Editing Methionine (section)

==Derivatives==

===''S''-Adenosylmethionine===
[[File:S-Adenosyl-L-methionin.svg|thumbnail|left|''S''-Adenosylmethionine is a cofactor derived from methionine.]]
{{Main|S-Adenosylmethionine}}
The methionine-derivative [[S-adenosyl methionine|''S''-adenosylmethionine]] (SAM-e) is a [[cofactor (biochemistry)|cofactor]] that serves mainly as a [[methyl]] donor. SAM-e is composed of an adenosyl molecule (via 5′ carbon) attached to the sulfur of methionine, therefore making it a [[sulfonium]] cation (i.e., three substituents and positive charge). The sulfur acts as a [[HSAB theory|soft Lewis acid]] (i.e., donor/electrophile) which allows the ''S''-methyl group to be transferred to an oxygen, nitrogen, or aromatic system, often with the aid of other cofactors such as [[cobalamin]] (vitamin B<sub>12</sub> in humans). Some enzymes use SAM-e to initiate a radical reaction; these are called [[radical SAM|radical SAM-e]] enzymes.
As a result of the transfer of the methyl group, ''S''-adenosylhomocysteine is obtained. In bacteria, this is either regenerated by methylation or is salvaged by removing the adenine and the homocysteine, leaving the compound dihydroxypentandione to spontaneously convert into [[autoinducer-2]], which is excreted as a waste product or quorum signal.{{cn|date=October 2024}}