Fig. 4: Crystal structure of flavone reductase (FLR).

a Crystal structure of FLR monomer. The N-terminal domain, and C-terminal subdomains 1 and 2 are colored slate-blue, red, and cyan, respectively. FMN is shown with magenta sticks. b Crystal structure of FLR dimmer and its interface. Monomers A and B are shown with ribbon cartoon and electron static surface potential, respectively. c Zoom-in views of the cofactor FMN- and apigenin-binding site. The pictures show the amino acid residues surrounding the active site, and Fo-Fc density map (blue), contoured at 1.0 σ level, from which the FMN and apigenin were omitted. d In vitro enzymatic activity assay (initial turnover rate) of the wild-type (WT) or mutants of FLR. Data are represented as mean ± SD (n = 3). Error bars show SDs. Statistical analysis was performed by a two-tailed Student’s t-test. ^***P < 0.001 versus FLR-WT. ns, not significant. e Structural superimposition of the FLR monomer with the WrbA protein (PDB:3B6I, colored gray). Tru-FLR, the truncated FLR protein. Initial turnover rates of purified FLR and Tru-FLR towards different substrates were measured. Data are presented as mean ± SD (n = 3). Error bars show SDs. Statistical analysis was performed by a two-tailed Student’s t-test. ^***P < 0.001 versus FLR.