title: Structure analysis of geranyl pyrophosphate methyltransferase and the proposed reaction mechanism of SAM-dependent C-methylation.
abstract: In the typical isoprenoid-biosynthesis pathway, condensation of the universal C(5)-unit precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) occurs via the common intermediates prenyl pyrophosphates (C(10)-C(20)). The diversity of isoprenoids reflects differences in chain length, cyclization and further additional modification after cyclization. In contrast, the biosynthesis of 2-methylisonorneol (2-MIB), which is responsible for taste and odour problems in drinking water, is unique in that it primes the enzymatic methylation of geranyl pyrophosphate (GPP) before cyclization, which is catalyzed by an S-adenosyl-L-methionine-dependent methyltransferase (GPPMT). The substrate of GPPMT contains a nonconjugated olefin and the reaction mechanism is expected to be similar to that of the steroid methyltransferase (SMT) family. Here, structural analysis of GPPMT in complex with its cofactor and substrate revealed the mechanisms of substrate recognition and possible enzymatic reaction. Using the structures of these complexes, methyl-group transfer and the subsequent proton-abstraction mechanism are discussed. GPPMT and SMTs contain a conserved glutamate residue that is likely to play a role as a general base. Comparison with the reaction mechanism of the mycolic acid cyclopropane synthase (MACS) family also supports this result. This enzyme represented here is the first model of the enzymatic C-methylation of a nonconjugated olefin in the isoprenoid-biosynthesis pathway. In addition, an elaborate system to avoid methylation of incorrect substrates is proposed.
authors.affiliation: Graduate School of Life Sciences, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Orapin
authors.initials: O
authors.lastname: Ariyawutthiphan
authors.forename: Toyoyuki
authors.initials: T
authors.lastname: Ose
authors.forename: Atsushi
authors.initials: A
authors.lastname: Minami
authors.forename: Sandip
authors.initials: S
authors.lastname: Shinde
authors.forename: Sandip
authors.initials: S
authors.lastname: Sinde
authors.forename: Muneya
authors.initials: M
authors.lastname: Tsuda
authors.forename: Yong-Gui
authors.initials: YG
authors.lastname: Gao
authors.forename: Min
authors.initials: M
authors.lastname: Yao
authors.forename: Hideaki
authors.initials: H
authors.lastname: Oikawa
authors.forename: Isao
authors.initials: I
authors.lastname: Tanaka
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002413:Cations, Divalent / Q000378:metabolism; D018360:Crystallography, X-Ray; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011106:Polyisoprenyl Phosphates / Q000378:metabolism*; D011487:Protein Conformation; D055503:Protein Multimerization; D012436:S-Adenosylmethionine / Q000378:metabolism*; D016415:Sequence Alignment; D013302:Streptomyces / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D002413:Cations, Divalent; D011106:Polyisoprenyl Phosphates; C015234:geranyl pyrophosphate; D012436:S-Adenosylmethionine; D008780:Methyltransferases