The crystal structures of the ferric and ferrous forms of the heme complex of HmuO, a heme oxygenase of Corynebacterium diphtheriae

Enzymological and structural characterization of Arabidopsis thaliana heme oxygenase-1

Arabidopsis thaliana heme oxygenase‐1 (AtHO‐1), a metabolic enzyme in the heme degradation pathway, serves as a prototype for study of the bilin‐related functions in plants. Past biological analyses revealed that AtHO‐1 requires ferredoxin‐NADP⁺ reductase (FNR) and ferredoxin for its enzymatic activity. Here, we characterized the binding and degradation of heme by AtHO‐1, and found that ferredoxin is a dispensable component of the reducing system that provides electrons for heme oxidation. Furthermore, we reported the crystal structure of heme‐bound AtHO‐1, which demonstrates both conserved and previously undescribed features of plant heme oxygenases. Finally, the electron transfer pathway from FNR to AtHO‐1 is suggested based on the known structural information.

Hydrogen Sulfide and Carbon Monoxide Tolerance in Bacteria

Hydrogen sulfide and carbon monoxide share the ability to be beneficial or harmful molecules depending on the concentrations to which organisms are exposed. Interestingly, humans and some bacteria produce small amounts of these compounds. Since several publications have summarized the recent knowledge of its effects in humans, here we have chosen to focus on the role of H₂S and CO on microbial physiology. We briefly review the current knowledge on how bacteria produce and use H₂S and CO. We address their potential antimicrobial properties when used at higher concentrations, and describe how microbial systems detect and survive toxic levels of H₂S and CO. Finally, we highlight their antimicrobial properties against human pathogens when endogenously produced by the host and when released by external chemical donors.

Crystal structure of higher plant heme oxygenase-1 and its mechanism of interaction with ferredoxin

Heme oxygenase (HO) converts heme to carbon monoxide, biliverdin, and free iron, products that are essential in cellular redox signaling and iron recycling. In higher plants, HO is also involved in the biosynthesis of photoreceptor pigment precursors. Despite many common enzymatic reactions, the amino acid sequence identity between plant-type and other HOs is exceptionally low (∼19.5%), and amino acids that are catalytically important in mammalian HO are not conserved in plant-type HOs. Structural characterization of plant-type HO is limited to spectroscopic characterization by electron spin resonance, and it remains unclear how the structure of plant-type HO differs from that of other HOs. Here, we have solved the crystal structure of Glycine max (soybean) HO-1 (GmHO-1) at a resolution of 1.06 Å and carried out the isothermal titration calorimetry measurements and NMR spectroscopic studies of its interaction with ferredoxin, the plant-specific electron donor. The high-resolution X-ray structure of GmHO-1 reveals several novel structural components: an additional irregularly structured region, a new water tunnel from the active site to the surface, and a hydrogen-bonding network unique to plant-type HOs. Structurally important features in other HOs, such as His ligation to the bound heme, are conserved in GmHO-1. Based on combined data from X-ray crystallography, isothermal titration calorimetry, and NMR measurements, we propose the evolutionary fine-tuning of plant-type HOs for ferredoxin dependency in order to allow adaptation to dynamic pH changes on the stroma side of the thylakoid membrane in chloroplast without losing enzymatic activity under conditions of fluctuating light.

Unique Electronic Structures of the Highly Ruffled Hemes in Heme-Degrading Enzymes of Staphylococcus aureus, IsdG and IsdI, by Resonance Raman and Electron Paramagnetic Resonance Spectroscopies

Staphylococcus aureus uses IsdG and IsdI to convert heme into a mixture of staphylobilin isomers, 15-oxo-β-bilirubin and 5-oxo-δ-bilirubin, formaldehyde, and iron. The highly ruffled heme found in the heme-IsdI and IsdG complexes has been proposed to be responsible for the unique heme degradation products. We employed resonance Raman (RR) and electron paramagnetic resonance (EPR) spectroscopies to examine the coordination and electronic structures of heme bound to IsdG and IsdI. Heme complexed to IsdG and IsdI is coordinated by a neutral histidine. The trans ligand is hydroxide in the ferric alkaline form of both proteins. In the ferric neutral form at pH 6.0, heme is six-coordinated with water as the sixth ligand for IsdG and is in the mixture of the five-coordinated and six-coordinated species for IsdI. In the ferrous CO-bound form, CO is strongly hydrogen bonded with a distal residue. The marker lines, ν₂ and ν₃, appear at frequencies that are distinct from other proteins having planar hemes. The EPR spectra for the ferric hydroxide and cyanide states might be explained by assuming the thermal mixing of the d-electron configurations, (d_xy)²(d_xz,d_yz)³ and (d_xz,d_yz)⁴(d_xy)¹. The fraction for the latter becomes larger for the ferric cyanide form. In the ferric neutral state at pH 6.0, the quantum mechanical mixing of the high and intermediate spin configurations might explain the peculiar frequencies of ν₂ and ν₃ in the RR spectra. The heme ruffling imposed by IsdG and IsdI gives rise to unique electronic structures of heme, which are expected to modulate the first and subsequent steps of the heme oxygenation.

