Heme Oxygenase Mechanism:  Evidence for an Electrophilic, Ferric Peroxide Species

Montellano, Paul R. Ortiz de

doi:10.1021/ar960207q

Cited by 222 publications

(261 citation statements)

References 51 publications

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“…Differences in the Proximal Pocket-In the rat apo-HO-1 structure, a large portion of the proximal helix (residues [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], which includes the proximal heme ligand (His 25 ), is not visible (24). In the human apo-HO-1 structure, the same region is clearly visible, with the proximal histidine pulled away from the heme pocket.…”

Section: Discussionmentioning

confidence: 99%

“…The mechanism of HO resembles that of cytochrome P450 in its ability to oxidize unactivated C-H bonds (14). However, in contrast to cytochrome P450 and heme peroxidases, the action of HO does not proceed through a ferryl intermediate (15,16), but appears rather to involve a peroxo ligand capable of attacking the ␣-meso bridge of the heme (Fig. 1).…”

Section: Heme Oxygenase (Ho)mentioning

confidence: 99%

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Comparison of the Heme-free and -bound Crystal Structures of Human Heme Oxygenase-1

Lad

Schüller

Shimizu

et al. 2003

Journal of Biological Chemistry

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112

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Heme oxygenase (HO) catalyzes the degradation of heme to biliverdin. The crystal structure of human HO-1 in complex with heme reveals a novel helical structure with conserved glycines in the distal helix, providing flexibility to accommodate substrate binding and product release (Schuller, D. J., Wilks, A., Ortiz de Montellano, P. R., and Poulos, T. L. (1999) Nat. Struct. Biol. 6, 860 -867). To structurally understand the HO catalytic pathway in more detail, we have determined the crystal structure of human apo-HO-1 at 2.1 Å and a higher resolution structure of human HO-1 in complex with heme at 1.5 Å. Although the 1.5-Å heme⅐HO-1 model confirms our initial analysis based on the 2.08-Å model, the higher resolution structure has revealed important new details such as a solvent H-bonded network in the active site that may be important for catalysis. Because of the absence of the heme, the distal and proximal helices that bracket the heme plane in the holo structure move farther apart in the apo structure, thus increasing the size of the active-site pocket. Nevertheless, the relative positioning and conformation of critical catalytic residues remain unchanged in the apo structure compared with the holo structure, but an important solvent H-bonded network is missing in the apoenzyme. It thus appears that the binding of heme and a tightening of the structure around the heme stabilize the solvent H-bonded network required for proper catalysis. Heme oxygenase (HO)1 catalyzes the oxidation of heme to biliverdin and is the key reaction in the recycling of porphyrin and iron in mammals. Biliverdin is reduced by biliverdin reductase to bilirubin, which is then conjugated with glucuronic acid and excreted (1, 2). Excretion of bilirubin is often deficient in newborn children, giving rise to neonatal jaundice and the potential for neurological damage (3). CO, the other product of the HO reaction, has been suggested to serve as a signaling molecule through the guanylate cyclase system in a manner similar to nitric oxide, although this remains a controversial topic (4, 5). Finally, the iron released by HO is normally recycled and represents the major source of this metal in heme homeostasis, whereas increased iron release due to elevated HO activity can trigger enhanced lipid and protein peroxidation (6, 7). There are two HO isoforms, denoted HO-1 and HO-2 (8 -10). A third isoform discovered in rats has been described (11), although the expressed protein is not an active HO, and therefore, its significance is currently unclear.Mammalian HO-1 is a membrane-bound protein.The expression of truncated, water-soluble, fully active forms of human (12) and rat (13) HO-1 missing the C-terminal 23-26 amino acid residues has facilitated major advances in structure/function studies of HO, especially mechanistic studies. The mechanism of HO resembles that of cytochrome P450 in its ability to oxidize unactivated C-H bonds (14). However, in contrast to cytochrome P450 and heme peroxidases, the action of HO does not proceed through a ferryl in...

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Section: Discussionmentioning

confidence: 99%

Section: Heme Oxygenase (Ho)mentioning

confidence: 99%

Comparison of the Heme-free and -bound Crystal Structures of Human Heme Oxygenase-1

Lad

Schüller

Shimizu

et al. 2003

Journal of Biological Chemistry

Self Cite

112

205

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“…The mechanism of this reaction has been proposed to involve electrophilic addition of the terminal oxygen in the ferric hydroperoxo complex to the ␣-meso-carbon, followed by elimination of the meso-proton to give the 5-hydroxyheme product (45). The evidence for this mechanism comes from observation of the ferric hydroperoxo complex in cryogenic experiments (46), from the demonstration that reaction of heme-hHO-1 with ethylhydroperoxide gives the 5-ethoxyheme product (47) and from the observation that an electron donating meso-methyl substituent favors, and an electron withdrawing substituent disfavors, reaction at the substituted meso-carbon (23,48).…”

