Scheme 3.

Schematic representation of the synthesis pathway of riboflavin (adapted from Fig. 2 in Sung, Lee, J. Photosci., [35]). Guanosine triphosphate (GTP) is converted to 2,5-diamino-6-(5′-phosphoribosylamino)-4-pyrimidineone (DAPO) by RibA, which is further converted in three steps to 5-amino-6-(D-ribitylamino)uracil (APDO). Another route starts from ribulose 5-phosphate, which is converted to 3,4-dihydroxy-2-butanone 4-phosphate (DHBP) by RibB. The enzyme RibH produces the product 6,7-dimethyl-8-ribityllumazine (lumazine) from DHBP and APDO. Lumazine is converted to riboflavin by RibE, which is subsequently transformed into flavin mononucleotide (FMN) by a riboflavin kinase.