MAT1A catalyzes a two-step reaction that involves the transfer of the adenosyl moiety of ATP to methionine to form S-adenosylmethionine and tripolyphosphate， which is subsequently cleaved to PPi and Pi. S-adenosylmethionine is the source of methyl groups for most biological methylations. MAT1A is found as a homotetramer（MAT I) or a homodimer（MAT III) whereas a third form， MAT II（gamma)， is encoded by the MAT2A gene. Mutations in MAT1A gene are associated with methionine adenosyltransferase deficiency. MAT1A expression also correlates with a differentiated phenotype， whereas liver cells expressing MAT2A present a dedifferentiated phenotype and lowered AdoMet synthesis. Likewise， NFkB and TNFa cause a switch from MAT1A to MAT2A expression in human hepatocellular carcinoma（HCC)， which facilitates cancer cell growth.