Recent Advances in the Understanding of the Reaction Chemistries of the Heme Catabolizing Enzymes HO and BVR Based on High Resolution Protein Structures

In mammals, catabolism of the heme group is indispensable for life. Heme is first cleaved by the enzyme Heme Oxygenase (HO) to the linear tetrapyrrole Biliverdin IXα (BV), and BV is then converted into bilirubin by Biliverdin Reductase (BVR). HO utilizes three Oxygen molecules (O2) and seven electrons supplied by NADPH-cytochrome P450 oxidoreductase (CPR) to open the heme ring and BVR reduces BV through the use of NAD(P)H. Structural studies of HOs, including substrate-bound, reaction intermediate-bound, and several specific inhibitor-bound forms, reveal details explaining substrate binding to HO and mechanisms underlying-specific HO reaction progression. Cryo-trapped structures and a time-resolved spectroscopic study examining photolysis of the bond between the distal ligand and heme iron demonstrate how CO, produced during the HO reaction, dissociates from the reaction site with a corresponding conformational change in HO. The complex structure containing HO and CPR provides details of how electrons are transferred to the heme-HO complex. Although the tertiary structure of BVR and its complex with NAD+ was determined more than 10 years ago, the catalytic residues and the reaction mechanism of BVR remain unknown. A recent crystallographic study examining cyanobacterial BVR in complex with NADP+ and substrate BV provided some clarification regarding these issues. Two BV molecules are bound to BVR in a stacked manner, and one BV may assist in the reductive catalysis of the other BV. In this review, recent advances illustrated by biochemical, spectroscopic, and crystallographic studies detailing the chemistry underlying the molecular mechanism of HO and BVR reactions are presented.

Coculturing of Mosquito-Microbiome Bacteria Promotes Heme Degradation in Elizabethkingia anophelis

Anopheles mosquito microbiomes are intriguing ecological niches. Within the gut, microbes adapt to oxidative stress due to heme and iron after blood meals. Although metagenomic sequencing has illuminated spatial and temporal fluxes of microbiome populations, limited data exist on microbial growth dynamics. Here, we analyze growth interactions between a dominant microbiome species, Elizabethkingia anophelis, and other Anopheles-associated bacteria. We find E. anophelis inhibits a Pseudomonas sp. via an antimicrobial-independent mechanism and observe biliverdins, heme degradation products, upregulated in cocultures. Purification and characterization of E. anophelis HemS demonstrates heme degradation, and we observe hemS expression is upregulated when cocultured with Pseudomonas sp. This study reveals a competitive microbial interaction between mosquito-associated bacteria and characterizes the stimulation of heme degradation in E. anophelis when grown with Pseudomonas sp.

A single mutation converts Alr5027 from cyanobacteria Nostoc sp. PCC 7120 to a heme-binding protein with heme-degrading ability

HutZ from Vibrio cholerae (VcHutZ) is a dimeric protein that catalyzes oxygen-dependent degradation of heme. The reaction mechanism is the same as that of canonical heme oxygenase (HO), but the structure of HutZ is quite different from that of HO. Thus, we postulate that HutZ has evolved via a different pathway from that of HO. The Alr5027 protein from cyanobacteria possessing proteins potentially related to ancestral proteins utilizing O₂ in enzymatic reactions is homologous to HutZ family proteins (67% similarity), but the heme axial ligand of HutZ is not conserved in Alr5027. To investigate whether Alr5027 can bind and degrade heme, we expressed Alr5027 in Escherichia coli and purified it. Although Alr5027 did not bind heme, replacement of Lys164, corresponding to the heme axial ligand of HutZ, with histidine conferred heme-binding capability. The K164H mutant produced verdoheme in the reaction with H₂O₂, indicating acquisition of heme-degradation ability. Among the mutants, the K164H mutant produced verdoheme most efficiently. Although the K164H mutant did not degrade heme through ascorbic acid, biliverdin, the final product of VcHutZ, was formed by treatment of verdoheme with ascorbic acid. An analysis of Trp103 fluorescence indicated elongation of the distance between protomers in this mutant compared with VcHutZ-the probable cause of the inefficiency of ascorbic acid-supported heme-degradation activity. Collectively, our findings indicate that a single lysine-to-histidine mutation converted Alr5027 to a heme-binding protein that can form verdoheme through H₂O₂, suggesting that HutZ family proteins have acquired the heme-degradation function through molecular evolution from an ancestor protein of Alr5027.

