Homocysteine metabolism enzymes (Hcy metabolism)

Homocysteine metabolism encompasses the biochemical pathways responsible for the regulation of homocysteine, a sulfur-containing amino acid derived from methionine. The system primarily consists of two pathways: the remethylation cycle, which converts homocysteine back to methionine via methionine synthase (MTR) and methylenetetrahydrofolate reductase (MTHFR), and the transsulfuration pathway, which converts it to cysteine through cystathionine beta-synthase (CBS) [PubMed: 15189115]. These processes are critical for maintaining cellular methylation potential and redox balance through the production of S-adenosylmethionine (SAM) and glutathione [UniProt: P35520]. Clinical deficiency in the required B-vitamin cofactors (B6, B9, B12) or genetic mutations in these enzymes lead to hyperhomocysteinemia, a condition strongly associated with increased risks of cardiovascular disease, stroke, and neural tube defects [NIH: StatPearls - Hyperhomocysteinemia]. Pharmacological management typically involves high-dose vitamin supplementation to lower homocysteine levels or the use of betaine to provide alternative methyl donors [PubChem: CID 247]. Additionally, the folate-dependent portion of this pathway is a major target for antifolate drugs like methotrexate, which are used to treat cancer and autoimmune diseases by disrupting nucleotide synthesis [PubMed: 12130706].

Other names

Methionine cycleTranssulfuration pathwayFolate-mediated one-carbon metabolismCystathionine beta-synthase (CBS)Methylenetetrahydrofolate reductase (MTHFR)Methionine synthase (MTR)Betaine-homocysteine S-methyltransferase (BHMT)

Cofactor supplementation (B-vitamins) to increase enzymatic flux; Substrate replacement (Betaine) for alternative remethylation; Competitive inhibition of folate-dependent enzymes (Antifolates).