Inhibition of S-adenosyl-L-methionine sterol-C-24-methyltransferase by analogues of a carbocationic ion high-energy intermediate. Structure activity relationships for C-25 heteroatoms (N, As, S) substituted triterpenoid derivatives

Design of high energy intermediate analogues to study sterol biosynthesis in higher plants

Several enzymes of plant sterol biosynthesis involve during their catalysis postulated or demonstrated carbocationic high energy intermediates (HEI). The aim of this study was to interfere with plant sterol biosynthesis by means of rationally designed species able to mimic these carbocationic HEI. It has been demonstrated previously that the design of transition state (TS) or HEI analogues could lead to powerful and specific inhibitors of enzymes. We applied this approach to the following target enzymes: 2,3-epoxy-2,3-dihydroqualene cyclase, AdoMet-cycloartenol-C-24-methyltransferase (AdoMet CMT), cycloeucalenol-obtusifoliol isomerase (COI) and Δ(8)-Δ(7)-sterol isomerase. Very potent inhibitors have been obtained in the four cases. As an example, analogues of cycloartenol substituted at C-25 by a charged heteroatom (N, As, S) have been synthesized and shown to be able to mimic the C-25 carbocationic HEI involved in the reaction catalyzed by the AdoMet CMT. These compounds were shown to be very potent and specific inhibitors of this enzyme both in vitro (Ki=2.10(-8) M, Ki/Km=10(-3)) and in vivo. The potent inhibitors described are powerful tools to control in vivo the sterol profile of plant cells and therefore to study the structural and functional roles of sterols in cell membranes. Moreover, these compounds constitute leader molecules of a new class of rationally designed inhibitors which could be of value in plant protection.

Proteomics - The key to understanding systems biology of Arabidopsis trichomes

Every multicellular organism consists of numerous organs, tissues and specific cell types. To gain detailed knowledge about the morphogenesis of these complex structures, it is inevitable to advance biochemical analyses to ultimate spatial and temporal resolution since individual cell types contribute differently to the overall performance of living objects. Single cell sampling combined with systems biological approaches was recently applied to investigations of Arabidopsis thaliana trichomes (leaf hairs). These are single celled structures that provide ideal model systems to address various aspects of plant cell development and differentiation at the level of individual cells. A previously suggested function of trichomes in plant stress responses could thus be confirmed. Furthermore, trichome-specific "omics" data collected in several laboratories are mutually conclusive which demonstrates the applicability of systems biological approaches at the single cell level.

Cloning, functional expression and phylogenetic analysis of plant sterol 24C-methyltransferases involved in sitosterol biosynthesis

Sterol 24C-methyltransferases (SMTs) constitute a group of sequence-related proteins that catalyze the distinct patterns of 24-alkyl sterols that occur throughout nature. Two SMT cDNAs (SMT2-1 and SMT2-2) were cloned by homology based PCR methods from young leaves of Glycine max (soybean) and the corresponding enzymes were expressed functionally in Escherichia coli. The full-length cDNA for SMT2-1 and SMT2-2 have open reading frames of 1086 bp and 1092 bp, respectively, and encode proteins of 361 and 363 residues with a calculated molecular mass of 40.3 and 40.4 kDa, respectively. The substrate preference of the two isoforms was similar yet they differed from SMT1; kinetically SMT2-1 and SMT2-2 generated k(cat) values for the optimal substrate 24(28)methylene lophenol of 0.8 min(-1) and 1.34 min(-1), respectively, compared to the activity of SMT1 that generated a k(cat) for the optimal substrate cycloartenol of 0.6 min(-1). SMT2-2 was purified to homogeneity and the subunit organization shown to be tetrameric in similar fashion to other cloned SMTs. Analysis of the accumulated products catalyzed by the recombinant enzymes demonstrated that soybean SMT2-1 and SMT2-2 operate transalkylation activities analogous to the soybean plant SMT1. Metabolite analyses correlated with transcript profiling of the three SMT isoforms during soybean maturation clearly demonstrated that SMT isoform expression determines specific C24-methyl to C24-ethyl ratios to flowering whereas with seed development there is a disconnection such that the SMT transcript levels decrease against an increase in sterol content; generally SMT2-2 is expressed more than SMT2-1 or SMT1. These observations suggest that the genes that encode SMT1 and SMT2 in sitosterol biosynthesis may have undergone divergent evolution. In support of this proposition, the genomic organization for SMT1 of fungi and protozoa align very closely with one another and to those of the plant SMT2; both sets of SMTs lack introns. Unexpectedly, the SMT1 from Glycine max and other embryophytes of diverse origin possess disparate intron-exon characteristics that can be shown relates back to the algae. Our results suggest that the order of SMT1 appearing before SMT2 in phytosterol synthesis arose recently in plant evolution in response to duplication of a more primitive SMT gene likely to have been bifunctional and catalytically promiscuous.

