Stereoselectivity of Each of the Three Steps of the Heme Oxygenase Reaction:  Hemin to meso-Hydroxyhemin, meso-Hydroxyhemin to Verdoheme, and Verdoheme to Biliverdin

Abstract

Heme oxygenase catalyzes the regiospecific oxidation of hemin to biliverdin IXalpha with concomitant liberation of CO and iron by three sequential monooxygenase reactions. The alpha-regioselectivity of heme oxygenase has been thought to result from the regioselective oxygenation of the heme alpha-meso position at the first step, which leads to the reaction pathway via meso-hydroxyheme IXalpha and verdoheme IXalpha intermediates. However, recent reports concerning heme oxygenase forming biliverdin isomers other than biliverdin IXalpha raise a question whether heme oxygenase can degrade meso-hydroxyhemin and isomers other than the alpha-isomers. In this paper, we investigated the stereoselectivity of each of the two reaction steps from meso-hydroxyhemin to verdoheme and verdoheme to biliverdin by using a truncated form of rat heme oxygenase-1 and the chemically synthesized four isomers of meso-hydroxyhemin and verdoheme. Heme oxygenase-1 converted all four isomers of meso-hydroxyhemin to the corresponding isomers of verdoheme. In contrast, only verdoheme IXalpha was converted to the corresponding biliverdin IXalpha. We conclude that the third step, but not the second, is stereoselective for the alpha-isomer substrate. The present findings on regioselectivities of the second and the third steps have been discussed on the basis of the oxygen activation mechanisms of these steps.

Crystal Structure of the Dioxygen-bound Heme Oxygenase from Corynebacterium diphtheriae

“…DISCUSSION HmuO, like HO-1, yields only the ␣-isomer of biliverdin (12). Regioselectivity of HO catalysis is defined at the first oxygenation step (50), where the remote hydroperoxo oxygen attacks the porphyrin ␣-meso-carbon to form ␣-meso-hydroxyheme (6,25). Previous studies have suggested that regiospecific degradation of heme by HO activity is primarily under the steric control.…”

O2- and H2O2-dependent Verdoheme Degradation by Heme Oxygenase

“…Even the binding site of O 2 is still uncertain because resonance structures of verdoheme involving a change in iron redox state (Fig. 2) Oxygen activation on the verdoheme iron has been proposed, analogous to the first step of meso-hydroxylation of heme in its ␣-regiospecificity (20). ␣-Selectivity of the first step is a result of steric hindrance imposed by the distal helix that covers the ␤-, ␥-, and ␦-meso-carbons but not the ␣-meso-carbon (23).…”

Catalytic antioxidants to treat amyotropic lateral sclerosis

“…Other protective mechanisms (e.g., metal-binding proteins, HSPs and so on) are upregulated to help the cells cope with the collateral damage of the haemorrhage. The ability of many synthetic metalloporphyrins to act as potent inducers may be related (in part) to the fact that they tend to be poor substrates for haem oxygenases [72,73] and thus they cannot be readily metabolised and cleared. The relatively long tissue residence time of some metalloporphyrins may also be related to this property and may help explain the potent adaptive resistance effects of these complexes.…”

“…Understanding why the metalloporphyrins ultimately fail and what might be done to prevent the loss of neuroprotection may be key to halting disease progression entirely. To the extent that metalloporphyrins are acting via an adaptive resistance mechanism, the dosing regimen may prove to be critical; for example, dosing to achieve the classic steady-state plasma level may not be the optimal approach, particularly considering that many of the porphyrins are largely unmetabolised [72,73] and that tissue residence times can be much greater than plasma residence [28]. Intermittent treatment with medium to high doses, continuous low dosing, dose escalation and combination with other agents are only a few of the strategies being examined to find a regimen that provides longer-term protection.…”