title: Catalytic mechanism of glycine N-methyltransferase.
abstract: Methyltransfer reactions are some of the most important reactions in biological systems. Glycine N-methyltransferase (GNMT) catalyzes the S-adenosyl-l-methionine- (SAM-) dependent methylation of glycine to form sarcosine. Unlike most SAM-dependent methyltransferases, GNMT has a relatively high value and is weakly inhibited by the product S-adenosyl-l-homocysteine (SAH). The major role of GNMT is believed to be the regulation of the cellular SAM/SAH ratio, which is thought to play a key role in SAM-dependent methyltransfer reactions. Crystal structures of GNMT complexed with SAM and acetate (a potent competitive inhibitor of Gly) and the R175K mutated enzyme complexed with SAM were determined at 2.8 and 3.0 A resolutions, respectively. With these crystal structures and the previously determined structures of substrate-free enzyme, a catalytic mechanism has been proposed. Structural changes occur in the transitions from the substrate-free to the binary complex and from the binary to the ternary complex. In the ternary complex stage, an alpha-helix in the N-terminus undergoes a major conformational change. As a result, the bound SAM is firmly connected to protein and a "Gly pocket" is created near the bound SAM. The second substrate Gly binds to Arg175 and is brought into the Gly pocket. Five hydrogen bonds connect the Gly in the proximity of the bound SAM and orient the lone pair orbital on the amino nitrogen (N) of Gly toward the donor methyl group (C(E)) of SAM. Thermal motion of the enzyme leads to a collision of the N and C(E) so that a S(N)2 methyltransfer reaction occurs. The proposed mechanism is supported by mutagenesis studies.
authors.affiliation: Department of Molecular Biosciences, University of Kansas, 1200 Sunnyside Avenue, Lawrence, Kansas 66045-7534, USA.
authors.forename: Yoshimi
authors.initials: Y
authors.lastname: Takata
authors.forename: Yafei
authors.initials: Y
authors.lastname: Huang
authors.forename: Junichi
authors.initials: J
authors.lastname: Komoto
authors.forename: Taro
authors.initials: T
authors.lastname: Yamada
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Konishi
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Ogawa
authors.forename: Tomoharu
authors.initials: T
authors.lastname: Gomi
authors.forename: Motoji
authors.initials: M
authors.lastname: Fujioka
authors.forename: Fusao
authors.initials: F
authors.lastname: Takusagawa
mesh_terms: D000595:Amino Acid Sequence; D002384:Catalysis; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D050938:Glycine N-Methyltransferase; D006860:Hydrogen Bonding; D008780:Methyltransferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016296:Mutagenesis; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism
chemical_list: D011994:Recombinant Proteins; D012436:S-Adenosylmethionine; D008780:Methyltransferases; D050938:Glycine N-Methyltransferase