Enzymological and structural characterization of Arabidopsis thaliana heme oxygenase‐1

Arabidopsis thaliana heme oxygenase‐1 (AtHO‐1), a metabolic enzyme in the heme degradation pathway, serves as a prototype for study of the bilin‐related functions in plants. Past biological analyses revealed that AtHO‐1 requires ferredoxin‐NADP(+) reductase (FNR) and ferredoxin for its enzymatic activity. Here, we characterized the binding and degradation of heme by AtHO‐1, and found that ferredoxin is a dispensable component of the reducing system that provides electrons for heme oxidation. Furthermore, we reported the crystal structure of heme‐bound AtHO‐1, which demonstrates both conserved and previously undescribed features of plant heme oxygenases. Finally, the electron transfer pathway from FNR to AtHO‐1 is suggested based on the known structural information.