SAR studies on azasterols as potential anti-trypanosomal and anti-leishmanial agents

There is an urgent need for the development of new drugs for the treatment of neglected tropical diseases such as human African trypanosomiasis, Chagas disease and leishmaniasis. Azasterols, have been shown to have activity against the parasites which cause these diseases. In this paper we report synthesis of new azasterols and subsequent analysis of the SAR. The chemistry focused on variations in the ester at the 3beta-position of the sterol and the position of the nitrogen in the side chain. The data allowed us to derive preliminary pharmacophore models for the activity of the azasterols against the parasites which cause these diseases.

Yeast sterol C24-methyltransferase: role of highly conserved tyrosine-81 in catalytic competence studied by site-directed mutagenesis and thermodynamic analysis

The role of Try-81 in the reaction catalyzed by Saccharomyces cerevisiae sterol 24-C-methyltransferase (Erg6p) was investigated kinetically and for product differences against a panel of position-81 mutants in which Tyr was substituted with Trp, Phe, Ile, Leu, Val and Ala. The residue chosen for mutation is one that was reported previously to accept fecosterol and yield a set 24-ethyl (idene) sterol products typical of plants, showing the amino acid residue is located close to the transient C25 carbocation intermediate in the active site. One group of mutants (aromatic) tested with the natural substrate zymosterol accelerated the C-methylation reaction (k(cat)/K(m)) whereas the other group of mutants (aliphatics) decreased catalytic competence as the amino acid side chain was downsized. Mutating to aromatic and assaying with the substrate analog designed as a suicide substrate 26,27-dehydrozymosterol favored C26-monol formation, whereas mutating to the aliphatic of smaller size favored C26-diol formation (a measure of enzyme alkylation). In no case was zymosterol converted to an intermediate that formed a C25-diol. Thermodynamic analysis (determination of E(a), DeltaG(double dagger), DeltaH(double dagger) and TDeltaS(double dagger)) for the C-methylation reaction performed by these enzymes assayed with the substrate and its analog or zymosterol paired with the "charged' high energy intermediate (HEI) analogs 24(R,S)25,epiminolanosterol and 25-azalanosterol or "neutral" membrane insert ergosterol showed that mutation to aromatics can reduce inhibitor potency (measured as K(m)/K(i)), yet catalysis can improve in Trp81 by the introduction of a gain in free energy associated with stabilization of the transition state of a rate-controlling step directed toward turnover. Alternatively, mutation to the smaller aliphatic amino acid side chains led to a destabilization in the active site structure which was accompanied by increases in the partition ratios associated with abortive complex formation. The results are explained by consideration of the functional differences attributed to Tyr81 substitution to aromatics and aliphatics of different size involved with cation-pi or hydrogen bonding interactions and in the activation barriers required of differing side chain conformations to orient the reactants in the direction of turnover versus enzyme inactivation.

Delta24(25)-sterol methenyltransferase: intracellular localization and azasterol sensitivity in Leishmania major promastigotes overexpressing the enzyme

Trypanosomatids contain predominantly ergostane-based sterols, which differ from cholesterol, the main sterol in mammalian cells, in the presence of a methyl group in the 24 position. The methylation is initiated by S-adenosyl-L-methionine:Delta(24 (25))-sterol methenyltransferase, an enzyme present in protozoa, but absent in mammals. The importance of this enzyme is underscored by its potential as a drug target in the treatment of the leishmaniases. Here, we report studies concerning the intracellular distribution of sterol methenyltransferase in Leishmania major promastigotes and overexpressing cells using a specific antibody raised against highly purified recombinant protein. It was found by immunofluorescence and electron microscopy studies that in L. major wild-type cells sterol methenyltransferase was primarily associated to the endoplasmic reticulum. In addition to this location, the protein was incorporated into translucent vesicles presumably of the endocytic pathway. We also found in this study that cells overproducing the enzyme do not have increased resistance to the sterol methenyltransferase inhibitor 22, 26 azasterol.

