) sterols. The stereochemical features as well as the observation of isotopically sensitive branching during the second C-methylation suggests that the two methylation steps can proceed by a change in chemical mechanism resulting from differences in sterol structure, concerted versus carbocation; the kinetic mechanism remains the same during the consecutive methylation of the Delta 24 bond.

Enzyme mechanisms for sterol C-methylations

The mechanisms by which sterol methyl transferases (SMT) transform olefins into structurally different C-methylated products are complex, prompting over 50 years of intense research. Recent enzymological studies, together with the latest discoveries in the fossil record, functional analyses and gene cloning, establish new insights into the enzymatic mechanisms of sterol C-methylation and form a basis for understanding regulation and evolution of the sterol pathway. These studies suggest that SMTs, originated shortly after life appeared on planet earth. SMTs, including those which ultimately give rise to 24 alpha- and 24 beta-alkyl sterols, align the si(beta)-face pi-electrons of the Delta(24)-double bond with the S-methyl group of AdoMet relative to a set of deprotonation bases in the active site. From the orientation of the conformationally flexible side chain in the SMT Michaelis complex, it has been found that either a single product is formed or cationic intermediates are partitioned into multiple olefins. The product structure and stereochemistry of SMT action is phylogenetically distinct and physiologically significant. SMTs control phytosterol homeostasis and their activity is subject to feedback regulation by specific sterol inserts in the membrane. A unified conceptual framework has been formulated in the steric-electric plug model that posits SMT substrate acceptability on the generation of single or double 24-alkylated side chains, which is the basis for binding order, stereospecificity and product diversity in this class of AdoMet-dependent methyl transferase enzymes. The focus of this review is the mechanism of the C-methylation process which, as discussed, can be altered by point mutations in the enzyme to direct the shape of sterol structure to optimize function.

Active site mapping and substrate channeling in the sterol methyltransferase pathway

Sterol methyltransferase (SMT) from Saccharomyces cerevisiae was purified from Escherichia coli BL21(DE3) and labeled with the mechanism-based irreversible inhibitor [3-3H]26,27-dehydrozymosterol (26,27-DHZ). A 5-kDa tryptic digest peptide fragment containing six acidic residues at positions Glu-64, Asp-65, Glu-68, Asp-79, Glu-82, and Glu-98 was determined to contain the substrate analog covalently attached to Glu-68 by Edman sequencing and radioanalysis using C18 reverse phase high performance liquid chromatography. Site-directed mutagenesis of the six acidic residues to leucine followed by activity assay of the purified mutants confirmed Glu-68 as the only residue to participate in affinity labeling. Equilibration studies indicated that zymosterol and 26,27-DHZ were bound to native and the E68L mutant with similar affinity, whereas S-adenosylmethionine was bound only to the native SMT, K(d) of about 2 microm. Analysis of the incubation products of the wild-type and six leucine mutants by GC-MS demonstrated that zymosterol was converted to fecosterol, 26,27-DHZ was converted to 26-homo-cholesta-8(9),23(24)E,26(26')-trienol as well as 26-homocholesta-8(9),26(26')-3beta,24beta-dienol, and in the case of D79L and E82L mutants, zymosterol was also converted to a new product, 24-methylzymosta-8,25(27)-dienol. The structures of the methylenecyclopropane ring-opened olefins were determined unambiguously by a combination of (1)H and (13)C NMR techniques. A K(m) of 15 microm, K(cat) of 8 x 10(-4) s(-1), and partition ratio of 0.03 was established for 26,27-DHZ, suggesting that the methylenecyclopropane can serve as a lead structure for a new class of antifungal agents. Taken together, partitioning that leads to loss of enzyme function is the result of 26,27-DHZ catalysis forming a highly reactive cationic intermediate that interacts with the enzyme in a region normally not occupied by the zymosterol high energy intermediate as a consequence of less than perfect control. Alternatively, the gain in enzyme function resulting from the production of a delta(25(27))-olefin originates with the leucine substitution directing substrate channeling along different reaction channels in a similar region at the active site.

