The Oxygen and Carbon Monoxide Reactions of Heme Oxygenase*

The O2and CO reactions with the heme, α-hydroxyheme, and verdoheme complexes of heme oxygenase have been studied. The heme complexes of heme oxygenase isoforms-1 and -2 have similar O2and CO binding properties. The O2affinities are very high,KO2= 30–80 μm−1, which is 30–90-fold greater than those of mammalian myoglobins. The O2association rate constants are similar to those for myoglobins (kO2′ = 7–20 μm−1s−1), whereas the O2dissociation rates are remarkably slow (kO2= 0.25 s−1), implying the presence of very favorable interactions between bound O2and protein residues in the heme pocket. The CO affinities estimated for both isoforms are only 1–6-fold higher than the corresponding O2affinities. Thus, heme oxygenase discriminates much more strongly against CO binding than either myoglobin or hemoglobin. The CO binding reactions with the ferrous α-hydroxyheme complex are similar to those of the protoheme complex, and hydroxylation at the α-meso position does not appear to affect the reactivity of the iron atom. In contrast, the CO affinities of the verdoheme complexes are >10,000 times weaker than those of the heme complexes because of a 100-fold slower association rate constant (kCO′  ≈ 0.004 μm−1s−1) and a 300-fold greater dissociation rate constant (kCO≈ 3 s−1) compared with the corresponding rate constants of the protoheme and α-hydroxyheme complexes. The positive charge on the verdoporphyrin ring causes a large decrease in reactivity of the iron.