Evaluation of Azasterols as Anti-Parasitics

In this article, the design and synthesis of some novel azasterols is described, followed by their evaluation against Trypanosoma brucei rhodesiense, T. cruzi, Leishmania donovani, and Plasmodium falciparum, the causative agents of human African trypanosomiasis, Chagas disease, leishmaniasis, and malaria, respectively. Some of the compounds showed anti-parasitic activity. In particular, a number of compounds appeared to very potently inhibit the growth of the blood stream form T. b. rhodesiense, with one compound giving an IC50 value of 12 nM. Clear structure activity relationships could be discerned. These compounds represent important leads for further optimization. Azasterols have previously been shown to inhibit sterol biosynthesis in T. cruzi and L. donovani by the inhibition of the enzyme sterol 24-methyltransferase. However, in this case, none of the compounds showed inhibition of the enzyme. Therefore, these compounds have an unknown mode of action.

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 delta24-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 (kcat/Km) for the substrate of the second C1-transfer activity by a factor of 5 compared to control and an increase of delta24(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 delta24(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.

Mechanistic analysis of a multiple product sterol methyltransferase implicated in ergosterol biosynthesis in Trypanosoma brucei

Sterol methyltransferase (SMT) plays a key role in sterol biosynthesis in different pathogenic organisms by setting the pattern of the side chain structure of the final product. This catalyst, absent in humans, provides critical pathway-specific enzymatic steps in the production of ergosterol in fungi or phytosterols in plants. The new SMT gene was isolated from Trypanosoma brucei genomic DNA and cloned into an Escherichia coli expression system. The recombinant SMT was purified to homogeneity to give a band at 40.0 kDa upon SDS-PAGE and showed a tetrameric subunit organization by gel chromatography. It has a pH optimum of 7.5, an apparent kcat value of 0.01 s(-1), and a Km of 47 +/- 4 microm for zymosterol. The products of the reaction were a mixture of C24-monoalkylated sterols, ergosta-8,24 (25)-dienol, ergosta-8,25 (27)-dienol, and ergosta-8,24 (28)-dienol (fecosterol), and an unusual double C24-alkylated sterol, 24,24-dimethyl ergosta-8,25 (27)-dienol, typically found in plants. Inhibitory profile studies with 25-azalanosterol (Ki value of 39 nm) or 24(R,S), 25-epiminolanosterol (Ki value of 49 nm), ergosterol (Ki value of 27 microm) and 26,27-dehydrozymosterol (Ki and kinact values of 29 microm and 0.26 min(-1), respectively) and data showing zymosterol as the preferred acceptor strongly suggest that the protozoan SMT has an active site topography combining properties of the SMT1 from plants and yeast (37-47% identity). The enzymatic activation of this and other SMTs reveals that the catalytic requirements for the C-methyl reaction are remarkably versatile, whereas the inhibition studies provide a powerful approach to rational design of new anti-sleeping sickness chemotherapeutic drugs.

Enzyme redesign and interactions of substrate analogues with sterol methyltransferase to understand phytosterol diversity, reaction mechanism and the nature of the active site

Several STM (sterol methyltransferase) genes have been cloned, sequenced and expressed in bacteria recently, making it possible to address questions of the relationship between sterol structure and function. The active site and mechanism of action of a set of phylogenetically diverse SMTs have been probed by site-directed mutagenesis as well as by using substrate and related analogues of the SMT-catalysed reaction. An active-site model has been developed that is in accord with the results presented, which is consistent with the hypothesis that SMTs are bifunctional enzymes kinetically responsible to bind Δ24-acceptor sterols of specific steric and electronic character and rigid orientation imposed by multiple hydrophobic active site contacts exacted from a common waxy core. Functional divergence influenced by the architectural role of sterols in membranes is considered to govern the evolution of product distribution and specificity of individual SMTs as discussed.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Preparation of transition-state analogues of sterol 24-methyl transferase as potential anti-parasitics

