Heme oxygenase-2. Properties of the heme complex of the purified tryptic fragment of recombinant human heme oxygenase-2

A Chemical Proteomic Map of Heme-Protein Interactions

Heme is an essential cofactor for many human proteins as well as the primary transporter of oxygen in blood. Recent studies have also established heme as a signaling molecule, imparting its effects through binding with protein partners rather than through reactivity of its metal center. However, the comprehensive annotation of such heme-binding proteins in the human proteome remains incomplete. Here, we describe a strategy which utilizes a heme-based photoaffinity probe integrated with quantitative proteomics to map heme-protein interactions across the proteome. In these studies, we identified 350+ unique heme-protein interactions, the vast majority of which were heretofore unknown and consist of targets from diverse functional classes, including transporters, receptors, enzymes, transcription factors, and chaperones. Among these proteins is the immune-related interleukin receptor-associated kinase 1 (IRAK1), where we provide preliminary evidence that heme agonizes its catalytic activity. Our findings should improve the current understanding of heme's regulation as well as its signaling functions and facilitate new insights of its roles in human disease.

Transcriptome assembly for a colour-polymorphic grasshopper (Gomphocerus sibiricus) with a very large genome size

BACKGROUND

The club-legged grasshopper Gomphocerus sibiricus is a Gomphocerinae grasshopper with a promising future as model species for studying the maintenance of colour-polymorphism, the genetics of sexual ornamentation and genome size evolution. However, limited molecular resources are available for this species. Here, we present a de novo transcriptome assembly as reference resource for gene expression studies. We used high-throughput Illumina sequencing to generate 5,070,036 paired-end reads after quality filtering. We then combined the best-assembled contigs from three different de novo transcriptome assemblers (Trinity, SOAPdenovo-trans and Oases/Velvet) into a single assembly.

RESULTS

This resulted in 82,251 contigs with a N50 of 1357 and a TransRate assembly score of 0.325, which compares favourably with other orthopteran transcriptome assemblies. Around 87% of the transcripts could be annotated using InterProScan 5, BLASTx and the dammit! annotation pipeline. We identified a number of genes involved in pigmentation and green pigment metabolism pathways. Furthermore, we identified 76,221 putative single nucleotide polymorphisms residing in 8400 contigs. We also assembled the mitochondrial genome and investigated levels of sequence divergence with other species from the genus Gomphocerus. Finally, we detected and assembled Wolbachia sequences, which revealed close sequence similarity to the strain pel wPip.

CONCLUSIONS

Our study has generated a significant resource for uncovering genotype-phenotype associations in a species with an extraordinarily large genome, while also providing mitochondrial and Wolbachia sequences that will be useful for comparative studies.

QM/MM Study of the Conversion of Oxophlorin into Verdoheme by Heme Oxygenase

Heme oxygenase is an enzyme that degrades heme, thereby recycling iron in most organisms, including humans. Pervious density functional theory (DFT) calculations have suggested that iron(III) hydroxyheme, an intermediate generated in the first step of heme degradation by heme oxygenase, is converted to iron(III) superoxo oxophlorin in the presence of dioxygen. In this article, we have studied the detailed mechanism of conversion of iron(III) superoxo oxophlorin to verdoheme by using combined quantum mechanics and molecular mechanics (QM/MM) calculations. The calculations employed the B3LYP method and the def2-QZVP basis set, considering dispersion effects with the DFT-D3 approach, obtaining accurate energies with large QM regions of almost 1000 atoms. The reaction was found to be exothermic by -35 kcal/mol, with a rate-determining barrier of 19 kcal/mol in the doublet state. The protein environment and especially water in the enzyme pocket significantly affects the reaction by decreasing the reaction activation energies and changing the structures by providing strategic hydrogen bonds.

Comparison of the Mechanisms of Heme Hydroxylation by Heme Oxygenases-1 and -2: Kinetic and Cryoreduction Studies

The two isoforms of human heme oxygenase (HO1 and HO2) catalyze oxidative degradation of heme to biliverdin, Fe, and CO. Unlike HO1, HO2 contains two C-terminal heme regulatory motifs (HRMs) centered at Cys265 and Cys282 that act as redox switches and, in their reduced dithiolate state, bind heme (Fleischhacker et al., Biochemistry , 2015 , 54 , 2693 - 2708 ). Here, we describe cryoreduction/annealing and electron paramagnetic resonance spectroscopic experiments to study the structural features of the oxyheme moiety in HO2 and to elucidate the initial steps in heme degradation. We conclude that the same mechanism of heme hydroxylation to α-meso-hydroxyheme is employed by both isoforms and that the HRMs do not affect the physicochemical properties of the oxy-Fe(II) and HOO-Fe(III) states of HO2. However, the absorption spectrum of oxy-Fe(II)-HO2 is slightly blue-shifted relative to that of HO1. Furthermore, heme hydroxylation proceeds three times more slowly, and the oxy-Fe(II) state is 100-fold less stable in HO2 than in HO1. These distinctions are attributed to slight structural variances in the two proteins, including differences in equilibrium between open versus closed conformations. Kinetic studies revealed that heme oxygenation by HO2 occurs solely at the catalytic core in that a variant of HO2 lacking the C-terminal HRM domain exhibits the same specific activity as one containing both the catalytic core and HRM domain; furthermore, a truncated variant containing only the HRM region binds but cannot oxidize heme. In summary, HO1 and HO2 share similar catalytic mechanisms, and the HRMs do not play a direct role in the HO2 catalytic cycle.

Characterization of zebrafish neuroglobin and cytoglobins 1 and 2: Zebrafish cytoglobins provide insights into the transition from six-coordinate to five-coordinate globins

Neuroglobin (Ngb) and cytoglobin (Cygb) are two six-coordinate heme proteins of unknown physiological function. Although studies on the mammalian proteins have elucidated aspects of Ngb and Cygb biophysics and indicated potential functions, the properties of non-mammalian Ngbs and Cygbs are largely uncharacterized. We have expressed the recombinant zebrafish proteins Ngb, Cygb1, and Cygb2 in Escherichia coli and characterized their nitrite reduction rates, spectral properties, autoxidation rate constants, redox potentials and lipid binding properties. The three zebrafish proteins can catalyze the reduction of nitrite to nitric oxide with a broad range of reaction rate constants. (Ngb, 0.68 ± 0.04 M(-1) s(-1); Cygb1, 28.6 ± 3.1 M(-1) s(-1); Cygb2, 0.94 ± 0.18 M(-1) s(-1)). We observe that zebrafish Ngb and Cygb2 have comparable spectral features to those of human Ngb and Cygb, consistent with a six-coordinate heme, whereas unexpectedly Cygb1 has a five-coordinate heme, a slower autoxidation and in general has properties more akin to oxygen transport proteins. In agreement with a possible oxygen carrier and nitrite reductase role, we detect mRNA transcript for Cygb1 but not Cygb2 or Ngb in zebrafish blood. Unlike human Cygb, neither of the zebrafish globins binds oleic acid with high affinity. This finding suggests that lipid binding may be a trait acquired later during evolution and not an ancestral property of cytoglobins. Altogether, our results uncover unexpected properties of zebrafish globins and reveal the pivotal role of cytoglobins in the transition of heme globins from six-coordinate to five-coordinate oxygen carriers and nitrite reductases.

