Riboflavin (1) is a redox cofactor involved in the catalysis of a wide diversity of biological oxidation reactions and flavoenzymology is now a well-studied area. In contrast, very little is known about the enzymes involved in riboflavin catabolism. Riboflavin breakdown proceeds in two stages. In the first stage, riboflavin lyase catalyzes the oxidative cleavage of ribose from the isoalloxazine heterocycle. Ribose is further metabolized but lumichrome (2) precipitates unaltered from the growth medium. The second stage converts lumichrome (2) to usable metabolites. This begins with the oxidation of the C7 methyl group of lumichrome. The pyrimidine ring is then degraded using three hydrolysis reactions. The resulting quinoxaline undergoes decarboxylation followed by hydroxylation to give 10. Dihydroxylation, catalyzed by a Rieske dioxygenase, completes the heterocycle degradation to give the highly substituted amido catechol 12. This then undergoes extradiol cleavage to give a product of currently unknown structure. My lecture will describe the isolation of riboflavin and lumichrome catabolic strains, the identification of the two catabolic operons, the reconstitution of the first 11 steps on the pathway and selected mechanistic studies.
Riboflavin Catabolism Support or Funding Information This research was supported by a grant from the Robert A. Welch Foundation (A0034).
