title: Cloning, mechanistic and functional analysis of a fungal sterol C24-methyltransferase implicated in brassicasterol biosynthesis.
abstract: The first committed step in the formation of 24-alkylsterols in the ascomycetous fungus Paracoccidiodes brasiliensis (Pb) has been shown to involve C24-methylation of lanosterol to eburicol (24(28)-methylene-24,25-dihydro-lanosterol) on the basis of metabolite co-occurrence. A similarity-based cloning strategy was employed to obtain the cDNA clone corresponding to the sterol C24-methyltransferase (SMT) implicated in the C24-methylation reaction. The resulting catalyst, prepared as a recombinant fusion protein (His/Trx/S), was expressed in Escherichia coli BL21(C43) and shown to possess a substrate specificity for lanosterol and to generate a single exocyclic methylene product. The full-length cDNA has an open reading frame of 1131 base pairs and encodes a protein of 377 residues with a calculated molecular mass of 42,502Da. The enzymatic C24-methylation gave a K(mapp) of 38microM and k(catapp) of 0.14min(-1). Quite unexpectedly, "plant" cycloartenol was catalyzed in high yield to 24(28)-methylene cycloartanol consistent with conformational arguments that favor that both cycloartenol and lanosterol are bound pseudoplanar in the ternary complex. Incubation of [27-(13)C]- or [24-(2)H]cycloartenol with PbSMT and analysis of the enzyme-generated product by a combination of (1)H and (13)CNMR and mass spectroscopy established the regiospecific conversion of the pro-Z methyl group of the Delta(24(25))-substrate to the pro-R isopropyl methyl group of the product and the migration of H24 to C25 on the Re-face of the original substrate double bond undergoing C24-methylation. Inhibition kinetics and products formed from the substrate analogs 25-azalanosterol (K(i) 14nM) and 26,27-dehydrolanosterol (K(i) 54muM and k(inact) of 0.24min(-1)) provide direct evidence for distinct reaction channeling capitalized by structural differences in the C24- and C26-sterol acceptors. 25-Azalanosterol was a potent inhibitor of cell growth (IC(50), 30nM) promoting lanosterol accumulation and 24-alkyl sterol depletion. Phylogenetic analysis of PbSMT with related SMTs of diverse origin together with the results of the present study indicate that the enzyme may have a similar complement of active-site amino acid residues compared to related yeast SMTs affording monofunctional C(1)-transfer behavior, yet there are sufficient differences in its overall amino acid composition and substrate-dependent partitioning pathways to group PbSMT into a fourth and new class of SMT.
authors.affiliation: Laboratorio de Biologia Molecular, Instituto de Ciencias Biologicas, Universidade Federal de Goias, 74001-970, Goiania, Goias, Brazil.
authors.forename: Maristela
authors.initials: M
authors.lastname: Pereira
authors.forename: Zhihong
authors.initials: Z
authors.lastname: Song
authors.forename: Ludier Kesser
authors.initials: LK
authors.lastname: Santos-Silva
authors.forename: Mathew H
authors.initials: MH
authors.lastname: Richards
authors.forename: Thi Thuy Minh
authors.initials: TT
authors.lastname: Nguyen
authors.forename: JiaLin
authors.initials: J
authors.lastname: Liu
authors.forename: Celia Maria
authors.initials: CM
authors.lastname: de Almeida Soares
authors.forename: Aline Helena
authors.initials: AH
authors.lastname: da Silva Cruz
authors.forename: Kulothungan
authors.initials: K
authors.lastname: Ganapathy
authors.forename: W David
authors.initials: WD
authors.lastname: Nes
mesh_terms: D000478:Alkylation; D000595:Amino Acid Sequence; D055162:Biocatalysis; D002775:Cholestadienols / Q000737:chemistry; D002849:Chromatography, Gas; D002851:Chromatography, High Pressure Liquid; D003001:Cloning, Molecular; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D007810:Lanosterol / Q000737:chemistry / Q000302:isolation & purification; D053858:Metabolic Networks and Pathways; D008780:Methyltransferases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010228:Paracoccidioides / Q000201:enzymology*; D010802:Phylogeny; D010840:Phytosterols / Q000096:biosynthesis* / Q000737:chemistry; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D015203:Reproducibility of Results; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D013379:Substrate Specificity; D014443:Tyrosine / Q000235:genetics
chemical_list: D002775:Cholestadienols; D010840:Phytosterols; D011994:Recombinant Proteins; D007810:Lanosterol; C003532:brassicasterol; D014443:Tyrosine; D008780:Methyltransferases