A Single Mutation in the Mycobacterium tuberculosis Heme-Degrading Protein, MhuD, Results in Different Products

Mycobacterium tuberculosis heme-degrading protein MhuD degrades heme to mycobilin isomers and iron, while its closest homologues from Staphylococcus aureus, IsdG and IsdI, degrade heme to staphylobilin isomers, formaldehyde, and iron. Superposition of the structures of the heme-bound complexes reveals that the heme molecule in the MhuD active site is rotated ∼90° about the tetrapyrrole plane with respect to IsdG and IsdI active site heme molecules. Therefore, the variation in IsdG/IsdI and MhuD chromophore products may be attributed to the different heme orientations. In MhuD, two arginines, Arg22 and Arg26, stabilize the heme propionates and may account for the heme orientation. Herein, we demonstrate that the MhuD-R26S variant alters the resulting chromophore product from mycobilin to biliverdin IXα (α-BV), whereas the R22S variant does not. Surprisingly, unlike canonical heme oxygenase (HO) that also degrades heme to α-BV, the MhuD-R26S variant produces the C1 product formaldehyde rather than carbon monoxide as observed for HO. The MhuD-R26S variant is an important tool for further probing the mechanism of action of MhuD and for studying the fate of the MhuD product in mycobacterium.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

From Host Heme To Iron: The Expanding Spectrum of Heme Degrading Enzymes Used by Pathogenic Bacteria

Iron is an essential nutrient for many bacteria. Since the metal is highly sequestered in host tissues, bound predominantly to heme, pathogenic bacteria often take advantage of heme uptake and degradation mechanisms to acquire iron during infection. The most common mechanism of releasing iron from heme is through oxidative degradation by heme oxygenases (HOs). In addition, an increasing number of proteins that belong to two distinct structural families have been implicated in aerobic heme catabolism. Finally, an enzyme that degrades heme anaerobically was recently uncovered, further expanding the mechanisms for bacterial heme degradation. In this analysis, we cover the spectrum and recent advances in heme degradation by infectious bacteria. We briefly explain heme oxidation by the two groups of recognized HOs to ground readers before focusing on two new types of proteins that are reported to be involved in utilization of heme iron. We discuss the structure and enzymatic function of proteins representing these groups, their biological context, and how they are regulated to provide a more complete look at their cellular role.

Haem oxygenase: A functionally diverse enzyme of photosynthetic organisms and its role in phytochrome chromophore biosynthesis, cellular signalling and defence mechanisms

Haem oxygenase (HO) is a universal enzyme that catalyses stereospecific cleavage of haem to BV IX α and liberates Fe⁺² ion and CO as by-product. Beside haem degradation, it has important functions in plants that include cellular defence, stomatal regulation, iron mobilization, phytochrome chromophore synthesis, and lateral root formation. Phytochromes are an extended family of photoreceptors with a molecular mass of 250 kDa and occur as a dimer made up of 2 equivalent subunits of 125 kDa each. Each subunit is made of two components: the chromophore, a light-capturing pigment molecule and the apoprotein. Biosynthesis of phytochrome (phy) chromophore includes the oxidative splitting of haem to biliverdin IX by an enzyme HO, which is the decisive step in the biosynthesis. In photosynthetic organisms, BVα is reduced to 3Z PΦB by a ferredoxin-dependent PΦB synthase that finally isomerised to PΦB. The synthesized PΦB assembles with the phytochrome apoprotein in the cytoplasm to generate holophytochrome. Thus, necessary for photomorphogenesis in plants, which has confirmed from the genetic studies, conducted on Arabidopsis thaliana and pea. Besides the phytochrome chromophore synthesis, the review also emphasises on the current advances conducted in plant HO implying its developmental and defensive role.

Function Coupling Mechanism of PhuS and HemO in Heme Degradation

Most bacteria possess only one heme-degrading enzyme for obtaining iron, however few bacteria such as Pseudomonas aeruginosa express two, namely PhuS and HemO. While HemO is a well-known heme oxygenase, previously we discovered that PhuS also possesses heme degradation activity and generates verdoheme, an intermediate of heme breakdown. To understand the coexistence of these two enzymes, using the DFT calculation we reveal that PhuS effectively enhances heme degradation through its participation in heme hydroxylation, the rate limiting reaction. Heme is converted to verdoheme in this reaction and the energy barrier for PhuS is substantially lower than for HemO. Thus, HemO is mainly involved in the ring opening reaction which converts verdoheme to biliverdin and free iron. Our kinetics experiments show that, in the presence of both PhuS and HemO, complete degradation of heme to biliverdin is enhanced. We further show that PhuS is more active than HemO using heme as a substrate and generates more CO. Combined experimental and theoretical results directly identify function coupling of this two-enzyme system, resulting in more efficient heme breakdown and utilization.

