Crystal Structures of Ferrous and CO-, CN--, and NO-Bound Forms of Rat Heme Oxygenase-1 (HO-1) in Complex with Heme: Structural Implications for Discrimination between CO and O2 in HO-1,

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Abstract
Heme oxygenase (HO) catalyzes heme degradation by utilizing O2 and reducing equivalents to produce biliverdin IXα, iron, and CO. To avoid product inhibition, the heme−HO complex (heme−HO) is structured to markedly increase its affinity for O2 while suppressing its affinity for CO. We determined the crystal structures of rat ferrous heme−HO and heme−HO bound to CO, CN-, and NO at 2.3, 1.8, 2.0, and 1.7 Å resolution, respectively. The heme pocket of ferrous heme−HO has the same conformation as that of the previously determined ferric form, but no ligand is visible on the distal side of the ferrous heme. Fe−CO and Fe−CN- are tilted, whereas the Fe−NO is bent. The structure of heme−HO bound to NO is identical to that bound to N3-, which is also bent as in the case of O2. Notably, in the CO- and CN--bound forms, the heme and its ligands shift toward the α-meso carbon, and the distal F-helix shifts in the opposite direction. These shifts allow CO or CN- to bind in a tilted fashion without a collision between the distal ligand and Gly139 O and cause disruption of one salt bridge between the heme and basic residue. The structural identity of the ferrous and ferric states of heme−HO indicates that these shifts are not produced on reduction of heme iron. Neither such conformational changes nor a heme shift occurs on NO or N3- binding. Heme−HO therefore recognizes CO and O2 by their binding geometries. The marked reduction in the ratio of affinities of CO to O2 for heme−HO achieved by an increase in O2 affinity [Migita, C. T., Matera, K. M., Ikeda-Saito, M., Olson, J. S., Fujii, H., Yoshimura, T., Zhou, H., and Yoshida, T. (1998) J. Biol. Chem.273, 945−949] is explained by hydrogen bonding and polar interactions that are favorable for O2 binding, as well as by characteristic structural changes in the CO-bound form.