Stereochemical fate of C-26 and C-27 during the conversion of isofucosterol to sitosterol and of 24-methylenecholesterol to campesterol and dihydrobrassicasterol in Oryza sativa cell cultures

Administration of pro-R-methyl-13C-labeled isofucosterol to cultured cells of Oryza sativa revealed that the pro-R and pro-S methyls at C-25 become the pro-R and pro-S methyls at C-25 of sitosterol, respectively. Similar administration experiments using pro-S-methyl-13C-labeled 24-methylenecholesterol established that the pro-R and pro-S methyls at C-25 of 24-methylenecholesterol become the pro-R and pro-S methyls of campesterol, and the pro-S and pro-R methyls of dihydrobrassicasterol, respectively. These results are compatible with our recently proposed 'syn-SE2' mechanism' for double bond isomerization of delta 24(28) into delta 24(25).

Sterol C-methyl transferase from Prototheca wickerhamii mechanism, sterol specificity and inhibition

The membrane-bound sterol methyl transferase (SMT) enzyme from Prototheca wickerhamii, a non-photosynthetic, yeast-like alga, was found to C-methylate appropriate delta24(25)-sterol acceptor molecules to delta25(27)-24beta-methyl products stereoselectively. Incubation with pairs of substrates--[2H3-methyl]AdoMet and cycloartenol, and AdoMet and [27-(13)C]lanosterol--followed by 1H and 13C NMR analysis of the isotopically labeled products demonstrated the si-face (beta-face attack) mechanism of C-methylation and the regiospecificity of delta25(27)-double bond formation from the pro-Z methyl group (C27) on lanosterol. The enzyme has a substrate preference for a sterol with a 3beta-hydroxyl group, a planar nucleus and a side chain oriented into a 'right-handed' structure (20R-chirality) characteristic of the native substrate, cycloartenol. The apparent native molecular weight of the SMT was determined to be approximately 154,000, as measured by Superose 6 FPLC. A series of sterol analogues which contain heteroatoms substituted for C24 and C25 or related structural modifications, including steroidal alkaloids, havs been used to probe further the active site and mechanism of action of the SMT enzyme. Sterol side chains containing isoelectronic modifications of a positively charged moiety in the form of an ammonium group substituted for carbon at C25, C24, C23 or C22 are particularly potent non-competitive inhibitors (Ki for the most potent inhibitor tested, 25-azacycloartanol, was ca. 2 nM, four orders of magnitude less than the Km for cycloartenol of 28 microM), supporting the intermediacy of the 24-methyl C24(25)-carbenium ion intermediate. Ergosterol, but neither cholesterol nor sitosterol, was found to inhibit SMT activity (Ki = 80 microM). The combination of results suggests that the interrelationships of substrate functional groups within the active center of a delta24(25) to delta25(27) 24beta-methyl-SMT could be approximated thereby allowing the rational design of C-methylation inhibitors to be formulated and tested.

Isolation and characterization of an active-site peptide from a sterol methyl transferase with a mechanism-based inhibitor

Chemical affinity labeling of pure sterol methyl transferase (SMT) from Saccharomyces cerevisiae using the mechanism-based irreversible inhibitor, [3-3H]26,27-dehydrozymosterol, inhibited the SMT with an apparent Ki of 1.1 microM and k(inact) of 1.52 min(-1). The protein-inhibitor adduct was subjected to cleavage with trypsin and the resulting covalently modified peptide was analyzed by Edman sequencing from the N-terminus. The radiochemically labeled ca. 5.0 kDa peptide fragment of the cleavage mixture was shown to be contiguous through 17 residues to a segment that includes a highly conserved hydrophobic motif (Region I, stretching between T78 and F91) characteristic of SMT enzymes. The results confirm that Region I is the sterol binding/active site.

