Structural Basis for the Thioredoxin-like Activity Profile of the Glutaredoxin-like NrdH-redoxin from Escherichia coli*

NrdH-redoxin is a representative of a class of small redox proteins that contain a conserved CXXC motif and are characterized by a glutaredoxin-like amino acid sequence and thioredoxin-like activity profile. The crystal structure of recombinantEscherichia coli NrdH-redoxin in the oxidized state has been determined at 1.7 Å resolution by multiwavelength anomalous diffraction. NrdH-redoxin belongs to the thioredoxin superfamily and is structurally most similar to E. coliglutaredoxin 3 and phage T4 glutaredoxin. The angle between the C-terminal helix α3 and strand β4, which differs between thioredoxin and glutaredoxin, has an intermediate value in NrdH-redoxin. The orientation of this helix is to a large extent determined by an extended hydrogen-bond network involving the highly conserved sequence motif⁶¹WSGFRP(D/E)⁶⁷, which is unique to this subclass of the thioredoxin superfamily. Residues that bind glutathione in glutaredoxins are in general not conserved in NrdH-redoxin, and no glutathione-binding cleft is present. Instead, NrdH-redoxin contains a wide hydrophobic pocket at the surface, similar to thioredoxin. Modeling studies suggest that NrdH-redoxin can interact with E. coli thioredoxin reductase at this pocket and also via a loop that is complementary to a crevice in the reductase in a similar manner as observed in the E. colithioredoxin-thioredoxin reductase complex.