There is an urgent need for new drugs to treat leishmaniasis and Chagas disease. One important drug target in these organisms is sterol biosynthesis. In these organisms the main endogenous sterols are ergosta- and stigmata-like compounds in contrast to the situation in mammals, which have cholesterol as the sole sterol. In this paper we discuss the design, synthesis and evaluation of potential transition state analogues of the enzyme Delta24(25)-methyltransferase (24-SMT). This enzyme is essential for the biosynthesis of ergosterol, but not required for the biosynthesis of cholesterol. A series of compounds were successfully synthesised in which mimics of the S-adenosyl methionine co-factor were attached to the sterol nucleus. Compounds were evaluated against recombinant Leishmania major 24-SMT and the parasites L. donovani and Trypanosoma cruzi in vitro, causative organisms of leishmaniasis and Chagas disease, respectively. Some of the compounds showed inhibition of the recombinant Leishmania major 24-SMT and induced growth inhibition of the parasites. Some compounds also showed anti-parasitic activity against L. donovani and T. cruzi, but no inhibition of the enzyme. In addition, some of the compounds had anti-proliferative activity against the bloodstream forms of Trypanosoma brucei rhodesiense, which causes African trypanosomiasis.

Elucidation of enzyme mechanisms using fluorinated substrate analogues

A great variety of biological reactions that are physiologically important are catalyzed by enzymes. Understanding the reaction course of these enzyme-catalyzed transformations are of significant importance since the insights gained from these experiments may facilitate the design of methods to control or mimic their actions. A common strategy to study enzyme catalyses is to use fluorinated substrate analogues as mechanistic probes, since fluorine is an effective hydroxyl group mimic and can also be used to replace a hydrogen atom. Using fluorinated substrate probes have enabled researchers to obtain crucial information regarding the catalytic mechanism of enzymatic reactions. Many of these compounds are good enzyme inhibitors and have been developed into clinically useful chemotherapeutic agents. This review will discuss some examples of the use of fluorine containing compounds as mechanistic probes/enzyme inhibitors, many of which are selected from our own work.

Sterol methyltransferase: functional analysis of highly conserved residues by site-directed mutagenesis

Sterol methyltransferase (SMT), the enzyme from Saccharomyces cerevisiae that catalyzes the conversion of sterol acceptor in the presence of AdoMet to C-24 methylated sterol and AdoHcy, was analyzed for amino acid residues that contribute to C-methylation activity. Site-directed mutagenesis of nine aspartate or glutamate residues and four histidine residues to leucine (amino acids highly conserved in 16 different species) and expression of the resulting mutant proteins in Escherichia coli revealed that residues at H90, Asp125, Asp152, Glu195, and Asp276 are essential for catalytic activity. Each of the catalytically impaired mutants bound sterol, AdoMet, and 25-azalanosterol, a high energy intermediate analogue inhibitor of C-methylation activity. Changes in equilibrium binding and kinetic properties of the mutant enzymes indicated that residues required for catalytic activity are also involved in inhibitor binding. Analysis of the pH dependence of log kcat/Km for the wild-type SMT indicated a pH optimum for activity between 6 and 9. These results and data showing that only the mutant H90L binds sterol, AdoMet, and inhibitor to similar levels as the wild-type enzyme suggest that H90 may act as an acceptor in the coupled methylation-deprotonation reaction. Circular dichroism spectra and chromatographic information of the wild-type and mutant enzymes confirmed retention of the overall conformation of the enzyme during the various experiments. Taken together, our studies suggest that the SMT active center is composed of a set of acidic amino acids at positions 125, 152, 195, and 276, which contribute to initial binding of sterol and AdoMet and that the H90 residue functions subsequently in the reaction progress to promote product formation.

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 and accumulation of sterols

In recent years, the impressive development of molecular genetics tools, the sequencing of the Arabidopsis thaliana genome, the availability of DNA or transposon tagged mutants, and the multiple possibilities offered by stable transformation with DNA in sense and antisense orientation have enabled the application of a strategy of gain or loss of function to study the sterol biosynthesis pathway. Here we describe the results obtained with these techniques. The results essentially confirm data obtained previously with sterol biosynthesis inhibitors (SBIs) and enable the precise dissection of biosynthetic pathways. We discuss the advantages and disadvantages of molecular genetics techniques as applied to sterol metabolism. The greater selectivity of these techniques constitutes an invaluable advantage and has led to the discovery of a role for sterols in plant development.