Chlamydomonas reinhardtii LFO1 Is an IsdG Family Heme Oxygenase

Heme is essential for respiration across all domains of life. However, heme accumulation can lead to toxicity if cells are unable to either degrade or export heme or its toxic by-products. Under aerobic conditions, heme degradation is performed by heme oxygenases, enzymes which utilize oxygen to cleave the tetrapyrrole ring of heme. The HO-1 family of heme oxygenases has been identified in both bacterial and eukaryotic cells, whereas the IsdG family has thus far been described only in bacteria. We identified a hypothetical protein in the eukaryotic green alga Chlamydomonas reinhardtii, which encodes a protein containing an antibiotic biosynthesis monooxygenase (ABM) domain consistent with those associated with IsdG family members. This protein, which we have named LFO1, degrades heme, contains similarities in predicted secondary structures to IsdG family members, and retains the functionally conserved catalytic residues found in all IsdG family heme oxygenases. These data establish LFO1 as an IsdG family member and extend our knowledge of the distribution of IsdG family members beyond bacteria. To gain further insight into the distribution of the IsdG family, we used the LFO1 sequence to identify 866 IsdG family members, including representatives from all domains of life. These results indicate that the distribution of IsdG family heme oxygenases is more expansive than previously appreciated, underscoring the broad relevance of this enzyme family. IMPORTANCE This work establishes a protein in the freshwater alga Chlamydomonas reinhardtii as an IsdG family heme oxygenase. This protein, LFO1, exhibits predicted secondary structure and catalytic residues conserved in IsdG family members, in addition to a chloroplast localization sequence. Additionally, the catabolite that results from the degradation of heme by LFO1 is distinct from that of other heme degradation products. Using LFO1 as a seed, we performed phylogenetic analysis, revealing that the IsdG family is conserved in all domains of life. Additionally, C. reinhardtii contains two previously identified HO-1 family heme oxygenases, making C. reinhardtii the first organism shown to contain two families of heme oxygenases. These data indicate that C. reinhardtii may have unique mechanisms for regulating iron homeostasis within the chloroplast.

Structural and mutational analyses of the Leptospira interrogans virulence-related heme oxygenase provide insights into its catalytic mechanism

Heme oxygenase from Leptospira interrogans is an important virulence factor. During catalysis, redox equivalents are provided to this enzyme by the plastidic-type ferredoxin-NADP⁺ reductase also found in L. interrogans. This process may have evolved to aid this bacterial pathogen to obtain heme-iron from their host and enable successful colonization. Herein we report the crystal structure of the heme oxygenase-heme complex at 1.73 Å resolution. The structure reveals several distinctive features related to its function. A hydrogen bonded network of structural water molecules that extends from the catalytic site to the protein surface was cleared observed. A depression on the surface appears to be the H⁺ network entrance from the aqueous environment to the catalytic site for O₂ activation, a key step in the heme oxygenase reaction. We have performed a mutational analysis of the F157, located at the above-mentioned depression. The mutant enzymes were unable to carry out the complete degradation of heme to biliverdin since the reaction was arrested at the verdoheme stage. We also observed that the stability of the oxyferrous complex, the efficiency of heme hydroxylation and the subsequent conversion to verdoheme was adversely affected. These findings underscore a long-range communication between the outer fringes of the hydrogen-bonded network of structural waters and the heme active site during catalysis. Finally, by analyzing the crystal structures of ferredoxin-NADP⁺ reductase and heme oxygenase, we propose a model for the productive association of these proteins.

A Ferric-Peroxo Intermediate in the Oxidation of Heme by IsdI

The canonical heme oxygenases (HOs) catalyze heme oxidation via a heme-bound hydroperoxo intermediate that is stabilized by a water cluster at the active site of the enzyme. In contrast, the hydrophobic active site of IsdI, a heme-degrading enzyme from Staphylococcus aureus, lacks a water cluster and is expected to oxidize heme by an alternative mechanism. Reaction of the IsdI-heme complex with either H2O2 or m-chloroperoxybenzoic acid fails to produce a specific oxidized heme iron intermediate, suggesting that ferric-hydroperoxo or ferryl derivatives of IsdI are not involved in the catalytic mechanism of this enzyme. IsdI lacks a proton-donating group in the distal heme pocket, so the possible involvement of a ferric-peroxo intermediate has been evaluated. Density functional theory (DFT) calculations indicate that heme oxidation involving a ferric-peroxo intermediate is energetically accessible, whereas the energy barrier for a reaction involving a ferric-hydroperoxo intermediate is too great in the absence of a proton donor. We propose that IsdI catalyzes heme oxidation through nucleophilic attack by the heme-bound peroxo species. This proposal is consistent with our previous demonstration by nuclear magnetic resonance spectroscopy that heme ruffling increases the susceptibility of the meso-carbon of heme to nucleophilic attack.

