THE 3-DIMENSIONAL STRUCTURE OF FLAVODOXIN REDUCTASE FROM ESCHERICHIA-COLI AT 1.7-ANGSTROM RESOLUTION

Flavodoxin reductase from Escherichia coli is an FAD-containing oxido- reductase that transports electrons between flavodoxin or ferredoxin a nd NADPH. Together with flavodoxin, the enzyme is involved in the redu ctive activation of three E. coli enzymes: cobalamin-dependent methion ine synthase, pyruvate formate lyase and anaerobic ribonucleotide redu ctase. An additional function for the oxidoreductase arrears to be to protect the bacteria against oxygen radicals. The three-dimensional st ructure of flavodoxin reductase has been solved by multiple isomorphou s replacement, and has been refined at 1.7 Angstrom to an R-value of 1 8.4% and R-free 24.8%. The monomeric molecule contains one beta-sandwi ch FAD domain and an alpha/beta NADP domain. The overall structure is similar to other reductases of the NADP-ferredoxin reductase family in spite of the low sequence similarities within the family. Flavodoxin reductase lacks the loop which is involved in the binding of the adeno sine moiety of FAD in other FAD binding enzymes of the superfamily but is missing in the FMN binding phthalate dioxygenase reductase. Instea d of this loop, the adenine interacts with an extra tryptophan at the C terminus. The FAD in flavodoxin reductase has an unusual bent confor mation with a hydrogen bond between the adenine and the isoalloxazine. This is probably the cause of the unusual spectrum of the enzyme. The re is a pronounced cleft close to the isoalloxazine that appears to be well suited for binding of flavodoxin/ferredoxin. Two extra short str ands of the NADP-binding domain probably act as an anchor point for th e binding of flavodoxin. (C) 1997 Academic Press Limited.