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.

The ratio of campesterol to sitosterol that modulates growth in Arabidopsis is controlled by STEROL METHYLTRANSFERASE 2;1

The Arabidopsis genome contains three distinct genes encoding sterol-C24-methyltransferases (SMTs) involved in sterol biosynthesis. The expression of one of them, STEROL METHYLTRANSFERASE 2;1, was modulated in 35S:SMT2;1 Arabidopsis in order to study its physiological function. Plants overexpressing the transgene accumulate sitosterol, a 24-ethylsterol which is thought to be the typical plant membrane reinforcer, at the expense of campesterol. These plants displayed a reduced stature and growth that could be restored by brassinosteroid treatment. Plants showing co-suppression of SMT2;1 were characterized by a predominant 24-methylsterol biosynthetic pathway leading to a high campesterol content and a depletion in sitosterol. Pleiotropic effects on development such as reduced growth, increased branching, and low fertility of high-campesterol plants were not modified by exogenous brassinosteroids, indicating specific sterol requirements to promote normal development. Thus SMT2;1 has a crucial role in balancing the ratio of campesterol to sitosterol in order to fit both growth requirements and membrane integrity.

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.

Plant sterol-C24-methyl transferases: different profiles of tobacco transformed with SMT1 or SMT2

Higher plant cells contain a mixture of 24-desmethyl, 24-methyl(ene), and 24-ethyl(idene) sterols in given proportions according to species but also to cell type. As a first step to investigate the function of such sterol compositions in the physiology of a plant, we have illustrated in the present work the coexistence of two distinct (S)-adenosyl-L-methionine sterol-C24-methyltransferases (SMT) in transgenic Nicotiana tabacum L. Indeed, modulation of the expression of the tobacco gene SMT1-1, which encodes a cycloartenol-C24-methyltransferase, results in variations of the proportion of cycloartenol and a concomitant effect on the proportion of 24-ethyl sterols. Overexpression in tobacco of the Arabidopsis thaliana (L.) Heynh. gene SMT2-1 which encodes a 24-methylene lophenol-C24(1)-methyltransferase, results in a dramatic modification of the ratio of 24-methyl cholesterol to sitosterol associated with a reduced growth, a topic discussed in the present work.

Role of highly conserved residues in the reaction catalyzed by recombinant Delta7-sterol-C5(6)-desaturase studied by site-directed mutagenesis

The role of 15 residues in the reaction catalyzed by Arabidopsis thaliana Delta7-sterol-C5(6)-desaturase (5-DES) was investigated using site-directed mutagenesis and expression of the mutated enzymes in an erg3 yeast strain defective in 5-DES. The mutated desaturases were assayed in vivo by sterol analysis and quantification of Delta5,7-sterols. In addition, the activities of the recombinant 5-DESs were examined directly in vitro in the corresponding yeast microsomal preparations. One group of mutants was affected in the eight evolutionarily conserved histidine residues from three histidine-rich motifs. Replacement of these residues by leucine or glutamic acid completely eliminated the desaturase activity both in vivo and in vitro, in contrast to mutations at seven other conserved residues. Thus, mutants H203L, H222L, H222E, P201A, G234A, and G234D had a 5-DES activity reduced to 2-20% of the wild-type enzyme, while mutants K115L, P175V, and P175A had a 5-DES activity and catalytical efficiency (V/K) that was similar to that of the wild-type. Therefore, these residues are not essential for the catalysis but contribute to the activity through conformational or other effects. One possible function for the histidine-rich motifs would be to provide the ligands for a presumed catalytic Fe center, as previously proposed for a number of integral membrane enzymes catalyzing desaturations and hydroxylations [Shanklin et al. (1994) Biochemistry 33, 12787-12794]. Another group of mutants was affected in residue 114 based on previous in vivo observations in A. thaliana indicating that mutant T114I was deficient in 5-DES activity. We show that the enzyme T114I has an 8-fold higher Km and 10-fold reduced catalytic efficiency. Conversely, the functionally conservative substituted mutant enzyme T114S displays a 28-fold higher Vmax value and an 8-fold higher Km value than the wild-type enzyme. Consequently, V/K for T114S was 38-fold higher than that for T114I. The data suggest that Thr 114 is involved in stabilization of the enzyme-substrate complex with a marked discrimination between the ground-state and the transition state of a rate-controlling step in the catalysis by the 5-DES.

