Cysteine and methionine metabolism

Cysteine and methionine metabolism represents a complex network of biochemical reactions responsible for the synthesis, interconversion, and degradation of sulfur-containing amino acids. Methionine is an essential amino acid that serves as the precursor for S-adenosylmethionine (SAM), which is the primary methyl donor for DNA, RNA, and protein methylation reactions (StatPearls, PMID: 31855344). The pathway also includes the transsulfuration route, where methionine-derived homocysteine is converted into cysteine, a rate-limiting precursor for the synthesis of glutathione, the cell's major antioxidant (KEGG, map00270). This metabolic network is critical for cellular redox homeostasis and polyamine synthesis. Clinically, dysregulation of this pathway is associated with significant pathologies; for instance, elevated homocysteine is a recognized risk factor for cardiovascular disease and stroke (NIH, PubMed). Furthermore, many cancer cells exhibit 'methionine addiction,' a metabolic dependency where they require exogenous methionine to survive, making enzymes in this pathway potential therapeutic vulnerabilities (NCBI, PMID: 30858548). While the pathway itself is not a single protein target, it contains several high-value drug targets, including methionine synthase and cystathionine beta-synthase. Drugs like methotrexate indirectly impact this pathway by disrupting the folate cycle, which is essential for methionine regeneration.

Other names

Sulfur amino acid metabolismMethionine cycleTranssulfuration pathwayOne-carbon metabolism

Pharmacological intervention typically involves the inhibition of specific enzymes within the pathway (e.g., dihydrofolate reductase) or the supplementation of pathway intermediates and cofactors to modulate metabolite flux and reduce toxic accumulations like homocysteine.