Section: Discussionmentioning

confidence: 99%

Human Heme Oxygenase Oxidation of 5- and 15-Phenylhemes

Wang

Niemevz

Lad

et al. 2004

Journal of Biological Chemistry

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Human heme oxygenase-1 (hHO-1) catalyzes the O 2 -dependent oxidation of heme to biliverdin, CO, and free iron. Previous work indicated that electrophilic addition of the terminal oxygen of the ferric hydroperoxo complex to the ␣-meso-carbon gives 5-hydroxyheme. Earlier efforts to block this reaction with a 5-methyl substituent failed, as the reaction still gave biliverdin IX␣. Surprisingly, a 15-methyl substituent caused exclusive cleavage at the ␥-meso-rather than at the normal, unsubstituted ␣-meso-carbon. No CO was formed in these reactions, but the fragment cleaved from the porphyrin eluded identification. We report here that hHO-1 cleaves 5-phenylheme to biliverdin IX␣ and oxidizes 15-phenylheme at the ␣-meso position to give 10-phenylbiliverdin IX␣. The fragment extruded in the oxidation of 5-phenylheme is benzoic acid, one oxygen of which comes from O 2 and the other from water. The 2.29-and 2.11-Å crystal structures of the hHO-1 complexes with 1-and 15-phenylheme, respectively, show clear electron density for both the 5-and 15-phenyl rings in both molecules of the asymmetric unit. The overall structure of 15-phenylheme-hHO-1 is similar to that of heme-hHO-1 except for small changes in distal residues 141-150 and in the proximal Lys 18 and Lys 22 . In the 5-phenylhemehHO-1 structure, the phenyl-substituted heme occupies the same position as heme in the heme-HO-1 complex but the 5-phenyl substituent disrupts the rigid hydrophobic wall of residues Met 34 , Phe 214 , and residues 26 -42 near the ␣-meso carbon. The results provide independent support for an electrophilic oxidation mechanism and support a role for stereochemical control of the reaction regiospecificity.Heme 1 oxygenase, the rate-limiting enzyme in the heme degradation pathway, oxidizes heme to biliverdin, carbon monoxide, and free iron (1) (Fig. 1). In humans and other mammals, the heme is cleaved exclusively at the ␣-meso position to give biliverdin IX␣. The biliverdin then is reduced to bilirubin by biliverdin reductase before it is conjugated with glucuronic acid and excreted in the bile (2). Bilirubin is a powerful physiological antioxidant (3), but is neurotoxic at the high concentrations found in Criggler Najar patients and neonatal jaundice (4). CO, the second product of the reaction, is a gaseous messenger molecule that is thought to be involved in apoptosis (5), atherosclerosis (6), psoriasis (7), vascular constriction (8), cellular protection (9), and chronic renal inflammation (10). Finally, the ferrous iron released by heme oxygenase is normally recycled and serves as the major source of this metal for hemoglobin and other hemoprotein synthesis, as only 1-3% of the iron utilized daily in the synthesis of red blood cells is obtained from the diet (11). Thus, in addition to its other functions, heme oxygenase plays a vital role in maintaining iron homeostasis.In mammals, there are three heme oxygenase isoforms, namely HO-1, HO-2, and HO-3. HO-1 is inducible and present in highest concentration in spleen and liver. It is a stress pro...

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“…a one-electron reduced form of the oxaporphyrin ring of ferrous verdoheme, to which O 2 binds at α-pyrrole carbon, has been proposed as a possible mechanism for the cleavage of the oxaporphyrin ring of ferrous verdoheme [21], as shown in Fig. 2.…”

Section: Introductionmentioning

confidence: 99%

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

Satō

Higashimoto

Sakamoto

et al. 2007

Journal of Inorganic Biochemistry

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The heme oxygenase (HO) reaction consists of three successive oxygenation reactions, i.e. heme to α-hydroxyheme, α-hydroxyheme to verdoheme, and verdoheme to biliverdin-iron chelate. Of these, the least understood step is the conversion of verdoheme to biliverdin-iron chelate. For the cleavage of the oxaporphyrin ring of ferrous verdoheme, involvement of a verdoheme π-neutral radical has been proposed. To probe this hypothetical mechanism in the HO reaction, we performed electrochemical reduction of ferrous verdoheme complexed with rat HO-1 under anaerobic conditions. On the basis of the electrochemical spectral changes, the midpoint potential for the oneelectron reduction of the oxaporphyrin ring of ferrous verdoheme was found to be −0.47 ± 0.01 V vs the normal hydrogen electrode (NHE). Because this potential is far lower than those of both flavins of NADPH-cytochrome P450 reductase, and of NADPH, it is concluded that the one-electron reduction of the oxaporphyrin ring of ferrous verdoheme is unlikely to occur and that the formation of the π-neutral radical cannot be the initial step in the degradation of verdoheme by HO. Rather, it appears more reasonable to consider an alternative mechanism in which binding of O 2 to the ferrous iron of verdoheme is the first step in the degradation of verdoheme.

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Heme Oxygenase Mechanism: Evidence for an Electrophilic, Ferric Peroxide Species

Cited by 222 publications

References 51 publications

Comparison of the Heme-free and -bound Crystal Structures of Human Heme Oxygenase-1

Comparison of the Heme-free and -bound Crystal Structures of Human Heme Oxygenase-1

Human Heme Oxygenase Oxidation of 5- and 15-Phenylhemes

Electrochemical reduction of ferrous α-verdoheme in complex with heme oxygenase-1

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