title: Characterization, mutagenesis and mechanistic analysis of an ancient algal sterol C24-methyltransferase: Implications for understanding sterol evolution in the green lineage.
abstract: 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.
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Brad A
authors.initials: BA
authors.lastname: Haubrich
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Emily K
authors.initials: EK
authors.lastname: Collins
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Alicia L
authors.initials: AL
authors.lastname: Howard
authors.affiliation: Department of Cell Biology, University of Texas Southwestern Medical School, Dallas, TX 75390, United States.
authors.forename: Qian
authors.initials: Q
authors.lastname: Wang
authors.affiliation: Department of Cell Biology, University of Texas Southwestern Medical School, Dallas, TX 75390, United States.
authors.forename: William J
authors.initials: WJ
authors.lastname: Snell
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Miller
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Crista D
authors.initials: CD
authors.lastname: Thomas
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States.
authors.forename: Stephanie K
authors.initials: SK
authors.lastname: Pleasant
authors.affiliation: Center for Chemical Biology and Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States. Electronic address: wdavid.nes@ttu.edu.
authors.forename: W David
authors.initials: WD
authors.lastname: Nes
mesh_terms: D000595:Amino Acid Sequence; D002696:Chlamydomonas / Q000201:enzymology; D000460:Chlorophyta / Q000737:chemistry; D008401:Gas Chromatography-Mass Spectrometry; D007553:Isotope Labeling; D019684:Magnoliopsida / Q000737:chemistry; D008780:Methyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015394:Molecular Structure; D016296:Mutagenesis; D019906:Nuclear Magnetic Resonance, Biomolecular; D012436:S-Adenosylmethionine / Q000378:metabolism; D013261:Sterols / Q000737:chemistry*
chemical_list: D013261:Sterols; D012436:S-Adenosylmethionine; D008780:Methyltransferases
keywords: Chlamydmonas reinhardtii green algae; Cholesterol; Ergosterol; SMT1; SMT2; Sterol C24-methyltransferase; Sterol evolution