Cysteine metabolism refers to the biological pathways that consume or create cysteine

Cysteine

Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

. The pathways of different amino acids and other metabolites interweave and overlap to creating complex systems.

Human cysteine metabolism

In human cysteine metabolism, L-cysteine is consumed in several ways as shown below. L-cysteine is also consumed in methionine

Methionine

Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

 and glutathione

Glutathione

Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...

 metabolism as well as pantothenate/CoA

COA

COA can refer to:*Codename Amscray*Cash on Arrival*Cause of action*CedarOpenAccounts*Center of Attention*Certificate of Appealability*Certificate of Approval for marriage or civil partnership in the United Kingdom*Certificate of Authenticity...

 biosynthesis.

L-cysteine consumption pathways

enzyme	→	product
cysteine dioxygenase Cysteine dioxygenase Cysteine dioxygenase ) is a mammalian non-heme iron enzyme that catalyzes the conversion of L-cysteine to cysteine sulfinic acid by incorporation of dioxygen....	→	3-sulfino-L-alanine (or cysteine sulfinic acid Cysteine sulfinic acid Cysteine sulfinic acid is an intermediate in cysteine metabolism.It is formed by cysteine dioxygenase.... )
amino-acid racemase Amino-acid racemase In enzymology, an amino-acid racemase is an enzyme that catalyzes the chemical reactionHence, this enzyme has one substrate, L-amino acid, and one product, D-amino acid....	→	D-cysteine
cysteine lyase Cysteine lyase In enzymology, a cysteine lyase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-cysteine and sulfite, whereas its two products are L-cysteate and hydrogen sulfide....	→	L-cysteate
cysteine—tRNA ligase	→	L-cysteinyl-tRNACys
cystine reductase Cystine reductase In enzymology, a cystine reductase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-cysteine and NAD+, whereas its 3 products are L-cystine, NADH, and H+....	→	L-cystine Cystine Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...
cysteine transaminase Cysteine transaminase In enzymology, a cysteine transaminase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-cysteine and 2-oxoglutarate, whereas its two products are mercaptopyruvate and L-glutamate....	→	3-mercapto-pyruvate

L-cysteine is the product of several processes as well. In addition to the reactions below, L-cysteine is also a product of glycine

Glycine

Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

, serine

Serine

Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

, and threonine

Threonine

Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...

 metabolism.

L-cysteine production pathways

starting chemical	→	enzyme
O-acetyl-L-serine	→	cysteine synthase Cysteine synthase In enzymology, a cysteine synthase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are O3-acetyl-L-serine and hydrogen sulfide, whereas its two products are L-cysteine and acetate....
L-cystine Cystine Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...	→	glutathione-cystine transhydrogenase Glutathione-cystine transhydrogenase In enzymology, a glutathione—cystine transhydrogenase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are glutathione and cystine, whereas its two products are glutathione disulfide and cysteine....
pyruvate	→	cystathionine γ-lyase
3-mercapto-pyruvate	→	cysteine transaminase Cysteine transaminase In enzymology, a cysteine transaminase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-cysteine and 2-oxoglutarate, whereas its two products are mercaptopyruvate and L-glutamate....

See also

• D-cysteine desulfhydrase
  D-cysteine desulfhydrase
  In enzymology, a D-cysteine desulfhydrase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are D-cysteine and H2O, whereas its 3 products are sulfide, NH3, and pyruvate....
  
  
• Sulphur metabolism