This family consists of several archaebacterial S-adenosylmethionine synthetase C(AdoMet synthetase or MAT) (EC 188.8.131.52). S-Adenosylmethionine (AdoMet) occupies a central role in the metabolism of all cells. The biological roles of AdoMet include acting as the primary methyl group donor, as a precursor to the polyamines, and as a progenitor of a 5'-deoxyadenosyl radical. S-Adenosylmethionine synthetase catalyzes the only known route of AdoMet biosynthesis. The synthetic process occurs in a unique reaction in which the complete triphosphate chain is displaced from ATP and a sulfonium ion formed. MATs from various organisms contain ~400-amino acid polypeptide chains.