Abstract
Glycine N-methyltransferase (EC 2. 1. 1. 20) catalyzes the transfer of the methyl group of S-adenosyl-L-methionine (AdoMet) to glycine to form sarcosine and S-adenosyl-L-homocysteine. As opposed to monomeric structure of most AdoMet-dependent methyltransferases, glycine N-methyltransferase is a tetramer consisting of identical subunits. Glycine N-methyltransferase exhibits sigmoidal rate behavior with respect to AdoMet and hyperbolic kinetics with respect to glycine. The sigmoidal kinetics arises from the cooperative binding of AdoMet to the catalytic site residing on each subunit. In the three-dimensional structure, the four spherical subunits are arranged to a flat square shape with a large hole in the center. Each subunit has a long N-terminal segment protruded from the main body. The segments interact with each other in the central hole, and this interaction appears to cause cooperativity in binding AdoMet. The AdoMet-binding region folds into a typical α/β structure as other methyltransferases of known structure. However, glycine N-methyltransferase possesses an additional domain that surrounds AdoMet. The unique structure of the AdoMet-binding site suggests that folate coenzymes are bound at the same site. The physiological role and regulation of glycine N-methyltransferase are briefly discussed.
