Hydroxyheme

The exact stoichiometric requirements involved in the formation of ferric verdoheme from a-meso-hydroxyheme have been controversial 274-276). Although there is general agreement that this process is oxygen-dependent, the suggestion that additional reducing equivalents are also required 274, 277) has been questioned 275, 276). The conversion of verdoheme to biliverdin is the least well-characterized step of the overall reaction, although a mechanism has been proposed 271). 

The reaction catalyzed by HO and involved in coupled oxidation of Mb results in release of 1 mol of CO during conversion of a-meso-hydroxyheme to verdoheme. In a number of Mb variants, the formation... 

F. Electronic Effects on the Reaction Regiochemistry The Second Stage a-meso-Hydroxyheme to Verdoheme The Third Stage Verdoheme to Biliverdin... 

The conversion of a-meso-hydroxyheme to verdoheme is an oxygen-dependent process because the HO-l a-meso-hydroxyheme complex, whether obtained by reconstitution of the apoenzyme with synthetic a-meso-hydroxyheme or from oxidation of the heme complex with H2O2, is stable under anaerobic conditions 104, 105). EPR analysis of the... 

Fig. 11. EPR of the radical in the a-meso-hydroxyheme-HO-1 complex (A) The Fe heme HO-l complex, (B) the a-meso-hydroxyheme-HO-1 complex formed anaerobically with 1 equiv of H2O2 (C) the a-meso-hydroxyheme complex after subtraction of the spectrum of unreacted heme from trace (B) and (D) the spectrum in B after the addition of CO to form the Fe hCO complex. The spectra are taken from Liu et al. (.104).

Fig. 12. Proposed mechanism for the HO-1-catalyzed conversion of Fe a-meso-hydroxyheme (shown deprotonated) to Fe verdoheme. In an equally good variant of this mechanism, the oxygen molecule binds to the Fe" before it binds to the carbon of the porphyrin to give the same peroxo-bridged intermediate.

The minimal mechanism required for the conversion of a-meso-hydroxyheme to verdoheme requires deprotonation of a-meso-hydroxyheme to produce an Fe radical species that binds oxygen to give an Fe ... 

Fig. 14. Manifold of reactive species produced from the reaction of a heme group with oxygen and two reducing equivalents. The rate of conversion of A to B limits the lifetime (and therefore reactivity) of the Fe peroxo anion. The rate of formation of the ferryl species C via the Fe -OOH complex B competes with the intramolecular hydroxylation reaction to give hydroxyheme. Reactions of the Fe -hydroperoxy complex B with exogenous electrophilic substrates must compete with conversion of the intermediate to both C and meso-hydroxyheme. The Fe -OOH complex B can also be formed directly with H2O,.

Optical absorption spectra of the hydroperoxo-ferric intermediate in HO64 show Soret maximum at 421 nm (5 nm red shifted as compared with 416 nm band for oxy-ferrous HO) and Q-bands at 530 and 557 nm. After annealing at 212-215 K, a new species is formed with Soret band at 406 nm characteristic for the o-meso-hydroxyheme. 

Davydov, R.M., T, Yoshida, M. Ikeda-Saito, and B.M. Hoffman (1999). Hydroperoxy-heme oxygenase generated by cryoreduction catalyzes the formation of a-meso-hydroxyheme as detected by EPR and ENDOR. J. Am. Chem. Soc. 121, 10656-10657. 

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