In enzymology, a mRNA -methyltransferase also known as mRNA cap guanine-N7 methyltransferase is an enzyme that catalyzes the chemical reaction S-adenosyl-L-methionine + GpppR-RNA ⇌ {\displaystyle \rightleftharpoons } S-adenosyl-L-homocysteine + m7GpppR-RNA Thus, the two substrates of this enzyme are S-adenosyl methionine and GpppR-RNA, whereas its two products are S-adenosylhomocysteine and m7GpppR-RNA. Wikipedia