Overexpression of an Arabidopsis cDNA encoding a sterol-C24(1)-methyltransferase in tobacco modifies the ratio of 24-methyl cholesterol to sitosterol and is associated with growth reduction

Higher plants synthesize 24-methyl sterols and 24-ethyl sterols in defined proportions. As a first step in investigating the physiological function of this balance, an Arabidopsis cDNA encoding an S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase, the typical plant enzyme responsible for the production of 24-ethyl sterols, was expressed in tobacco (Nicotiana tabacum L.) under the control of a constitutive promoter. Transgenic plants displayed a novel 24-alkyl-Delta5-sterol profile: the ratio of 24-methyl cholesterol to sitosterol, which is close to 1 in the wild type, decreased dramatically to values ranging from 0.01 to 0.31. In succeeding generations of transgenic tobacco, a high S-adenosyl-L-methionine 24-methylene lophenol-C24(1)-methyltransferase enzyme activity and, consequently, a low ratio of 24-methyl cholesterol to sitosterol, was associated with reduced growth compared with the wild type. However, this new morphological phenotype appeared only below the threshold ratio of 24-methyl cholesterol to sitosterol of approximately 0.1. Because the size of cells was unchanged in small, transgenic plants, we hypothesize that a radical decrease of 24-methyl cholesterol and/or a concomitant increase of sitosterol would be responsible for a change in cell division through as-yet unknown mechanisms.

Identification of cDNAs encoding sterol methyl-transferases involved in the second methylation step of plant sterol biosynthesis

Two methyl transfers are involved in the course of plant sterol biosynthesis and responsible for the formation of 24-alkyl sterols (mainly 24-ethyl sterols) which play major roles in plant growth and development. The first methyl transfer applies to cycloartenol, the second one to 24-methylene lophenol. Five cDNA clones encoding two Arabidopsis thaliana, two Nicotiana tabacum and one Ricinus communis S-adenosyl-L-methionine (AdoMet) sterol methyltransferases (SMT) were isolated. The deduced amino acid sequences of A. thaliana and N. tabacum SMT are about 80% identical in all possible combinations. In contrast they are about 40% identical with the deduced amino acid sequence of R. communis SMT and the published Glycine max sequence. Both A. thaliana and one N. tabacum SMT cDNAs were expressed in a yeast null mutant erg6, deficient in AdoMet zymosterol C24-methyltransferase and containing C24-non-alkylated sterols. In all cases, several 24-ethylidene sterols were synthesized. A thorough study of the sterolic composition of erg6 expressing the A. thaliana cDNA 411 (erg6-4118-pYeDP60) showed 24-methylene and 24-ethylidene derivatives of 4-desmethyl, 4alpha-methyl and 4,4-dimethyl sterols as well as 24-methyl and 24-ethyl derivatives of 4-desmethyl sterols. The structure of 5alpha-stigmasta-8, Z-24(24(1))-dien-3beta-ol, the major sterol of transformed yeasts, was demonstrated by 400 MHz 1H NMR. Microsomes from erg6-4118-pYeDP60 were shown to possess AdoMet-dependent sterol-C-methyltransferase activity. Delipidated preparations of these microsomes converted cycloartenol into 24-methylene cycloartanol and 24-methylene lophenol into 24-ethylidene lophenol, thus allowing the first identification of a plant sterol-C-methyltransferase cDNA. The catalytic efficiency of the expressed SMT was 17-times higher with 24-methylene lophenol than with cycloartenol. This result provides evidence that the A. thaliana cDNA 411 (and most probably the 3 plant SMT cDNAs presenting 80% identity with it) encodes a 24-methylene lophenol-C-24(1) methyltransferase catalyzing the second methylation step of plant sterol biosynthesis.