Protein/protein interactions in the mammalian heme degradation pathway: heme oxygenase-2, cytochrome P450 reductase, and biliverdin reductase

Heme oxygenase (HO) catalyzes the rate-limiting step in the O2-dependent degradation of heme to biliverdin, CO, and iron with electrons delivered from NADPH via cytochrome P450 reductase (CPR). Biliverdin reductase (BVR) then catalyzes conversion of biliverdin to bilirubin. We describe mutagenesis combined with kinetic, spectroscopic (fluorescence and NMR), surface plasmon resonance, cross-linking, gel filtration, and analytical ultracentrifugation studies aimed at evaluating interactions of HO-2 with CPR and BVR. Based on these results, we propose a model in which HO-2 and CPR form a dynamic ensemble of complex(es) that precede formation of the productive electron transfer complex. The (1)H-(15)N TROSY NMR spectrum of HO-2 reveals specific residues, including Leu-201, near the heme face of HO-2 that are affected by the addition of CPR, implicating these residues at the HO/CPR interface. Alanine substitutions at HO-2 residues Leu-201 and Lys-169 cause a respective 3- and 22-fold increase in K(m) values for CPR, consistent with a role for these residues in CPR binding. Sedimentation velocity experiments confirm the transient nature of the HO-2 · CPR complex (K(d) = 15.1 μM). Our results also indicate that HO-2 and BVR form a very weak complex that is only captured by cross-linking. For example, under conditions where CPR affects the (1)H-(15)N TROSY NMR spectrum of HO-2, BVR has no effect. Fluorescence quenching experiments also suggest that BVR binds HO-2 weakly, if at all, and that the previously reported high affinity of BVR for HO is artifactual, resulting from the effects of free heme (dissociated from HO) on BVR fluorescence.

Coordinated expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 and heme oxygenase 2: evidence for a regulatory link between glycolysis and heme catabolism

Heme is an essential requirement for cell survival. Heme oxygenase (HO) is the rate-limiting enzyme in heme catabolism and consists of two isozymes, HO-1 and HO-2. To identify the protein that regulates the expression or function of HO-1 or HO-2, we searched for proteins that interact with both isozymes, using protein microarrays. We thus identified 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4) that synthesizes or degrades fructose-2,6-bisphosphate, a key activator of glycolysis, depending on cellular microenvironments. Importantly, HO-2 and PFKFB4 are predominantly expressed in haploid spermatids. Here, we show a drastic reduction in expression levels of PFKFB4 mRNA and protein and HO-2 mRNA in HepG2 human hepatoma cells in responses to glucose deprivation (≤ 2.5 mM), which occurred concurrently with remarkable induction of HO-1 mRNA and protein. Knockdown of HO-2 expression in HepG2 cells, using small interfering RNA, caused PFKFB4 mRNA levels to decrease with a concurrent increase in HO-1 expression. Thus, in HepG2 cells, HO-1 expression was increased, when expression levels of HO-2 and PFKFB4 mRNAs were decreased. Conversely, overexpression of HO-2 in HepG2 cells caused the level of co-expressed PFKFB4 protein to increase. These results suggest a potential regulatory role for HO-2 in ensuring PFKFB4 expression. Moreover, in D407 human retinal pigment epithelial cells, glucose deprivation decreased the expression levels of PFKFB4, HO-1, and HO-2 mRNAs. Thus, glucose deprivation consistently down-regulated the expression of PFKFB4 and HO-2 mRNAs in both HepG2 cells and RPE cells. We therefore postulate that PFKFB4 and HO-2 are expressed in a coordinated manner to maintain glucose homeostasis.

Inactivation of the heme degrading enzyme IsdI by an active site substitution that diminishes heme ruffling

IsdG and IsdI are paralogous heme degrading enzymes from the bacterium Staphylococcus aureus. Heme bound by these enzymes is extensively ruffled such that the meso-carbons at the sites of oxidation are distorted toward bound oxygen. In contrast, the canonical heme oxygenase family degrades heme that is bound with minimal distortion. Trp-66 is a conserved heme pocket residue in IsdI implicated in heme ruffling. IsdI variants with Trp-66 replaced with residues having less bulky aromatic and alkyl side chains were characterized with respect to catalytic activity, heme ruffling, and electrochemical properties. The heme degradation activity of the W66Y and W66F variants was approximately half that of the wild-type enzyme, whereas the W66L and W66A variants were inactive. A crystal structure and NMR spectroscopic analysis of the W66Y variant reveals that heme binds to this enzyme with less heme ruffling than observed for wild-type IsdI. The reduction potential of this variant (-96 ± 7 mV versus standard hydrogen electrode) is similar to that of wild-type IsdI (-89 ± 7 mV), so we attribute the diminished activity of this variant to the diminished heme ruffling observed for heme bound to this enzyme and conclude that Trp-66 is required for optimal catalytic activity.

Heme oxygenase isoforms differ in their subcellular trafficking during hypoxia and are differentially modulated by cytochrome P450 reductase

Heme oxygenase (HO) degrades heme in concert with NADPH cytochrome P450 reductase (CPR) which donates electrons to the reaction. Earlier studies reveal the importance of the hydrophobic carboxy-terminus of HO-1 for anchorage to the endoplasmic reticulum (ER) which facilitates the interaction with CPR. In addition, HO-1 has been shown to undergo regulated intramembrane proteolysis of the carboxy-terminus during hypoxia and subsequent translocation to the nucleus. Translocated nuclear HO-1 was demonstrated to alter binding of transcription factors and to alter gene expression. Little is known about the homologous membrane anchor of the HO-2 isoform. The current work is the first systematic analysis in a eukaryotic system that demonstrates the crucial role of the membrane anchor of HO-2 for localization at the endoplasmic reticulum, oligomerization and interaction with CPR. We show that although the carboxy-terminal deletion mutant of HO-2 is found in the nucleus, translocation of HO-2 to the nucleus does not occur under conditions of hypoxia. Thus, we demonstrate that proteolytic regulation and nuclear translocation under hypoxic conditions is specific for HO-1. In addition we show for the first time that CPR prevents this translocation and promotes oligomerization of HO-1. Based on these findings, CPR may modulate gene expression via the amount of nuclear HO-1. This is of particular relevance as CPR is a highly polymorphic gene and deficiency syndromes of CPR have been described in humans.

