Cystathionine is an intermediate in the synthesis of

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

. Cystathionine is produced by the

transsulfuration pathway The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine. Two transsulfurylation pathways are known: the ''forward'' and the ''reverse''. The ''forward ...

which converts

homocysteine Homocysteine 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 Cε methyl group. In th ...

into cystathionine. Cystathionine is then used by the enzymes

cystathionine gamma-lyase The enzyme cystathionine γ-lyase (EC 4.4.1.1, CTH or CSE; also cystathionase; systematic name L-cystathionine cysteine-lyase (deaminating; 2-oxobutanoate-forming)) breaks down cystathionine into cysteine, 2-oxobutanoate ( α-ketobutyrate), and ...

(CTH),

cysteine dioxygenase Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the conversion of L-cysteine to cysteine sulfinic acid (cysteine sulfinate). CDO plays an important role in cysteine catabolism, regulating intracellular levels of cysteine and res ...

(CDO), and sulfinoalanine decarboxylase to produce

hypotaurine Hypotaurine is a sulfinic acid that is an intermediate in the biosynthesis of taurine. Like taurine, it also acts as an endogenous neurotransmitter via action on the glycine receptors. It is an osmolyte with antioxidant properties. Hypotaurine is ...

and then

taurine Taurine (), or 2-aminoethanesulfonic acid, is an organic compound that is widely distributed in animal tissues. It is a major constituent of bile and can be found in the large intestine, and accounts for up to 0.1% of total human body weight. It ...

. Alternately, the cysteine from the cystathionine gamma-lyase can be used by the enzymes

glutamate–cysteine ligase Glutamate—cysteine ligase (GCL) ), previously known as γ-glutamylcysteine synthetase (GCS), is the first enzyme of the cellular glutathione (GSH) biosynthetic pathway that catalyzes the chemical reaction: L-glutamate + L-cysteine + ATP \rig ...

(GCL) and

glutathione synthetase Glutathione synthetase (GSS) () is the second enzyme in the glutathione (GSH) biosynthesis pathway. It catalyses the condensation of gamma-glutamylcysteine and glycine, to form glutathione. Glutathione synthetase is also a potent antioxidan ...

(GSS) to produce

glutathione Glutathione (GSH, ) is an antioxidant in plants, animals, fungi, and some bacteria and archaea. Glutathione is capable of preventing damage to important cellular components caused by sources such as reactive oxygen species, free radicals, pero ...

. An excess of cystathionine in the urine is called

cystathioninuria Cystathioninuria, also called cystathionase deficiency, is an autosomal recessive metabolic disorder. It is characterized by an abnormal accumulation of plasma cystathionine leading to excess cystathionine in the urine. Hereditary cystathioninuria ...

. Biosynthetically, cystathionine is generated from

and

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

cystathionine beta synthase Cystathionine-β-synthase, also known as CBS, is an enzyme () that in humans is encoded by the ''CBS'' gene. It catalyzes the first step of the transsulfuration pathway, from homocysteine to cystathionine: :Serine, L-serine + homocysteine, L-homo ...

(upper reaction in the diagram below). It is then cleaved into

and α-ketobutyrate by

(lower reaction). Cysteine biosynthesis unlabeled

References

{{Amino acid metabolism intermediates Sulfur amino acids