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.

Stereochemistry of yeast delta 24-sterol methyl transferase

S-Adenosyl-l-methionine: delta 24-sterol methyl transferase (24-SMT) mediates introduction of the C-28 carbon of yeast sterols. It has been shown that sulfonium analogues of the presumptive cationic intermediates of the methylenation reaction are potent in vivo and in vitro inhibitors of this process. In the presence of these inhibitors, cultures of yeast produced increased proportions of zymosterol, the natural substrate of the enzyme, while proportions of ergosterol and ergostatetraenol were decreased. New C27-sterol metabolites were also found. The in vivo inhibitory power of the analogues [I50 (microM)] was determined from the proportion of C-24 methylated sterols to C-24 nonmethylated sterols in treated cultures to be in the following order: 25-thiacholesterol iodide (0.07) > 24(S)-methyl-25-thiacholesteryl iodide (0.14) > 24(R)-methyl-25-thiacholesteryl iodide (0.25). Kinetic inhibition as revealed by radiolabeled S-adenosyl-l-methionine (SAM), crude enzyme and 25-thiacholesteryl iodide revealed this inhibitor to be uncompetitive with respect to zymosterol and competitive with respect to SAM. The greater inhibitory power of 24(S)-methyl-25-thiacholesteryl iodide compared to 24(R)-methyl-25-thiacholesteryl iodide suggests that methyl donation to delta 24 occurs from the si face. When considered in conjunction with Arigoni's previous work, the present results infer the methylenation mediated by yeast 24-SMT proceeds by alkylation from the si face of delta 24 followed by migration of a hydrogen from C-24 to C-25 across the re face and final loss of a hydrogen from C-28 on the re face.

Antifungal sterol biosynthesis inhibitors

During the course of the last decade, the development of SBIs, and particularly sterol biomethylation inhibitors, has been based on the rational design approach. Successful though this approach has been in elucidating sterol biomethylation enzymology, its limitations are becoming apparent from the findings that: (i) 24,25-double bond metabolism gives rise to cholesterol and ergosterol in a mechanistically similar manner, (ii) 25-azasterols are harmful to human physiology, and (iii) side-chain modified sterols designed to inhibit the SMT enzyme in S. cerevisiae may be ineffective or operate by another kinetic mechanism in a related organism, rendering it therapeutically nonuseful. Nevertheless, it may be possible to ultimately capitalize on the unique aspects of sterol biomethylation chemistry and enzymology to design taxa-specific inhibitors. With increased understanding of the structure and function of SMT enzymes in different fungi, it should be possible to prepare novel mechanism-based inactivators to control SMT activity uniquely and with high specific activity.

Sterol biosynthesis: strong inhibition of maize delta 5,7-sterol delta 7-reductase by novel 6-aza-B-homosteroids and other analogs of a presumptive carbocationic intermediate of the reduction reaction

A series of mono- and diazasteroids have been synthesized as analogs of a predicted carbocationic intermediate of delta 5,7-sterol delta 7-reductase (delta 7-SR). 6-Aza-B-homo-5 alpha-cholest-7-en-3 beta-ol (4), a novel compound whose synthesis is described for the first time, and 6,7-diaza-5 alpha-cholest-8(14)-en-3 beta-ol (6) were shown to be very powerful inhibitors of delta 7-SR in a preparation isolated from maize (Zea mays) (K(i),app = 50-70 nM, Ki,app/Km,app = 1.0 x 10(-4) to 1.3 x 10(-4). The data are consistent with a carbonium ion mechanism for the reduction; compounds 4 and 6 probably act as reaction intermediate analogs. Compound 4, in contrast to compound 6, displayed in the same microsomal preparation more than 50-fold selectivity for inhibition of the delta 7-SR versus delta 8-delta 7-sterol isomerase, cycloeucalenol isomerase, and delta 8,14-sterol delta 14-reductase, the mechanism of these four enzymes involving presumptive cationic intermediates centered respectively at C7, C8, C9, and C14. These observations highlight the paramount importance of the location of the positively charged nitrogen atom(s) in the B-ring structure for selectivity among these enzymes involving structurally close cationic reaction intermediates. Efficient in vivo inhibition of sterol biosynthesis in bramble cell suspension cultures by a low concentration of compound 4 was demonstrated and confirmed the in vitro properties of this derivative.)