Electronic properties of the highly ruffled heme bound to the heme degrading enzyme IsdI

IsdI, a heme-degrading protein from Staphylococcus aureus, binds heme in a manner that distorts the normally planar heme prosthetic group to an extent greater than that observed so far for any other heme-binding protein. To understand better the relationship between this distinct structural characteristic and the functional properties of IsdI, spectroscopic, electrochemical, and crystallographic results are reported that provide evidence that this heme ruffling is essential to the catalytic activity of the protein and eliminates the need for the water cluster in the distal heme pocket that is essential for the activity of classical heme oxygenases. The lack of heme orientational disorder in (1)H-NMR spectra of the protein argues that the catalytic formation of β- and δ-biliverdin in nearly equal yield results from the ability of the protein to attack opposite sides of the heme ring rather than from binding of the heme substrate in two alternative orientations.

Spectroscopic insights into axial ligation and active-site H-bonding in substrate-bound human heme oxygenase-2

Heme oxygenases (HOs) are monooxygenases that catalyze the first step in heme degradation, converting heme to biliverdin with concomitant release of Fe(II) and CO from the porphyrin macrocycle. Two heme oxygenase isoforms, HO-1 and HO-2, exist that differ in several ways, including a complete lack of Cys residues in HO-1 and the presence of three Cys residues as part of heme-regulatory motifs (HRMs) in HO-2. HRMs in other heme proteins are thought to directly bind heme, or to otherwise regulate protein stability or activity; however, it is not currently known how the HRMs exert these effects on HO-2 function. To better understand the properties of this vital enzyme and to elucidate possible roles of its HRMs, various forms of HO-2 possessing distinct alterations to the HRMs were prepared. In this study, variants with Cys265 in a thiol form are compared with those with this residue in an oxidized (part of a disulfide bond or existing as a sulfenate moiety) form. Absorption and magnetic circular dichroism spectroscopic data of these HO-2 variants clearly demonstrate that a new low-spin Fe(III) heme species characteristic of thiolate ligation is formed when Cys265 is reduced. Additionally, absorption, magnetic circular dichroism, and resonance Raman data collected at different temperatures reveal an intriguing temperature dependence of the iron spin state in the heme-HO-2 complex. These findings are consistent with the presence of a hydrogen-bonding network at the heme's distal side within the active site of HO-2 with potentially significant differences from that observed in HO-1.

Interactions of multiple gas-transducing systems: hallmarks and uncertainties of CO, NO, and H2S gas biology

The diverse physiological actions of the "biologic gases," O2, CO, NO, and H2S, have attracted much interest. Initially viewed as toxic substances, CO, NO, and H2S play important roles as signaling molecules. The multiplicity of gas actions and gas targets and the difficulty in measuring local gas concentrations obscures detailed mechanisms whereby gases exert their actions, and many questions remain unanswered. It is now readily apparent, however, that heme-based proteins play central roles in gas-generation/reception mechanisms and provide a point where multiple gases can interact. In this review, we consider a number of key issues related to "gas biology," including the effective tissue concentrations of these gases and the importance and significance of the physical proximity of gas-producing and gas-receptor/sensors. We also take an integrated approach to the interaction of gases by considering the physiological significance of CO, NO, and H2S on mitochondrial cytochrome c oxidase, a key target and central mediator of mitochondrial respiration. Additionally, we consider the effects of biologic gases on mitochondrial biogenesis and "suspended animation." By evaluating gas-mediated control functions from both in vitro and in vivo perspectives, we hope to elaborate on the complex multiple interactions of O2, NO, CO, and H2S.

Targeting heme oxygenase: therapeutic implications for diseases of the cardiovascular system

Heme oxygenase (HO) is important in attenuating the overall production of reactive oxygen species through its ability to degrade heme and to produce carbon monoxide, biliverdin/bilirubin, and release of free iron. Excess free heme catalyzes the formation of reactive oxygen species, which leads to endothelial cell (EC) dysfunction as seen in numerous pathologic vascular conditions including systemic hypertension and diabetes, as well as in ischemia/reperfusion injury.The up-regulation of HO-1 can be achieved through the use of pharmaceutical agents such as metalloporphyrins and statins. In addition, atrial natriuretic peptide and nitric oxide donors are important modulators of the heme-HO system, either through induction of HO-1 or the increased biologic activity of its products. Gene therapy and gene transfer, including site- and organ-specific targeted gene transfer have become powerful tools for studying the potential role of the 2 isoforms of HO, HO-1/HO-2, in the treatment of cardiovascular disease, as well as diabetes. HO-1 induction by pharmacological agents or the in vitro gene transfer of human HO-1 into ECs increases cell cycle progression and attenuates angiotensin II, tumor necrosis factor-alpha, and heme-mediated DNA damage; administration in vivo corrects blood pressure elevation after angiotensin II exposure. Delivery of human HO-1 to hyperglycemic rats significantly lowers superoxide levels and prevents EC damage and sloughing of vascular EC into the circulation. In addition, administration of human HO-1 to rats in advance of ischemia/reperfusion injury considerably reduces tissue damage.The ability to up-regulate HO-1 either through pharmacological means or through the use of gene therapy may offer therapeutic strategies for the prevention of cardiovascular disease in the future. This review discusses the implications of HO-1 delivery during the early stages of cardiovascular system injury or in early vascular pathology, and suggests that pharmacological agents that regulate HO activity or HO-1 gene delivery itself may become powerful tools for preventing the onset or progression of various cardiovascular diseases.

Evidence for heme oxygenase activity in a heme peroxidase

The heme peroxidase and heme oxygenase enzymes share a common heme prosthetic group but catalyze fundamentally different reactions, the first being H(2)O(2)-dependent oxidation of substrate using an oxidized Compound I intermediate, and the second O(2)-dependent degradation of heme. It has been proposed that these enzymes utilize a common reaction intermediate, a ferric hydroperoxide species, that sits at a crossroads in the mechanism and beyond which there are two mutually exclusive mechanistic pathways. Here, we present evidence to support this proposal in a heme peroxidase. Hence, we describe kinetic data for a variant of ascorbate peroxidase (W41A) which reacts slowly with tert-butyl hydroperoxide and does not form the usual peroxidase Compound I intermediate; instead, structural data show that a product is formed in which the heme has been cleaved at the alpha-meso position, analogous to the heme oxygenase mechanism. We interpret this to mean that the Compound I (peroxidase) pathway is shut down, so that instead the reaction intermediate diverts through the alternative (heme oxygenase) route. A mechanism for formation of the product is proposed and discussed in the light of what is known about the heme oxygenase reaction mechanism.

