1NBH: Structure Of Glycine N-methyltransferase Complexed With S- Adenosylmethionine And Acetate, Gnmt:sam:ace

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.
PDB ID: 1NBHDownload
MMDB ID: 22105
PDB Deposition Date: 2002/12/2
Updated in MMDB: 2012/12
Experimental Method:
x-ray diffraction
Resolution: 2.8  Å
Source Organism:
Similar Structures:
Biological Unit for 1NBH: tetrameric; determined by author
Molecular Components in 1NBH
Label Count Molecule
Proteins (4 molecules)
Glycine N-methyltransferase(Gene symbol: Gnmt)
Molecule annotation
Chemicals (8 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB