The one carbon cycle and the transsulphuration pathway. The one carbon cycle (upper part) provides activated methyl groups for transmethylation reactions: methionine is the carbon unit donor, being activated by adenosylation to SAMe. Release of the carbon unit to any acceptor generates S-adenosyl-homocysteine and then Hcy. Hcy is subsequently remethylated (recycled) to methionine. Alternatively, Hcy may enter the transsulphuration pathway (lower part) by forming a complex with serine by the highly regulated cystathionine β-synthase (4), leading to the synthesis of GSH.