2,5-Diamino-6-(5'-triphosphoryl-3',4'-trihydroxy-2'-oxopentyl)-amino-4-oxopyrimidine + 2 S-Adenosyl-L-methionine <=> 6-(3'-Triphosphoryl-1'-methylglyceryl)-7-methyl-7,8-dihydrobiopterin + 2 S-Adenosyl-L-homocysteine + H2O