In enzymology, a caffeate O-methyltransferase is an enzyme that catalyzes the chemical reaction S-adenosyl-L-methionine + 3,4-dihydroxy-trans-cinnamate ⇌ {\displaystyle \rightleftharpoons } S-adenosyl-L-homocysteine + 3-methoxy-4-hydroxy-trans-cinnamateThus, the two substrates of this enzyme are S-adenosyl methionine and 3,4-dihydroxy-trans-cinnamate, whereas its two products are S-adenosylhomocysteine and 3-methoxy-4-hydroxy-trans-cinnamate. Wikipedia