(2S)-2-hydroxy-3-oxobutyl phosphate + 5-amino-6-(D-ribitylamino)uracil --> 6,7-dimethyl-8-(1-D-ribityl)lumazine + H⁺ + 2 H₂O + phosphate (according to UniProt)

2 6,7-dimethyl-8-(1-D-ribityl)lumazine = riboflavin 4-(1-D-ribitylamino)-5-amino-2,6-dihydroxypyrimidine (according to Swiss-Prot)