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homocysteine
Homocysteine is the product of methyl group transfer to S-adenosyl methionine (SAM) in the biosynthesis of cysteine from methionine. The methyl group is transferred back to cysteine to reform methionine in a vitamin-B12-dependent reaction catalyzed by N5-methyltetrahydrofolate:L-homocysteine S-methyltransferase.
Immediate disposal of homocysteine also occurs via condensation with serine to form cystathionine in a reaction catalyzed by cystathionine beta-synthase. Cystathionine may then be converted to cysteine by cystathioninase to complete the trans-sulfuration pathway.
Another reaction disposing of homocysteine is catalyzed by betaine-homocysteine methyltransferase.
Homocystine is the disulfide of homocysteine.
Interactions
molecular events
Related
betaine-homocysteine S-methyltransferase
cystathionine beta-synthase (serine sulfhydrase, beta-thionase, CBS)
folate metabolism
homocysteine in serum/plasma
hyperhomocysteinemia
methionine metabolism (trans-sulfuration pathway)
methionine synthase; 5-methyltetrahydrofolate:homocysteine methyltransferase; vitamin-B12 dependent methionine synthase; tetrahydropteroyl-glutamate methyltransferase; MS (MTR)
S-adenosyl methionine (AdoMet, SAM, ademetionine)
Specific
homocystine
General
sulfur amino acid
thiol; sulhydryl compound; mercaptan
Database Correlations
PUBCHEM correlations
References
- Tietz Textbook of Clinical Chemistry, 2nd ed.
Burtis CA & Ashwood ER (eds), WB Saunders Co,
Philadelphia PA, 1993
- Medical Knowledge Self Assessment Program (MKSAP) 11, American
College of Physicians, Philadelphia 1998
Substructure-of
S-adenosylhomocysteine