Cysteine metabolism is comprised of the biological pathways that consume or create cysteine. The pathways of different amino acids and other metabolites interweave and overlap to creating complex systems.

Human cysteine metabolism

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In human cysteine metabolism, L-cysteine is consumed in several ways as shown below. L-cysteine is also consumed in methionine and glutathione metabolism as well as pantothenate/CoA biosynthesis.

L-cysteine consumption pathways

enzyme	→	product
cysteine dioxygenase	→	3-sulfino-L-alanine (or cysteine sulfinic acid)
amino-acid racemase	→	D-cysteine
cysteine lyase	→	L-cysteate
cysteine—tRNA ligase	→	L-cysteinyl-tRNACys
cystine reductase	→	L-cystine
cysteine transaminase	→	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, serine, and threonine metabolism.

L-cysteine production pathways

starting chemical	→	enzyme
O-acetyl-L-serine	→	cysteine synthase
L-cystine	→	glutathione-cystine transhydrogenase
pyruvate	→	cystathionine γ-lyase
3-mercapto-pyruvate	→	cysteine transaminase