Glycine Betaine

Glycine betaine is an oxidation product of choline in a biosynthetic pathway leading to methionine (see here). Glycine betaine can donate a methyl group to homocysteine to form methionine. This reaction is the only known biological methylation that does not involve S-adenosylmethionine.

See also: S-Adenosylmethionine and Biological Methylation, Metabolism of Sulfur-Containing Amino Acids