In enzymology, a quercetin 3-O-methyltransferase (EC 2.1.1.76) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + 3,5,7,3',4'-pentahydroxyflavone ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + 3-methoxy-5,7,3',4'-tetrahydroxyflavone

Thus, the two substrates of this enzyme are S-adenosyl methionine and 3,5,7,3',4'-pentahydroxyflavone, whereas its two products are S-adenosylhomocysteine and 3-methoxy-5,7,3',4'-tetrahydroxyflavone.

This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:3,5,7,3',4'-pentahydroxyflavone 3-O-methyltransferase. Other names in common use include flavonol 3-O-methyltransferase, and flavonoid 3-methyltransferase. This enzyme participates in flavonoid biosynthesis.

References

• Foucher AL, McIntosh A, Douce G, Wastling J, Tait A, Turner CM (2006). "A proteomic analysis of arsenical drug resistance in Trypanosoma brucei". Proteomics. 6 (9): 2726–32. doi:10.1002/pmic.200500419. PMID 16526094.
• De Luca V, Ibrahim RK (1985). "Enzymatic synthesis of polymethylated flavonols in Chrysosplenium americanum. I. Partial purification and some properties of S-adenosyl-L-methionine:flavonol 3-, 6-, 7-, and 4'-O-methyltransferases". Arch. Biochem. Biophys. 238 (2): 596–605. doi:10.1016/0003-9861(85)90205-X. PMID 3994393.
• De Luca V, Ibrahim RK (1985). "Enzymatic synthesis of polymethylated flavonols in Chrysosplenium americanum. II. Substrate interaction and product inhibition studies of flavonol 3-, 6-, and 4'-O-methyltransferases". Arch. Biochem. Biophys. 238 (2): 606–18. doi:10.1016/0003-9861(85)90206-1. PMID 3994394.
• Ibrahim RK; De Luca V (1982). "Polymethylated flavonol synthesis is catalyzed by distinct O-methyltransferases". Naturwissenschaften. 69 (1): 41–42. doi:10.1007/BF00441101.