Heme oxygenase catalyzes the oxidative cleavage of protoheme to biliverdin, the first step of heme metabolism utilizing O2 and NADPH. We determined the crystal structures of rat heme oxygenase-1 (HO-1)-heme and selenomethionyl HO-1-heme complexes. Heme is sandwiched between two helices with the δ-meso edge of the heme being exposed to the surface. Gly143N forms a hydrogen bond to the distal ligand of heme, OH-. The distance between Gly143N and the ligand is shorter than that in the human HO-1-heme complex. This difference may be related to a pH-dependent change of the distal ligand of heme. Flexibility of the distal helix may control the stability of the coordination of the distal ligand to heme iron. The possible role of Gly143 in the heme oxygenase reaction is discussed. Copyright (C) 2000 Federation of European Biochemical Societies.
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