Ligand-binding studies on heavy riboflavin synthase of Bacillus subtilis

Heavy riboflavin synthase is a complex enzyme consisting of three alpha subunits and approximately 60 beta subunits. Ligand-binding studies were performed with a variety of substrate and product analogues by analytical ultracentrifugation and by equilibrium dialysis. Nonlinear binding curves indicate the involvement of non-equivalent binding sites which could be assigned to the alpha and beta subunits by comparison with light riboflavin synthase (subunit composition alpha 3) and with aggregates of isolated beta subunits. The beta subunit binding site shows a high degree of stereospecificity. Tightly binding ligands must have a ribityl side chain and a pyrimidine or pteridine moiety with polar substituents.