Structures and solution properties of two novel periplasmic sensor domains with c-type heme from chemotaxis proteins of Geobacter sulfurreducens: implications for signal transduction

Periplasmic sensor domains from two methyl-accepting chemotaxis proteins from Geobacter sulfurreducens (encoded by genes GSU0935 and GSU0582) were expressed in Escherichia coli. The sensor domains were isolated, purified, characterized in solution, and their crystal structures were determined. In the crystal, both sensor domains form swapped dimers and show a PAS-type fold. The swapped segment consists of two helices of about 45 residues at the N terminus with the hemes located between the two monomers. In the case of the GSU0582 sensor, the dimer contains a crystallographic 2-fold symmetry and the heme is coordinated by an axial His and a water molecule. In the case of the GSU0935 sensor, the crystals contain a non-crystallographic dimer, and surprisingly, the coordination of the heme in each monomer is different; monomer A heme has His-Met ligation and monomer B heme has His-water ligation as found in the GSU0582 sensor. The structures of these sensor domains are the first structures of PAS domains containing covalently bound heme. Optical absorption, electron paramagnetic resonance and NMR spectroscopy have revealed that the heme groups of both sensor domains are high-spin and low-spin in the oxidized and reduced forms, respectively, and that the spin-state interconversion involves a heme axial ligand replacement. Both sensor domains bind NO in their ferric and ferrous forms but bind CO only in the reduced form. The binding of both NO and CO occurs via an axial ligand exchange process, and is fully reversible. The reduction potentials of the sensor domains differ by 95 mV (-156 mV and -251 mV for sensors GSU0582 and GSU0935, respectively). The swapped dimerization of these sensor domains and redox-linked ligand switch might be related to the mechanism of signal transduction by these chemotaxis proteins.

Hypoxia and heme oxygenases: oxygen sensing and regulation of expression

Heme is an essential molecule for life, as it is involved in sensing and using oxygen. Heme must be synthesized and degraded within an individual nucleated cell. Physiologic heme degradation is catalyzed by two functional isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, yielding carbon monoxide, iron, and biliverdin, an immediate precursor to bilirubin. HO-1 is an inducible enzyme, but the expression level of HO-2 is maintained in a narrow range. Characteristically, human HO-1 contains no Cys residue, whereas human HO-2 contains three Cys residues, each of which might be involved in heme binding. These features suggest separate physiologic roles of HO-1 and HO-2. Recent studies have shown that the expression levels of HO-1 and HO-2 are reduced under hypoxia, depending on the cell types. Moreover, we have proposed HO-2 as a potential O(2) sensor, because HO-2-deficient mice show hypoxemia and a blunted hypoxic ventilatory response with normal hypercapnic ventilatory response. HO-2-deficient mice also show hypertrophy of the pulmonary venous myocardium and enlargement of the carotid body. These morphometric changes are attributable to chronic hypoxemia. Here, we update the understanding of the regulation of HO-1 and HO-2 expression and summarize the regulatory role of HO-2 in the intercellular communication.

Heme as a magnificent molecule with multiple missions: heme determines its own fate and governs cellular homeostasis

Heme is a prosthetic group of various types of proteins, such as hemoglobin, myoglobin, cytochrome c, cytochrome p450, catalase and peroxidase. In addition, heme is involved in a variety of biological events by modulating the function or the state of hemoproteins. For example, protein synthesis is inhibited in erythroid cells under heme deficiency, as the consequence of the activation of heme-regulated inhibitor (HRI). Iron concentration in the cell is sensed and regulated by the heme-mediated oxidization and subsequent degradation of iron regulatory protein 2 (IRP2). Heme also binds to certain types of potassium channels, thereby inhibiting transmembrane K(+) currents. Importantly, heme determines its own fate; namely, heme regulates its synthesis and degradation through the feedback mechanisms, by which intracellular heme level is precisely maintained. Heme reduces heme synthesis by suppressing the expression of non-specific 5-aminolevulinate synthase (ALAS1) and stimulates heme breakdown by inducing heme oxygenase (HO)-1 expression. ALAS1 and HO-1 are the rate limiting enzymes in heme biosynthesis and catabolism, respectively. Accordingly, under the heme-rich condition, heme binds to cysteine-proline (CP) motifs of ALAS1 and those of transcriptional repressor Bach1, thereby leading to repression of mitochondrial transport of ALAS1 and induction of HO-1 transcription, respectively. Moreover, chemosensing functions of HO-2 containing CP motifs, another isozyme of HO, have been unveiled recently. In this review article, we summarize and update the pleiotropic effects of heme on various biological events and the regulatory network of heme biosynthesis and catabolism.

Evidence that the heme regulatory motifs in heme oxygenase-2 serve as a thiol/disulfide redox switch regulating heme binding

Heme oxygenase (HO) catalyzes the O(2)- and NADPH-dependent conversion of heme to biliverdin, CO, and iron. The two forms of HO (HO-1 and HO-2) share similar physical properties but are differentially regulated and exhibit dissimilar physiological roles and tissue distributions. Unlike HO-1, HO-2 contains heme regulatory motifs (HRMs) (McCoubrey, W. K., Jr., Huang, T. J., and Maines, M. D. (1997) J. Biol. Chem. 272, 12568-12574). Here we describe UV-visible, EPR, and differential scanning calorimetry experiments on human HO-2 variants containing single, double, and triple mutations in the HRMs. Oxidized HO-2, which contains an intramolecular disulfide bond linking Cys(265) of HRM1 and Cys(282) of HRM2, binds heme tightly. Reduction of the disulfide bond increases the K(d) for ferric heme from 0.03 to 0.3 microm, which is much higher than the concentration of the free heme pool in cells. Although the HRMs markedly affect the K(d) for heme, they do not alter the k(cat) for heme degradation and do not bind additional hemes. Because HO-2 plays a key role in CO generation and heme homeostasis, reduction of the disulfide bond would be expected to increase intracellular free heme and decrease CO concentrations. Thus, we propose that the HRMs in HO-2 constitute a thiol/disulfide redox switch that regulates the myriad physiological functions of HO-2, including its involvement in the hypoxic response in the carotid body, which involves interactions with a Ca(2+)-activated potassium channel.