Heme utilization by pathogenic bacteria: not all pathways lead to biliverdin

The eukaryotic heme oxygenases (HOs) (E.C. 1.14.99.3) convert heme to biliverdin, iron, and carbon monoxide (CO) in three successive oxygenation steps. Pathogenic bacteria require iron for survival and infection. Extracellular heme uptake from the host plays a critical role in iron acquisition and virulence. In the past decade, several HOs required for the release of iron from extracellular heme have been identified in pathogenic bacteria, including Corynebacterium diphtheriae, Neisseriae meningitides, and Pseudomonas aeruginosa. The bacterial enzymes were shown to be structurally and mechanistically similar to those of the canonical eukaryotic HO enzymes. However, the recent discovery of the structurally and mechanistically distinct noncanonical heme oxygenases of Staphylococcus aureus and Mycobacterium tuberculosis has expanded the reaction manifold of heme degradation. The distinct ferredoxin-like structural fold and extreme heme ruffling are proposed to give rise to the alternate heme degradation products in the S. aureus and M. tuberculosis enzymes. In addition, several “heme-degrading factors” with no structural homology to either class of HOs have recently been reported. The identification of these “heme-degrading proteins” has largely been determined on the basis of in vitro heme degradation assays. Many of these proteins were reported to produce biliverdin, although no extensive characterization of the products was performed. Prior to the characterization of the canonical HO enzymes, the nonenzymatic degradation of heme and heme proteins in the presence of a reductant such as ascorbate or hydrazine, a reaction termed “coupled oxidation”, served as a model for biological heme degradation. However, it was recognized that there were important mechanistic differences between the so-called coupled oxidation of heme proteins and enzymatic heme oxygenation. In the coupled oxidation reaction, the final product, verdoheme, can readily be converted to biliverdin under hydrolytic conditions. The differences between heme oxygenation by the canonical and noncanonical HOs and coupled oxidation will be discussed in the context of the stabilization of the reactive Fe^III–OOH intermediate and regioselective heme hydroxylation. Thus, in the determination of heme oxygenase activity in vitro, it is important to ensure that the reaction proceeds through successive oxygenation steps. We further suggest that when bacterial heme degradation is being characterized, a systems biology approach combining genetics, mechanistic enzymology, and metabolite profiling should be undertaken.

Crystallographic and spectroscopic insights into heme degradation by Mycobacterium tuberculosis MhuD

Mycobacterium heme utilization degrader (MhuD) is a heme-degrading protein from Mycobacterium tuberculosis responsible for extracting the essential nutrient iron from host-derived heme. MhuD has been previously shown to produce unique organic products compared to those of canonical heme oxygenases (HOs) as well as those of the IsdG/I heme-degrading enzymes from Staphylococcus aureus. Here, we report the X-ray crystal structure of cyanide-inhibited MhuD (MhuD-heme-CN) as well as detailed (1)H nuclear magnetic resonance (NMR), UV/vis absorption, and magnetic circular dichroism (MCD) spectroscopic characterization of this species. There is no evidence for an ordered network of water molecules on the distal side of the heme substrate in the X-ray crystal structure, as was previously reported for canonical HOs. The degree of heme ruffling in the crystal structure of MhuD is greater than that observed for HO and less than that observed for IsdI. As a consequence, the Fe 3dxz-, 3dyz-, and 3dxy-based MOs are very close in energy, and the room-temperature (1)H NMR spectrum of MhuD-heme-CN is consistent with population of both a (2)Eg electronic state with a (dxy)(2)(dxz,dyz)(3) electron configuration, similar to the ground state of canonical HOs, and a (2)B2g state with a (dxz,dyz)(4)(dxy)(1) electron configuration, similar to the ground state of cyanide-inhibited IsdI. Variable temperature, variable field MCD saturation magnetization data establishes that MhuD-heme-CN has a (2)B2g electronic ground state with a low-lying (2)Eg excited state. Our crystallographic and spectroscopic data suggest that there are both structural and electronic contributions to the α-meso regioselectivity of MhuD-catalyzed heme cleavage. The structural distortion of the heme substrate observed in the X-ray crystal structure of MhuD-heme-CN is likely to favor cleavage at the α- and γ-meso carbons, whereas the spin density distribution may favor selective oxygenation of the α-meso carbon.

Heme oxygenation and the widening paradigm of heme degradation

Heme degradation through the action of heme oxygenase (HO) is unusual in that it utilizes heme as both a substrate and cofactor for its own degradation. HO catalyzes the oxygen-dependent degradation of heme to biliverdin with the release of CO and "free" iron. The characterization of HO enzymes from humans to bacteria reveals a similar overall structural fold that contributes to the unique reaction manifold. The heme oxygenases share a similar heme-dependent activation of O2 to the ferric hydroperoxide as that of the cytochrome P450s and peroxidases. However, whereas the P450s promote cleavage of the ferric hydroperoxide OO bond to the oxoferryl species the HOs stabilize the ferric hydroperoxide promoting hydroxylation at the heme edge. The alternate reaction pathway in HO is achieved through the conformational flexibility and extensive hydrogen bond network within the heme binding site priming the heme for hydroxylation. Until recently it was believed that all heme degrading enzymes converted heme to biliverdin and iron, with the release of carbon monoxide (CO). However, the recent discovery of the bacterial IsdG-like heme degrading proteins of Staphylococcus aureus, Bacillus anthracis and Mycobacterium tuberculosis has expanded the reaction manifold of heme oxidation. Characterization of the heme degradation products in the IsdG-like reaction suggests a mechanism distinct from the classical HOs. In the following review we will discuss the structure-function of the canonical HOs as it relates to the emerging alternate reaction manifold of the IsdG-like proteins.

