Methionine is a sulfur-containing amino acid that is butyric acid bearing an amino substituent at position 2 and a methylthio substituent at position 4. It has a role as an Escherichia coli metabolite, a Saccharomyces cerevisiae metabolite, a plant metabolite, a Daphnia magna metabolite and an algal metabolite. It is an alpha-amino acid and a sulfur-containing amino acid. It derives from a butyric acid. It is a conjugate base of a methioninium. It is a conjugate acid of a methioninate. It is a tautomer of a methionine zwitterion.