Crystal structure of NAD(P)H : flavin oxidoreductase from Escherichia coli

Flavin reductases use flavins as substrates and are distinct from flavoenzy mes which have tightly bound flavins, The reduced flavin can serve to reduc e ferric complexes and iron proteins. In Escherichia coli, reactivation of ribonucleotide reductase is achieved by reduced flavins produced by flavin reductase, The crystal structure of E. coli flavin reductase reveals that t he enzyme structure is similar to the structures of the ferredoxin reductas e family of flavoproteins despite very low sequence similarities. The main difference between flavin reductase and structurally related flavoproteins is that there is no binding site for the AMP moiety of FAD. The direction o f the helix in the flavin binding domain, corresponding to the phosphate bi nding helix in the flavoproteins, is also slightly different and less suita ble for phosphate binding. Interactions for flavin substrates are instead p rovided by a hydrophobic isoalloxazine binding site that also contains a se rine and a threonine, which form hydrogen bonds to the isoalloxazine of bou nd riboflavin in a substrate complex.