Structure of the Escherichia coli O157:H7 Heme Oxygenase ChuS in Complex with Heme and Enzymatic Inactivation by Mutation of the Heme Coordinating Residue His-193

Heme oxygenases catalyze the oxidation of heme to biliverdin, CO, and free iron. For pathogenic microorganisms, heme uptake and degradation are critical mechanisms for iron acquisition that enable multiplication and survival within hosts they invade. Here we report the first crystal structure of the pathogenic Escherichia coli O157:H7 heme oxygenase ChuS in complex with heme at 1.45 A resolution. When compared with other heme oxygenases, ChuS has a unique fold, including structural repeats and a beta-sheet core. Not surprisingly, the mode of heme coordination by ChuS is also distinct, whereby heme is largely stabilized by residues from the C-terminal domain, assisted by a distant arginine from the N-terminal domain. Upon heme binding, there is no large conformational change beyond the fine tuning of a key histidine (His-193) residue. Most intriguingly, in contrast to other heme oxygenases, the propionic side chains of heme are orientated toward the protein core, exposing the alpha-meso carbon position where O(2) is added during heme degradation. This unique orientation may facilitate presentation to an electron donor, explaining the significantly reduced concentration of ascorbic acid needed for the reaction. Based on the ChuS-heme structure, we converted the histidine residue responsible for axial coordination of the heme group to an asparagine residue (H193N), as well as converting a second histidine to an alanine residue (H73A) for comparison purposes. We employed spectral analysis and CO measurement by gas chromatography to analyze catalysis by ChuS, H193N, and H73A, demonstrating that His-193 is the key residue for the heme-degrading activity of ChuS.

Down-regulation of heme oxygenase-2 is associated with the increased expression of heme oxygenase-1 in human cell lines

Intracellular heme concentrations are maintained in part by heme degradation, which is catalyzed by heme oxygenase. Heme oxygenase consists of two structurally related isozymes, HO-1 and HO-2. Recent studies have identified HO-2 as a potential oxygen sensor. To gain further insights into the regulatory role of HO-2 in heme homeostasis, we analyzed the expression profiles of HO-2 and the biochemical consequences of HO-2 knockdown with specific short interfering RNA (siRNA) in human cells. Both HO-2 mRNA and protein are expressed in the eight human cancer cell lines examined, and HO-1 expression is detectable in five of the cell lines, including HeLa cervical cancer and HepG2 hepatoma. Down-regulation of HO-2 expression with siRNA against HO-2 (siHO-2) caused induction of HO-1 expression at both mRNA and protein levels in HeLa and HepG2 cells. In contrast, knockdown of HO-1 expression did not noticeably influence HO-2 expression. HO-2 knockdown prolonged the half-life of HO-1 mRNA twofold in HeLa cells. Transient transfection assays in HeLa cells revealed that the 4.5-kb human HO-1 gene promoter was activated with selective knockdown of HO-2 in a sequence-dependent manner. Moreover, HO-2 knockdown caused heme accumulation in HeLa and HepG2 cells only when exposed to exogenous hemin. HO-2 knockdown may mimic a certain physiological change that is important in the maintenance of cellular heme homeostasis. These results suggest that HO-2 may down-regulate the expression of HO-1, thereby directing the co-ordinated expression of HO-1 and HO-2.

Heme oxygenase-1 as a potential therapeutic target for hepatoprotection

Heme oxygenase (HO), the rate limiting enzyme in the breakdown of heme into carbon monoxide (CO), iron and bilirubin, has recently received overwhelming research attention. To date three mammalian HO isozymes have been identified, and the only inducible form is HO-1 while HO-2 and HO-3 are constitutively expressed. Advances in unveiling signal transduction network indicate that a battery of redox-sensitive transcription factors, such as activator protein-1 (AP-1), nuclear factor-kappa B (NF-kappaB) and nuclear factor E2-related factor-2 (Nrf2), and their upstream kinases including mitogen-activated protein kinases play an important regulatory role in HO-1 gene induction. The products of the HO-catalyzed reaction, particularly CO and biliverdin/bilirubin have been shown to exert protective effects in several organs against oxidative and other noxious stimuli. In this context, it is interesting to note that induction of HO-1 expression contributes to protection against liver damage induced by several chemical compounds such as acetaminophen, carbon tetrachloride and heavy metals, suggesting HO-1 induction as an important cellular endeavor for hepatoprotection. The focus of this review is on the significance of targeted induction of HO-1 as a potential therapeutic strategy to protect against chemically-induced liver injury as well as hepatocarcinogenesis.

Hypoxia reduces the expression of heme oxygenase-2 in various types of human cell lines. A possible strategy for the maintenance of intracellular heme level

Heme oxygenase consists of two structurally related isozymes, heme oxygenase-1 and and heme oxygenase-2, each of which cleaves heme to form biliverdin, iron and carbon monoxide. Expression of heme oxygenase-1 is increased or decreased depending on cellular microenvironments, whereas little is known about the regulation of heme oxygenase-2 expression. Here we show that hypoxia (1% oxygen) reduces the expression levels of heme oxygenase-2 mRNA and protein after 48 h of incubation in human cell lines, including Jurkat T-lymphocytes, YN-1 and K562 erythroleukemia, HeLa cervical cancer, and HepG2 hepatoma, as judged by northern blot and western blot analyses. In contrast, the expression level of heme oxygenase-1 mRNA varies under hypoxia, depending on the cell line; it was increased in YN-1 cells, decreased in HeLa and HepG2 cells, and remained undetectable in Jurkat and K562 cells. Moreover, heme oxygenase-1 protein was decreased in YN-1 cells under the conditions used, despite the induction of heme oxygenase-1 mRNA under hypoxia. The heme oxygenase activity was significantly decreased in YN-1, K562 and HepG2 cells after 48 h of hypoxia. To explore the mechanism for the hypoxia-mediated reduction of heme oxygenase-2 expression, we showed that hypoxia shortened the half-life of heme oxygenase-2 mRNA (from 12 h to 6 h) in YN-1 cells, without affecting the half-life of heme oxygenase-1 mRNA (9.5 h). Importantly, the heme contents were increased in YN-1, HepG2 and HeLa cells after 48 h of incubation under hypoxia. Thus, the reduced expression of heme oxygenase-2 may represent an important adaptation to hypoxia in certain cell types, which may contribute to the maintenance of the intracellular heme level.

Dynamic changes in expression of heme oxygenases in mouse heart and liver during hypoxia

Heme oxygenase cleaves heme to form biliverdin, carbon monoxide (CO), and iron, and consists of two structurally related isozymes, HO-1 and HO-2. HO-2 is also known as a potential oxygen sensor. Here we show that the relative CO content in arterial blood, which reflects the total amount of endogenous heme degradation, dynamically changes in mice during acclimatization to normobaric hypoxia (10% O2), with the two peaks at 1 day and 21 days of hypoxia. The expression levels of HO-1 and HO-2 proteins were decreased by 20% and 40%, respectively, in the mouse liver at 7 days of hypoxia, which returned to the basal levels at 14 days. On the other hand, HO-1 and HO-2 proteins were increased 2-fold and 1.3-fold, respectively, in the heart at 28 days of hypoxia. Thus, hypoxia induces or represses the expression of HO-1 and HO-2 in vivo, depending on cellular microenvironments.

