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.