Figure 1.

The methionine cycle in T cells. After entering T cells, methionine flows into the methionine cycle. The first step of the methionine cycle is the conversion of methionine into S-adenosylmethionine (SAM) by methionine adenosyltransferase II (MATII). SAM is converted into S-adenosylhomocysteine (SAH) after donating a methyl group for methylation reactions. This step is mediated by Methyltransferases (MTs). SAH is then hydrolyzed by S-adenosyl-L-homocysteine hydrolase (SAHH) to generate homocysteine. Finally, homocysteine receives a methyl group from the folate cycle or betaine to become methionine, these reactions are mediated by 5-methyltetrahydrofolate: homocysteine methyltransferase (MTR) and betaine-homocysteine methyltransferase (BHMT) respectively. The methionine cycle is interconnected with three important metabolic pathways by providing substrates. These pathways include the folate cycle, the transsulfuration pathway, the methionine salvage pathway, and polyamine synthesis, all of which support important cellular functions. DMG, dimethylglycine; 5-methyl-THF, 5-methyltetrahydrofolate; THF, tetrahydrofolate; Glu-Cys, γ-glutamylcysteine; GSH, glutathione; MTA, 5′-Methylthioadenosine.