Identification of an Escherichia coli O157:H7 heme oxygenase with tandem functional repeats

Heme oxygenases (HOs) catalyze the oxidation of heme to biliverdin, carbon monoxide (CO), and free iron. Iron acquisition is critical for invading microorganisms to enable survival and growth. Here we report the crystal structure of ChuS, which displays a previously uncharacterized fold and is unique compared with other characterized HOs. Despite only 19% sequence identity between the N- and C-terminal halves, these segments of ChuS represent a structural duplication, with a root-mean-square deviation of 2.1 A between the two repeats. ChuS is capable of using ascorbic acid or cytochrome P450 reductase-NADPH as electron sources for heme oxygenation. CO detection confirmed that ChuS is a HO, and we have identified it in pathogenic Escherichia coli O157:H7. Based on sequence analysis, this HO is present in many bacteria, although not in the E. coli K-12 strain. The N- and C-terminal halves of ChuS are each a functional HO.

O(2)- and H(2)O(2)-dependent verdoheme degradation by heme oxygenase: reaction mechanisms and potential physiological roles of the dual pathway degradation

Heme oxygenase (HO) catalyzes the catabolism of heme to biliverdin, CO, and a free iron through three successive oxygenation steps. The third oxygenation, oxidative degradation of verdoheme to biliverdin, has been the least understood step despite its importance in regulating HO activity. We have examined in detail the degradation of a synthetic verdoheme IXalpha complexed with rat HO-1. Our findings include: 1) HO degrades verdoheme through a dual pathway using either O(2) or H(2)O(2); 2) the verdoheme reactivity with O(2) is the lowest among the three O(2) reactions in the HO catalysis, and the newly found H(2)O(2) pathway is approximately 40-fold faster than the O(2)-dependent verdoheme degradation; 3) both reactions are initiated by the binding of O(2) or H(2)O(2) to allow the first direct observation of degradation intermediates of verdoheme; and 4) Asp(140) in HO-1 is critical for the verdoheme degradation regardless of the oxygen source. On the basis of these findings, we propose that the HO enzyme activates O(2) and H(2)O(2) on the verdoheme iron with the aid of a nearby water molecule linked with Asp(140). These mechanisms are similar to the well established mechanism of the first oxygenation, meso-hydroxylation of heme, and thus, HO can utilize a common architecture to promote the first and third oxygenation steps of the heme catabolism. In addition, our results infer the possible involvement of the H(2)O(2)-dependent verdoheme degradation in vivo, and potential roles of the dual pathway reaction of HO against oxidative stress are proposed.

Hypoxemia and blunted hypoxic ventilatory responses in mice lacking heme oxygenase-2

Heme oxygenase (HO) catalyzes physiological heme degradation and consists of two structurally related isozymes, HO-1 and HO-2. Here we show that HO-2-deficient (HO-2(-/-)) mice exhibit hypoxemia and hypertrophy of the pulmonary venous myocardium associated with increased expression of HO-1. The hypertrophied venous myocardium may reflect adaptation to persistent hypoxemia. HO-2(-/-) mice also show attenuated ventilatory responses to hypoxia (10% O2) with normal responses to hypercapnia (10% CO2), suggesting the impaired oxygen sensing. Importantly, HO-2(-/-) mice exhibit normal breathing patterns with normal arterial CO2 tension and retain the intact alveolar architecture, thereby excluding hypoventilation and shunting as causes of hypoxemia. Instead, ventilation-perfusion mismatch is a likely cause of hypoxemia, which may be due to partial impairment of the lung chemoreception probably at pulmonary artery smooth muscle cells. We therefore propose that HO-2 is involved in oxygen sensing and responsible for the ventilation-perfusion matching that optimizes oxygenation of pulmonary blood.

The heme oxygenase dilemma in cellular homeostasis: new insights for the feedback regulation of heme catabolism

Heme must be synthesized and degraded within an individual nucleated cell. Heme degradation is catalyzed by the two isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, eventually yielding biliverdin/bilirubin, CO, and iron. These products possess important physiological roles but are potentially toxic to cells. Characteristically, human HO-1 contains no Cys residues, whereas HO-2 contains the potential heme-binding motifs of the Cys-Pro dipeptide. Expression of HO-1 is inducible or repressible, depending on cell types or cellular microenvironments, but expression levels of HO-2 are fairly constant. Thus, the main regulation of heme catabolism is a problem of the balance between induction and repression of HO-1. Notably, HO-1 expression is induced by heme in all mammalian cells examined, but is repressed by hypoxia in certain types of cultured human cells. The recent discovery of Bach1 as a heme-regulated and hypoxia-inducible repressor for transcription of the HO-1 gene has provided a missing link in the feedback control of heme catabolism. On the other hand, the human HO-1 gene promoter contains the (GT)n repeat polymorphism and a single nucleotide polymorphism (-427A --> T), both of which may contribute to fine-tuning of the transcription. Importantly, long (GT)n alleles are associated with susceptibility to smoking-induced emphysema or coronary artery disease, but may provide with resistance to cerebral malaria. The latter finding suggests a novel therapeutic strategy with inhibitors of HO-1 for the treatment of cerebral malaria. We discuss the potential regulatory role of Bach1 and HO-2 in heme catabolism and update the understanding of the regulation of HO-1 expression.

Crystal structures of the NO- and CO-bound heme oxygenase from Neisseriae meningitidis. Implications for O2 activation

Heme oxygenases catalyze the oxidation of heme to biliverdin, carbon monoxide, and free iron while playing a critical role in mammalian heme homeostasis. Pathogenic bacteria such as Neisseriae meningitidis also produce heme oxygenase as part of a mechanism to mine host iron. The key step in heme oxidation is the regioselective oxidation of the heme alpha-meso-carbon by an activated Fe(III)-OOH complex. The structures of various diatomic ligands bound to the heme iron can mimic the dioxygen complex and provide important insights on the mechanism of O2 activation. Here we report the crystal structures of N. meningitidis heme oxygenase (nm-HO) in the Fe(II), Fe(II)-CO, and Fe(II)-NO states and compare these to the NO complex of human heme oxygenase-1 (Lad, L., Wang, J., Li, H., Friedman, J., Bhaskar, B., Ortiz de Montellano, P. R., and Poulos, T. L. (2003) J. Mol. Biol. 330, 527-538). Coordination of NO or CO results in a reorientation of Arg-77 that enables Arg-77 to participate in an active site H-bonded network involving a series of water molecules. One of these water molecules directly H-bonds to the Fe(II)-linked ligand and very likely serves as the proton source required for oxygen activation. Although the active site residues differ between nm-HO and human HO-1, the close similarity in the H-bonded water network suggests a common mechanism shared by all heme oxygenases.

