Homocysteine methylation pathway

The **homocysteine methylation pathway** is not a single molecule or receptor but rather a metabolic network responsible for the conversion of homocysteine back to methionine through the transfer of a methyl group. This process is central to one-carbon metabolism and involves several enzymes—most notably **methionine synthase** (which requires vitamin B12 as a cofactor) and **betaine-homocysteine methyltransferase** (active mainly in liver and kidney), with 5-methyltetrahydrofolate or betaine serving as the primary methyl donors. The activity of this pathway is tightly linked with folate metabolism via the enzyme methylenetetrahydrofolate reductase (**MTHFR**) which generates 5-methyltetrahydrofolate from dietary folic acid. Homocysteine arises from dietary methionine after its activation to S‑adenosylmethionine (**SAM**), which donates its methyl group in numerous cellular reactions including DNA, RNA, protein, phospholipid, neurotransmitter, and creatinine synthesis. Disruption in this cycle—often due to deficiencies in vitamin cofactors or genetic variants affecting key enzymes—leads to elevated plasma homocysteine levels (*hyperhomocysteinemia*), which are associated with increased risk for cardiovascular diseases, neurodegeneration, pregnancy complications, congenital defects, and other pathologies due to oxidative stress and impaired cellular function[1][2][5][6]. Because "Homocysteine methylation pathway" refers broadly to an interconnected set of biochemical reactions rather than a discrete molecular target such as an enzyme or receptor protein—and because it cannot be directly targeted by drugs except through modulation via nutrient supplementation—it should not be classified as a canonical therapeutic target. Instead, individual enzymes within this pathway (e.g., *methionine synthase*, *MTHFR*) may serve as specific drug targets or biomarkers. Thus, > The entry "Homocysteine methylation pathway" is not itself a valid molecular target but describes an essential metabolic process involving multiple proteins/enzymes that can individually serve as therapeutic targets or biomarkers depending on context.[1][2][3][4][5]

Other names

Methionine cycleHomocysteine remethylation pathwayMethylation cycleOne-carbon metabolism (context-dependent)

Cofactor supplementation to enhance remethylation of homocysteine to methionine[1][2][5][6]