Heme oxygenase-1, intermediates in verdoheme formation and the requirement for reduction equivalents

Conversion of heme to verdoheme by heme oxygenase-1 (HO-1) is thought to involve α-meso-hydroxylation and elimination of the meso-carbon as CO, a reaction supported by both H₂O₂ and NADPH-cytochrome P450 reductase/O₂. Anaerobic reaction of the heme-HO-1 complex with 1 eq of H₂O₂ produces an enzyme-bound intermediate identified by spectroscopic methods as α- mesohydroxyheme. This is the first direct evidence for HO-1-catalyzed formation of α-meso-hydroxyheme. α-meso-Hydroxyheme exists as a mixture of Fe(III) phenolate, Fe(III) keto anion, and Fe(II) keto π neutral radical resonance structures. EPR shows that complexation with CO enhances the Fe(II) π neutral radical component. Reaction of the α-meso-hydroxyheme-HO-1 complex with O₂ generates Fe(III) verdoheme, which can be reduced in the presence of CO to the Fe(II) verdoheme-CO complex. Thus, conversion of α- meso-hydroxyheme to Fe(III) verdoheme, in contrast to a previous report (Matera, K. M., Takahashi, S., Fujii, H., Zhou, H., Ishikawa, K., Yoshimura, T., Rousseau, D. L., Yoshida, T., and Ikeda-Saito, M. (1996) J. Biol. Chem. 271, 6618-6624), does not require a reducing equivalent. An electron is only required to reduce ferric to ferrous verdoheme in the first step of its conversion to biliverdin.