Crystal structures of the ferric, ferrous, and ferrous-NO forms of the Asp140Ala mutant of human heme oxygenase-1: catalytic implications

Site-directed mutagenesis studies have shown that Asp140 in both human and rat heme oxygenase-1 is critical for enzyme activity. Here, we report the D140A mutant crystal structure in the Fe(III) and Fe(II) redox states as well as the Fe(II)-NO complex as a model for the Fe(II)-oxy complex. These structures are compared to the corresponding wild-type structures. The mutant and wild-type structures are very similar, except for the distal heme pocket solvent structure. In the Fe(III) D140A mutant one water molecule takes the place of the missing Asp140 carboxylate side-chain and a second water molecule, novel to the mutant, binds in the distal pocket. Upon reduction to the Fe(II) state, the distal helix running along one face of the heme moves closer to the heme in both the wild-type and mutant structures thus tightening the active site. NO binds to both the wild-type and mutant in a bent conformation that orients the NO O atom toward the alpha-meso heme carbon atom. A network of water molecules provides a H-bonded network to the NO ligand, suggesting a possible proton shuttle pathway required to activate dioxygen for catalysis. In the wild-type structure, Asp140 exhibits two conformations, suggesting a dynamic role for Asp140 in shuttling protons from bulk solvent via the water network to the iron-linked oxy complex. On the basis of these structures, we consider why the D140A mutant is inactive as a heme oxygenase but active as a peroxidase.

Expression and characterization of cyanobacterium heme oxygenase, a key enzyme in the phycobilin synthesis. Properties of the heme complex of recombinant active enzyme

An efficient bacterial expression system of cyanobacterium Synechocystis sp. PCC 6803 heme oxygenase gene, ho-1, has been constructed, using a synthetic gene. A soluble protein was expressed at high levels and was highly purified, for the first time. The protein binds equimolar free hemin to catabolize the bound hemin to ferric-biliverdin IX alpha in the presence of oxygen and reducing equivalents, showing the heme oxygenase activity. During the reaction, verdoheme intermediate is formed with the evolution of carbon monoxide. Though both ascorbate and NADPH-cytochrome P450 reductase serve as an electron donor, the heme catabolism assisted by ascorbate is considerably slow and the reaction with NADPH-cytochrome P450 reductase is greatly retarded after the oxy-heme complex formation. The optical absorption spectra of the heme-enzyme complexes are similar to those of the known heme oxygenase complexes but have some distinct features, exhibiting the Soret band slightly blue-shifted and relatively strong CT bands of the high-spin component in the ferric form spectrum. The heme-enzyme complex shows the acid-base transition, where two alkaline species are generated. EPR of the nitrosyl heme complex has established the nitrogenous proximal ligand, presumably histidine 17 and the obtained EPR parameters are discriminated from those of the rat heme oxygenase-1 complex. The spectroscopic characters as well as the catabolic activities strongly suggest that, in spite of very high conservation of the primary structure, the heme pocket structure of Synechocystis heme oxygenase isoform-1 is different from that of rat heme oxygenase isoform-1, rather resembling that of bacterial heme oxygenase, H mu O.

Interaction of nitric oxide with human heme oxygenase-1

NO and CO may complement each other as signaling molecules in some physiological situations. We have examined the binding of NO to human heme oxygenase-1 (hHO-1), an enzyme that oxidizes heme to biliverdin, CO, and free iron, to determine whether inhibition of hHO-1 by NO can contribute to the signaling interplay of NO and CO. An Fe(3+)-NO hHO-1-heme complex is formed with NO or the NO donors NOC9 or 2-(N,N-diethylamino)-diazenolate-2-oxide.sodium salt. Resonance Raman spectroscopy shows that ferric hHO-1-heme forms a 6-coordinated, low spin complex with NO. The nu(N-O) vibration of this complex detected by Fourier transform IR is only 4 cm(-1) lower than that of the corresponding metmyoglobin (met-Mb) complex but is broader, suggesting a greater degree of ligand conformational freedom. The Fe(3+)-NO complex of hHO-1 is much more stable than that of met-Mb. Stopped-flow studies indicate that k(on) for formation of the hHO-1-heme Fe(3+)-NO complex is approximately 50-times faster, and k(off) 10 times slower, than for met-Mb, resulting in K(d) = 1.4 microm for NO. NO thus binds 500-fold more tightly to ferric hHO-1-heme than to met-Mb. The hHO-1 mutations E29A, G139A, D140A, S142A, G143A, G143F, and K179A/R183A do not significantly diminish the tight binding of NO, indicating that NO binding is not highly sensitive to mutations of residues that normally stabilize the distal water ligand. As expected from the K(d) value, the enzyme is reversibly inhibited upon exposure to pathologically, and possibly physiologically, relevant concentrations of NO. Inhibition of hHO-1 by NO may contribute to the pleiotropic responses to NO and CO.

Heme oxygenase as an intrinsic defense system in vascular wall: implication against atherogenesis

Recent developments in our understanding of the atherosclerotic process and factors that trigger ischemic cardiovascular disease have led to the consideration of antioxidative responses or exogenous antioxidants, which are proposed to inhibit multiple proatherogenic and prothrombotic events in arterial wall. Heme oxygenases (HO), an enzyme essential for heme degradation, have been shown to have such antioxidative properties via the production of bile pigments, carbon monoxide and ferritin induction. We have demonstrated that mildly oxidized LDL markedly induces HO-1, an inducible form of HO, in human aortic endothelial and smooth muscle cell cocultures and that its induction results in the attenuation of monocyte chemotaxis induced by mildly oxidized LDL. We also confirmed abundant expression of HO-1 in human, murine and rabbit atherosclerotic lesions. By modulating HO activities in LDL-receptor knockout mice and Watanabe heritable hyperlipidemic rabbits during their atherosclerotic lesion developments, anti-atherogenic properties of HO have demonstrated as judged by the quantitative analyses of atherosclerotic lesion formation. HO expression was inversely correlated with the levels of plasma and tissue lipid peroxides. HO also influenced on nitric oxide pathway. These observations may suggest that HO, induced during atherosclerotic process, functions as an intrinsic protective pathway in vascular wall.

Heme oxygenase: evolution, structure, and mechanism

Heme oxygenase has evolved to carry out the oxidative cleavage of heme, a reaction essential in physiological processes as diverse as iron reutilization and cellular signaling in mammals, synthesis of essential light-harvesting pigments in cyanobacteria and higher plants, and the acquisition of iron by bacterial pathogens. In all of these processes, heme oxygenase has evolved a similar structural and mechanistic scaffold to function within seemingly diverse physiological pathways. The heme oxygenase reaction is catalytically distinct from that of other hemoproteins such as the cytochromes P450, peroxidases, and catalases, but shares a hemoprotein scaffold that has evolved to generate a distinct activated oxygen species. In the following review we discuss the evolution of the structural and functional properties of heme oxygenase in light of the recent crystal structures of the mammalian and bacterial enzymes.

