The oxygen and carbon monoxide reactions of heme oxygenase

The O₂ and CO reactions with the heme, α-hydroxy-heme, and verdoheme complexes of heme oxygenase have been studied. The heme complexes of heme oxygenase isoforms-1 and -2 have similar O₂ and CO binding properties. The O₂ affinities are very high, K(O₂) = 30-80 μM^-1, which is 30-90-fold greater than those of mammalian myoglobins. The O₂ association rate constants are similar to those for myoglobins (k(O₂)/'= 7-20 μM^-1 s^{- 1}), whereas the O₂ dissociation rates are remarkably slow (k(O₂) = 0.25 s^-1), implying the presence of very favorable interactions between bound O₂ and protein residues in the heme pocket. The CO affinities estimated for both isoforms are only 1-6-fold higher than the corresponding O₂ affinities. 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 (k(CO)/' ≃ 0.004 μM^-1 s^{- 1}) and a 300-fold greater dissociation rate constant (k(CO) ≃ 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.