4,4,14 alpha-trimethyl 9 beta,19-cyclo-5 alpha-26-homocholesta-24,26-dien-3 beta-ol: a potent mechanism-based inactivator of delta 24(25)- to delta 25(27)-sterol methyl transferase

The title compound (4A) was synthesized and tested as a mechanism-based inactivator of the sterol methyl transferase (SMT) enzyme from Prototheca wickerhamii. Using cycloartenol as substrate, 4A was found to exhibit time-dependent inactivation kinetics, generating a Ki value of 30 microM and Kinact value of 0.30 min-1.

Overexpression, purification, and stereochemical studies of the recombinant (S)-adenosyl-L-methionine: delta 24(25)- to delta 24(28)-sterol methyl transferase enzyme from Saccharomyces cerevisiae

The ERG6 gene that encodes (S)-adenosyl-L-methionine: delta 24(25)-to delta 24(28)-sterol methyl transferase (SMT) enzyme from Saccharomyces cerevisiae was introduced into plasmid pET23a(+) and the resulting native protein was overexpressed in BL21 (DE3) host cells under control of a T7 promoter. This enzyme was purified to apparent homogeneity by ammonium sulfate precipitation, anion exchange, and hydrophobic interaction chromatography. N-Terminal sequence analysis of the first 10 amino acids of the purified SMT protein confirmed the identity of the start triplet and expected primary structure. The enzyme exhibited a turnover number of 0.01/s and an isoelectric point of 5.95. A combination of Superose 6 chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified SMT enzyme possessed a native molecular weight of 172,000 and was tetrameric. The purified SMT enzyme generated kinetics in which velocity versus substrate curves relative to zymosterol (preferred sterol acceptor molecule) and AdoMet were sigmoidal rather than hyperbolic, indicating enzyme cooperativity among the subunits. Studies on product formation using [27-13C]zymosterol and [2H3-methyl]AdoMet incubated with the pure SMT enzyme confirmed the reaction mechanism of sterol methylation to involve a 1,2-hydride shift of H-24 to C-25 from the Re-face of the original 24,25- double bond. Deduced amino acid sequence comparisons of the SMT polypeptide from S. cerevisiae with related sterol methyl transferase enzymes of plant and fungal origin indicate that there is a significant degree of similarity between these enzymes. Specifically, there is a conserved sequence (in yeast from amino acids ca. 79 to 92 which contains an YEXGWG motif; referred to as Region I) that is not present in other AdoMet-dependent methyl transferase enzymes.

Substrate-based inhibitors of the (S)-adenosyl-L-methionine:delta24(25)- to delta24(28)-sterol methyl transferase from Saccharomyces cerevisiae

A series of 31 side-chain-modified analogs of cholesterol, zymosterol, lanosterol, and cycloartenol and the steroidal alkaloids solasodine and solanidine were studied as inhibitors of (S)-adenosyl-L-methionine:delta24(25)-sterol methyl transferase (SMT) enzyme activity from Saccharomyces cerevisiae. Two classes of sterol methylation inhibitors were tested: substrate analogs, including mechanism-based inhibitors, and transition state analogs. Several novel sterol methylation inhibitors that contained an aza, aziridine, or ammonium group in the sterol side chain were prepared and tested for the first time. The degree and kinetic pattern of methylation inhibition were found to be influenced by the position and nature of the variant functional group introduced into the side chain. The most potent inhibitors of SMT enzyme activity were transition state analog inhibitors (Ki values of 5 to 10 nM) that mimicked the structure and conformation of the natural substrate presumed to form in the ternary complex generated in the transition state. Steroidal alkaloids were potent competitive inhibitors with Ki values ranging from 2 to 30 microM, which is about the Kmapp of zymosterol, ca. 27 microM. An isosteric analog of the natural substrate, zymosterol, in which the 26/27-gem-dimethyl groups were joined to form a cyclopropylidene function is shown to be a potent irreversible mechanism-based inactivator of SMT enzyme activity that exhibits competitive-type inhibition, Ki 48 microM with a K(inact) of 1.52 min(-1). Mechanistic implications of these results provide new insights into the topology of the ternary complex involving sterol-AdoMet-enzyme.