Identification and characterization of an S-adenosyl-L-methionine: delta 24-sterol-C-methyltransferase cDNA from soybean

In plants, the dominant sterols are 24-alkyl sterols, which play multiple roles in plant growth and development, i.e. as membrane constituents and as precursors to steroid growth regulators such as brassinosteroids. The initial step in the conversion of the phytosterol intermediate cycloartenol to the 24-alkyl sterols is catalyzed by S-adenosyl-L-methionine: delta 24-sterol-C-methyl-transferase (SMT), a rate-limiting enzyme for phytosterol biosynthesis. A cDNA clone (SMT1) encoding soybean SMT was isolated from an etiolated hypocotyl cDNA library by immunoscreening using an anti-(plasma membrane) serum. The deduced amino acid sequence of the SMT1 cDNA contained three conserved regions found in S-adenosyl-L-methionine-dependent methyltransferases. The overall structure of the polypeptide encoded by the SMT1 cDNA is most similar to the predicted amino acid sequence of the yeast ERG6 gene, the putative SMT structural gene. The polypeptide encoded by the SMT1 cDNA was expressed as a fusion protein in Escherichia coli and shown to possess SMT activity. The growing soybean vegetative tissues had higher levels of SMT transcript than mature vegetative tissues. Young pods and immature seeds had very low levels of the SMT transcript. The SMT transcript was highly expressed in flowers. The expression of SMT transcript was suppressed in soybean cell suspension cultures treated with yeast elicitor. The transcriptional regulation of SMT in phytosterol biosynthesis is discussed.

Transformation of Saccharomyces cerevisiae with a cDNA encoding a sterol C-methyltransferase from Arabidopsis thaliana results in the synthesis of 24-ethyl sterols

Using an EST-cDNA probe, a full-length cDNA (411) sequence of 1411 bp was isolated from A. thaliana. This sequence contained features typical of methyltransferases in general and in particular showed 38% identity with ERG6, a S. cerevisiae gene which encodes the zymosterol-C-24-methyltransferase. A yeast vector containing this ORF (4118-pYeDP60) was used to transform a wild type S. cerevisiae which accumulates predominantly ergosterol, a 24-methyl sterol as well as a mutant erg6 null mutant accumulating principally zymosterol, a sterol non-alkylated at C-24. In both cases, several 24-ethyl- and 24-ethylidene sterols were synthetized indicating that the 4118 cDNA encodes a plant sterol C-methyltransferase able to perform two sequential methylations of the sterol side chain.

Mechanism and structural requirements for transformation of substrates by the (S)-adenosyl-L-methionine:delta 24(25)-sterol methyl transferase from Saccharomyces cerevisiae

The mechanism of action and active site of the enzyme (S)-adenosyl-L-methionine:delta 24(25)-sterol methyl transferase (SMT) from Saccharomyces cerevisiae strain GL7 have been probed with AdoMet, (S)-adenosyl-L-homocysteine, a series of 35 sterol substrates as acceptor molecules and a series of 10 substrate and high energy intermediate (HEI) sterol analogues as inhibitors of biomethylation. The SMT was found to be selective for sterol, both regio- and stereochemically. The presence of an unhindered 24,25-bond, an equatorially-oriented polar group at C-3 (which must act as a proton acceptor) attached to a planar nucleus and a freely rotating side chain were obligatory structural features for sterol binding/catalysis; no essential requirement or significant harmful effects could be found for the introduction of an 8(9)-bond, 14 alpha-methyl or 9 beta,19-cyclopropyl group. Alternatively, methyl groups at C-4 prevented productive sterol binding to the SMT. Initial velocity, product inhibition, and dead-end experiments demonstrated a rapid-equilibrium random bi bi mechanism. Deuterium isotope effects developed from SMT assays containing mixtures of [3-3H]zymosterol with AdoMet or [methyl-2H3]AdoMet confirmed the operation of a random mechanism, kappa H/kappa D = 1.3. From this combination of results, the spatial relationship of the sterol substrate to AdoMet could be approximated and the topology of the sterol binding to the SMT thereby formulated.