Structures of the substrate-free and product-bound forms of HmuO, a heme oxygenase from corynebacterium diphtheriae: x-ray crystallography and molecular dynamics investigation

Heme oxygenase catalyzes the degradation of heme to biliverdin, iron, and carbon monoxide. Here, we present crystal structures of the substrate-free, Fe(3+)-biliverdin-bound, and biliverdin-bound forms of HmuO, a heme oxygenase from Corynebacterium diphtheriae, refined to 1.80, 1.90, and 1.85 Å resolution, respectively. In the substrate-free structure, the proximal and distal helices, which tightly bracket the substrate heme in the substrate-bound heme complex, move apart, and the proximal helix is partially unwound. These features are supported by the molecular dynamic simulations. The structure implies that the heme binding fixes the enzyme active site structure, including the water hydrogen bond network critical for heme degradation. The biliverdin groups assume the helical conformation and are located in the heme pocket in the crystal structures of the Fe(3+)-biliverdin-bound and the biliverdin-bound HmuO, prepared by in situ heme oxygenase reaction from the heme complex crystals. The proximal His serves as the Fe(3+)-biliverdin axial ligand in the former complex and forms a hydrogen bond through a bridging water molecule with the biliverdin pyrrole nitrogen atoms in the latter complex. In both structures, salt bridges between one of the biliverdin propionate groups and the Arg and Lys residues further stabilize biliverdin at the HmuO heme pocket. Additionally, the crystal structure of a mixture of two intermediates between the Fe(3+)-biliverdin and biliverdin complexes has been determined at 1.70 Å resolution, implying a possible route for iron exit.

Heme uptake and metabolism in bacteria

All but a few bacterial species have an absolute need for heme, and most are able to synthesize it via a pathway that is highly conserved among all life domains. Because heme is a rich source for iron, many pathogenic bacteria have also evolved processes for sequestering heme from their hosts. The heme biosynthesis pathways are well understood at the genetic and structural biology levels. In comparison, much less is known about the heme acquisition, trafficking, and degradation processes in bacteria. Gram-positive and Gram-negative bacteria have evolved similar strategies but different tactics for importing and degrading heme, likely as a consequence of their different cellular architectures. The differences are manifested in distinct structures for molecules that perform similar functions. Consequently, the aim of this chapter is to provide an overview of the structural biology of proteins and protein-protein interactions that enable Gram-positive and Gram-negative bacteria to sequester heme from the extracellular milieu, import it to the cytosol, and degrade it to mine iron.

Crystal structure of HutZ, a heme storage protein from Vibrio cholerae: A structural mismatch observed in the region of high sequence conservation

Background

HutZ is the sole heme storage protein identified in the pathogenic bacterium Vibrio cholerae and is required for optimal heme utilization. However, no heme oxygenase activity has been observed with this protein. Thus far, HutZ’s structure and heme-binding mechanism are unknown.

Results

We report the first crystal structure of HutZ in a homodimer determined at 2.0 Å resolution. The HutZ structure adopted a typical split-barrel fold. Through a docking study and site-directed mutagenesis, a heme-binding model for the HutZ dimer is proposed. Very interestingly, structural superimposition of HutZ and its homologous protein HugZ, a heme oxygenase from Helicobacter pylori, exhibited a structural mismatch of one amino acid residue in β6 of HutZ, although residues involved in this region are highly conserved in both proteins. Derived homologous models of different single point variants with model evaluations suggested that Pro¹⁴⁰ of HutZ, corresponding to Phe²¹⁵ of HugZ, might have been the main contributor to the structural mismatch. This mismatch initiates more divergent structural characteristics towards their C-terminal regions, which are essential features for the heme-binding of HugZ as a heme oxygenase.

Conclusions

HutZ’s deficiency in heme oxygenase activity might derive from its residue shift relative to the heme oxygenase HugZ. This residue shift also emphasized a limitation of the traditional template selection criterion for homology modeling.

Crystallographic studies of heme oxygenase complexed with an unstable reaction intermediate, verdoheme

This article discusses the accuracy of X-ray structural studies of heme oxygenase (HO) in complex with an unstable intermediate, verdoheme. Heme degradation by HO proceeds through three successive steps of O(2) activation. The mechanism of the third step, the ring opening of verdoheme, has been the least understood. Recent structural studies of the verdoheme-HO complex provide detailed information concerning this mechanism. Due to X-ray-induced photoreduction and the instability of verdoheme, it has been difficult to obtain an accurate structure for the ferrous verdoheme-HO complex. Therefore, accurate structural studies, including analysis of the electronic state of the verdoheme-HO complex, are needed to elucidate the proper reaction mechanism.

