homocysteine

Introduction

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.

More general terms

• sulfur amino acid
• thiol; sulhydryl compound; mercaptan

More specific terms

• homocystine

Additional terms

• 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)

Database

• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=91552
• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=778