Abstract
Flavin reductases (FRs) catalyze the reduction of free flavins using NADH or NADPH. In recent years, a new family of short-chain FRs have been identified in a variety of bacteria and archaea. Here we have determined the crystal structure of an archaeal short-chain FR, HpaC from an aerobic thermoacidophilic crenarchaeon Sulfolobus tokodaii strain 7. The HpaC molecule exists as a homodimer with one FMN for each monomer. On the other hand, PheA2, the most structurally similar protein to HpaC, contains FAD rather than FMN. Structural comparison of these proteins has revealed that the short loop at residues 82-83 and the successive η1 helix in HpaC are closer to the bound flavin than those in PheA2. As a result, there is not sufficient space to accommodate the AMP moiety of FAD in HpaC, and thus HpaC prefers FMN to FAD. (Communicated by Masanori OTSUKA, M.J.A.)
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