Single turnover of substrate-bound ferric cysteine dioxygenase with superoxide anion: enzymatic reactivation, product formation, and a transient intermediate

Cysteine dioxygenase (CDO) is a non-heme mononuclear iron enzyme that catalyzes the O(2)-dependent oxidation of L-cysteine (Cys) to produce cysteine sulfinic acid (CSA). In this study we demonstrate that the catalytic cycle of CDO can be "primed" by one electron through chemical oxidation to produce CDO with ferric iron in the active site (Fe(III)-CDO, termed 2). While catalytically inactive, the substrate-bound form of Fe(III)-CDO (2a) is more amenable to interrogation by UV-vis and EPR spectroscopy than the 'as-isolated' Fe(II)-CDO enzyme (1). Chemical-rescue experiments were performed in which superoxide (O(2)(•-)) anions were introduced to 2a to explore the possibility that a Fe(III)-superoxide species represents the first intermediate within the catalytic pathway of CDO. In principle, O(2)(•-) can serve as a suitable acceptor for the remaining 3-electrons necessary for CSA formation and regeneration of the active Fe(II)-CDO enzyme (1). Indeed, addition of O(2)(•-) to 2a resulted in the rapid formation of a transient species (termed 3a) observable at 565 nm by UV-vis spectroscopy. The subsequent decay of 3a is kinetically matched to CSA formation. Moreover, a signal attributed to 3a was also identified using parallel mode X-band EPR spectroscopy (g ~ 11). Spectroscopic simulations, observed temperature dependence, and the microwave power saturation behavior of 3a are consistent with a ground state S = 3 from a ferromagnetically coupled (J ~ -8 cm(-1)) high-spin ferric iron (S(A) = 5/2) with a bound radical (S(B) = 1/2), presumably O(2)(•-). Following treatment with O(2)(•-), the specific activity of recovered CDO increased to ~60% relative to untreated enzyme.

Crystal structure and biochemical features of EfeB/YcdB from Escherichia coli O157: ASP235 plays divergent roles in different enzyme-catalyzed processes

EfeB/YcdB is a member of the dye-decolorizing peroxidase (DyP) protein family. A recent study has shown that this protein can extract iron from heme without breaking the tetrapyrrole ring. We report the crystal structure of EfeB from Escherichia coli O157 bound to heme at 1.95 Å resolution. The EfeB monomer contains two domains. The heme molecule is located in a large hydrophobic pocket in the C-terminal domain. A long loop connecting the two domains extensively interacts with the heme, which is a distinctive structural feature of EfeB homologues. A large tunnel formed by this loop and the β-sheet of C-terminal domain provides a potential cofactor/substrate binding site. Biochemical data show that the production of protoporphyrin IX (PPIX) is closely related to the peroxidation activity. The mutant D235N keeps nearly the same activity of guaiacol peroxidase as the wild-type protein, whereas the corresponding mutation in the classic DyP protein family completely abolished the peroxidation activity. These results suggest that EfeB is a unique member of the DyP protein family. In addition, dramatically enhanced fluorescence excitation and emission of EfeB-PPIX was observed, implying this protein may be used as a red color fluorescence marker.

Heme proteins--diversity in structural characteristics, function, and folding

The characteristics of heme prosthetic groups and their binding sites have been analyzed in detail in a data set of nonhomologous heme proteins. Variations in the shape, volume, and chemical composition of the binding site, in the mode of heme binding and in the number and nature of heme-protein interactions are found to result in significantly different heme environments in proteins with different functions in biology. Differences are also seen in the properties of the apo states of the proteins. The apo states of proteins that bind heme permanently in their functional form show some disorder, ranging from local unfolding in the heme binding pocket to complete unfolding to give a random coil. In contrast, proteins that bind heme transiently are fully folded in their apo and holo states, presumably allowing both apo and holo forms to remain biologically active resisting aggregation or proteolysis. The principles identified here provide a framework for the design of de novo proteins that will exhibit tight heme ligand binding and for the identification of the function of structural genomic target proteins with heme ligands.

