Transsulfuration pathway
Encyclopedia
The transsulfuration pathway is a metabolic pathway
involving the interconversion of cysteine
and homocysteine
, through the intermediate cystathionine
. In eukarotes, such as humans, the transsulfuration pathway is critical for creating cysteine from the essential amino acid methionine. Methionine is first converted to homocysteine by demethylation, which is then converted to the amino acid cysteine via the transsulfuration pathway. While methionine is considered an essential amino acid, cysteine becomes an essential amino acid when the transsulfuration pathway is defective.
In organisms that synthesize cysteine in sulfur assimilation
such as bacteria and yeast, the transsulfuration pathway works in reverse. The production of homocysteine through transsulfuration allows the conversion of this intermediate to methionine
, through a methylation reaction carried out by methionine synthase
.
Metabolic pathway
In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function...
involving the interconversion of 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...
and homocysteine
Homocysteine
Homocysteine is a non-protein amino acid with the formula HSCH2CH2CHCO2H. It is a homologue of the amino acid cysteine, differing by an additional methylene group. It is biosynthesized from methionine by the removal of its terminal Cε methyl group...
, through the intermediate cystathionine
Cystathionine
Cystathionine is an intermediate in the synthesis of cysteine.It is generated from homocysteine and serine by cystathionine beta synthase.It is cleaved into cysteine and α-ketobutyrate by cystathionine gamma-lyase....
. In eukarotes, such as humans, the transsulfuration pathway is critical for creating cysteine from the essential amino acid methionine. Methionine is first converted to homocysteine by demethylation, which is then converted to the amino acid cysteine via the transsulfuration pathway. While methionine is considered an essential amino acid, cysteine becomes an essential amino acid when the transsulfuration pathway is defective.
In organisms that synthesize cysteine in sulfur assimilation
Sulfur assimilation
Sulfur is an essential element for growth and physiological functioning of plants. However, its content strongly varies between plant species and it ranges from 0.1 to 6 % of the plants' dry weight. Sulfates taken up by the roots are the major sulfur source for growth, though it has to be reduced...
such as bacteria and yeast, the transsulfuration pathway works in reverse. The production of homocysteine through transsulfuration allows the conversion of this intermediate to 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...
, through a methylation reaction carried out by methionine synthase
5-methyltetrahydrofolate-homocysteine methyltransferase
Methionine synthase also known as MS, MeSe, MetH is an enzyme that in humans is encoded by the MTR gene . This enzyme is responsible for the regeneration of methionine from homocysteine...
.