Use of riboflavin-binding protein to investigate steric and electronic relationships in flavin analogs and models

We have examined the affinity of two recently synthesized flavin analogs for the isoalloxazine binding site of riboflavin-binding protein (RBP). The results showed that pyrimidopterines could bind to RBP [K(d) 160-250 μM]. This suggested that, at the FMN or FAD level, these analogs might also bind to other apoflavoproteins, thereby providing a high potential probe for flavin enzymology. In contrast, 4a,5-ring-opened isoalloxazines did not bind to RBP. However, 1,10a-ring-opened flavins bind with considerably avidity [K(d) about 40 nM]. Evidence is presented which indicates that the 4a,5-ring-opened species adopted a nonplanar configuration which, in turn, was responsible for the lack of affinity to RBP. Steric and electronic consequences of a 4a,5 ring opening are discussed in relation to flavin-dependent phenolic hydroxylases.