Mechanism of heme degradation by heme oxygenase

Heme oxygenase catalyzes the three step-wise oxidation of hemin to alpha-biliverdin, via alpha-meso-hydroxyhemin, verdoheme, and ferric iron-biliverdin complex. This enzyme is a simple protein which does not have any prosthetic groups. However, heme and its two metabolites, alpha-meso-hydroxyhemin and verdoheme, combine with the enzyme and activate oxygen during the heme oxygenase reaction. In the conversion of hemin to alpha-meso-hydroxyhemin, the active species of oxygen is Fe-OOH, which self-hydroxylates heme to form alpha-meso-hydroxyhemin. This step determines the alpha-specificity of the reaction. For the formation of verdoheme and liberation of CO from alpha-meso-hydroxyhemin, oxygen and one reducing equivalent are both required. However, the ferrous iron of the alpha-meso-hydroxyheme is not involved in the oxygen activation and unactivated oxygen is reacted on the 'activated' heme edge of the porphyrin ring. For the conversion of verdoheme to the ferric iron-biliverdin complex, both oxygen and reducing agents are necessary, although the precise mechanism has not been clear. The reduction of iron is required for the release of iron from the ferric iron-biliverdin complex to complete total heme oxygenase reaction.

Histidine 20, the crucial proximal axial heme ligand of bacterial heme oxygenase Hmu O from Corynebacterium diphtheriae

The hemin complex of Hmu O, a 24-kDa soluble heme degradation enzyme in Corynebacterium diphtheriae, is coordinated axially to a neutral imidazole of a proximal histidine residue in Hmu O. To identify which of the eight histidines in Hmu O is the proximal heme ligand, we have constructed and expressed the plasmids for eight His --> Ala Hmu O mutants. Reconstituted with hemin, the active site structures and enzymatic activity of these mutants have been examined by EPR, resonance Raman, and optical absorption spectroscopy. EPR of the NO-bound ferrous heme-Hmu O mutant complexes reveals His(20) as the proximal heme ligand in Hmu O, and this is confirmed by resonance Raman results from the ligand-free ferrous heme-H20A. All eight His --> Ala mutants bind hemin stoichiometrically, proving that none of the histidines is essential for hemin-Hmu O formation. However, His(20) is crucial to Hmu O catalysis. Its absence by point mutation has inhibited the conversion of hemin to biliverdin. The ferric heme-H20A complex is pentacoordinate. Resonance Raman of the CO-bound ferrous heme-H20A corroborates this and reveals an Fe-C-O bending mode, delta(Fe-C-O), the first reported for a pentacoordinate CO-bound hemeprotein. The appearance of delta(Fe-C-O) in C. diphtheriae Hmu O H20A but not mammalian HO-1 mutant H25A indicates that the heme environment between the two heme oxygenases is different.

The heme complex of Hmu O, a bacterial heme degradation enzyme from Corynebacterium diphtheriae. Structure of the catalytic site

Hmu O, a heme degradation enzyme in Corynebacterium diphtheriae, forms a stoichiometric complex with iron protoporphyrin IX and catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. Using a multitude of spectroscopic techniques, we have determined the axial ligand coordination of the heme-Hmu O complex. The ferric complex shows a pH-dependent reversible transition between a water-bound hexacoordinate high spin neutral pH form and an alkaline form, having high spin and low spin states, with a pK(a) of 9. (1)H NMR, EPR, and resonance Raman of the heme-Hmu O complex establish that a neutral imidazole of a histidine residue is the proximal ligand of the complex, similar to mammalian heme oxygenase. EPR of the deoxy cobalt porphyrin IX-Hmu O complex confirms this proximal histidine coordination. Oxy cobalt-Hmu O EPR reveals a hydrogen-bonding interaction between the O(2) and an exchangeable proton in the Hmu O distal pocket and two distinct orientations for the bound O(2). Mammalian heme oxygenase has only one O(2) orientation. This difference and the mixed spin states at alkaline pH indicate structural differences in the distal environment between Hmu O and its mammalian counterpart.

Heme degradation as catalyzed by a recombinant bacterial heme oxygenase (Hmu O) from Corynebacterium diphtheriae

Hmu O, a heme degradation enzyme in the pathogen Corynebacterium diphtheriae, catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. A bacterial expression system using a synthetic gene coding for the 215-amino acid, full-length Hmu O has been constructed. Expressed at very high levels in Escherichia coli BL21, the enzyme binds hemin stoichiometrically to form a hexacoordinate high spin hemin-Hmu O complex. When ascorbic acid is used as the electron donor, Hmu O converts hemin to biliverdin with alpha-hydroxyhemin and verdoheme as intermediates. The overall conversion rate to biliverdin is approximately 4-fold slower than that by rat heme oxygenase (HO) isoform 1. Reaction of the hemin-Hmu O complex with hydrogen peroxide yields a verdoheme species, the recovery of which is much less compared with rat HO-1. Reaction of the hemin complex with meta-chloroperbenzoic acid generates a ferryl oxo species. Thus, the catalytic intermediate species and the nature of the active form in the first oxygenation step of Hmu O appear to be similar to those of the mammalian HO. However, the considerably slow catalytic rate and low level of verdoheme recovery in the hydrogen peroxide reaction suggest that the active-site structure of Hmu O is different from that of its mammalian counterpart.

Identification of histidine 45 as the axial heme iron ligand of heme oxygenase-2

A truncated, soluble, and enzymatically active form of human heme oxygenase-2 (DeltaHHO2) was expressed in Escherichia coli. To identify the axial heme ligand of HO-2, His-45 to Ala (DeltaH45A) and His-152 to Ala (DeltaH152A) mutants have been prepared using this expression system. DeltaH45A could form a 1:1 complex with hemin but was completely devoid of the heme degradation activity. A 5-coordinate-type ferrous NO EPR spectrum was observed for the heme-DeltaH45A complex. The DeltaH152A mutant was expressed as an inclusion body and was recovered from the lysis pellet by dissolution in urea followed by dialysis. The solubilized fraction obtained, however, was composed of a mixture of a functional enzyme and an inactive fraction. The inactive fraction was removed by Sephadex G-75 column chromatography since it eluted out of the column at the void volume. The gel filtration-purified DeltaH152A exhibited spectroscopic and enzymatic properties identical to those of wild-type. We conclude, in contrast to the previous reports (McCoubrey and Maines (1993) Arch. Biochem. Biophys. 302, 402-408; McCoubrey, W. K., Jr., Huang, T. J., and Maines, M. (1997) J. Biol. Chem. 272, 12568-12574), that His-45, but not His-152, in heme oxygenase isoform-2 is the proximal ligand of the heme and is essential for the heme degradation activity of the enzyme. His-152 appears to play a structural role in stabilization of the heme oxygenase protein.