Homocysteine

Homocysteine
Names
	IUPAC name 2-Amino-4-sulfanylbutanoic acid
Identifiers
CAS Number	454-29-5 (racemate) ; 6027-13-0 (L-isomer) ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:17230 ;
ChEMBL	ChEMBL310604 ;
ChemSpider	757 ;
ECHA InfoCard	100.006.567
EC Number	207-222-9;
KEGG	C05330 ;
PubChem CID	778;
UNII	S7IJP4A89K (racemate) ; 0LVT1QZ0BA (L-isomer) ;
CompTox Dashboard (EPA)	DTXSID70859946 ;
	InChI InChI=1S/C4H9NO2S/c5-3(1-2-8)4(6)7/h3,8H,1-2,5H2,(H,6,7) Key: FFFHZYDWPBMWHY-UHFFFAOYSA-N ; InChI=1/C4H9NO2S/c5-3(1-2-8)4(6)7/h3,8H,1-2,5H2,(H,6,7) Key: FFFHZYDWPBMWHY-UHFFFAOYAS;
	SMILES C(CS)C(C(=O)O)N;
Properties
Chemical formula	C4H9NO2S
Molar mass	135.18 g/mol
Appearance	White crystalline powder
Melting point	234–235 °C (453–455 °F; 507–508 K) (decomposes)
Solubility in water	soluble
log P	−2.56
Acidity (pKa)	2.25
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H302
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Homocysteine (/ˌhoʊmoʊˈsɪstiːn/; symbol Hcy) is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (−CH₂−). It is biosynthesized from methionine by the removal of its terminal C^ε methyl group.

Although the production of homocysteine is a normal part of the metabolism of methionine, an excess of homocysteine can be harmful. There are two primary ways for organisms such as humans to metabolize homocysteine: remethylation and transsulfuration.

Remethylation adds a methyl group to the homocysteine molecule, converting homocysteine back into methionine. There are two known remethylation pathways. One pathway requires vitamin B₉ (folate) and B₁₂ (cobalamin), which drive the MTR (methionine synthase) and MTRR (methionine synthase reductase) enzymes. The other pathway uses TMG (trimethylglycine) to drive the BHMT (betaine-homocysteine methyltransferase) enzyme.

Transsulfuration converts homocysteine to cystathionine. This pathway requires vitamin B₆ to drive the CBS (cystathionine beta synthase) enzyme. Cystathionine is the immediate precursor of the amino acid cysteine, which (along with glutamate and glycine), is incorporated into the tripeptide glutathione, a major antioxidant in the human body.

Homocysteine is therefore an important metabolic substrate. However, excessive levels of homocysteine can result in hyperhomocysteinemia, which is regarded as an indicator of cardiovascular disease risk. Homocysteine likely contributes to atherogenesis, which can result in ischemic injury. Therefore, hyperhomocysteinemia is a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood. Hyperhomocysteinemia has also been correlated with the occurrence of blood clots, heart attacks, and strokes, although it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has also been associated with early-term spontaneous abortions and with neural tube defects.