Dioxygen activation for the self-degradation of heme: reaction mechanism and regulation of heme oxygenase

Heme oxygenase (HO) catalyzes the regiospecific conversion of heme to biliverdin, CO, and free iron through three successive oxygenation reactions. HO catalysis is unique in that all three O(2) activations are performed by the substrate itself. This Forum Article overviews our current understanding on the structural and biochemical properties of HO catalysis, especially its first and third oxygenation steps. The HO first step, regiospecific hydroxylation of the porphyrin alpha-meso-carbon atom, is of particular interest because of its sharp contrast to O(2) activation by cytochrome P450. HO was proposed to utilize the FeOOH species but not conventional ferryl hemes as a reactive intermediate for self-hydroxylation. We have succeeded in preparing and characterizing the FeOOH species of HO at low temperature, and our analyses of its reaction, together with mutational and crystallographic studies, reveal that protonation of FeOOH by a distal water molecule is critical in promoting the unique self-hydroxylation. The second oxygenation is a rapid, spontaneous autooxidation of the reactive alpha-meso-hydroxyheme in which the HO enzyme does not play a critical role. Further O(2) activation by verdoheme cleaves its porphyrin macrocycle to form biliverdin and free ferrous iron. This third step has been considered to be a major rate-determining step of HO catalysis to regulate the enzyme activity. Our reaction analysis strongly supports the FeOOH verdoheme as the key intermediate of the ring-opening reaction. This mechanism is very similar to that of the first meso-hydroxylation, and the distal water is suggested to enhance the third step as expected from the similarity. The HO mechanistic studies highlight the catalytic importance of the distal hydrogen-bonding network, and this manuscript also involves our attempts to develop HO inhibitors targeting the unique distal structure.

Solution 1H NMR characterization of substrate-free C. diphtheriae heme oxygenase: pertinence for determining magnetic axes in paramagnetic substrate complexes

Proton 2D NMR was used to confirm in solution a highly conserved portion of the molecular structure upon substrate loss for the heme oxygenase from the pathogenic bacterium Corynebacterium diphtheriae, HmuO. The chemical shifts for the conserved portion of the structure are assessed as references for the dipolar shifts needed to determine the orientation of the paramagnetic susceptibility tensor, chi, in paramagnetic substrate complexes of HmuO. It is shown that the chemical shifts for the structurally conserved portion of substrate-free HmuO serve as excellent references for residues with only small to moderate sized dipolar shifts in the cyanide-inhibited substrate complex of HmuO, yielding an orientation of chi that is essentially the same as conventionally obtained from large dipolar shifts based on empirical estimates of the diamagnetic reference. The implications of these diamagnetic chemical shifts for characterizing the hydrogen bonding in the physiologically relevant, resting-state, high-spin aquo complex are discussed. The pattern of labile proton exchange in the distal H-bond network of substrate-free HmuO allowed comparison of changes in dynamic stability of tertiary contacts in the substrate-free and substrate-bound HmuO and with the same complexes of human heme oxygenase.

Haem oxygenase (HO): an overlooked enzyme of plant metabolism and defence

Haem oxygenase (HO) degrades free haem released from haem proteins with the generation of ferrous iron (Fe2+), biliverdin-IXalpha (BV-IXalpha), and carbon monoxide (CO). The mechanism of haem cleavage has been conserved between plants and other organisms even though the function, subcellular localization, and cofactor requirements of HO differ substantially. The crystal structure of HO1, a monomeric protein, has been extensively reported in mammals, pathogenic bacteria, and cyanobacteria, but no such reports are available for higher plant HOs except a predicted model for pea HO1. Along with haem degradation, HO performs various cellular processes including iron acquisition/mobilization, phytochrome chromophore synthesis, cell protection, and stomatal regulation. To date, four HO genes (HO1, HO2, HO3, and HO4) have been reported in plants. HO1 has been well explored in cell metabolism; however, the divergent roles of the other three HOs is less known. The transcriptional up-regulation of HO1 in plants responds to many agents, such as light, UV, iron deprivation, reactive oxygen species (ROS), abscisic acid (ABA), and haematin. Recently the HO1/CO system has gained more attention due to its physiological cytoprotective role in plants. This review focuses on the recent advances made in plant HO research involving its role in environmental stresses. Moreover, the review emphasizes physiological, biochemical, and molecular aspects of this enzyme in plants.

How active-site protonation state influences the reactivity and ligation of the heme in chlorite dismutase

Chlorite dismutase catalyzes O(2) release from chlorite with exquisite efficiency and specificity. The spectroscopic properties, ligand binding affinities, and steady-state kinetics of chlorite dismutase from Dechloromonas aromatica were examined over pH 3-11.5 to gain insight into how the protonation state of the heme environment influences dioxygen formation. An acid-base transition was observed by UV/visible and resonance Raman (rR) spectroscopy with a pK(a) of 8.7, 2-3 pH units below analogous transitions observed in typical His-ligated peroxidases. This transition marks the conversion of a five-coordinate high-spin Fe(III) to a mixed high/low-spin ferric hydroxide, as confirmed by rR spectroscopy. The two Fe-OH stretching frequencies are quite low, consistent with a weak Fe-OH bond, despite the nearly neutral imidazole side chain of the proximal histidine ligand. The hydroxide is proposed to interact strongly with a distal H-bond donor, thereby weakening the Fe-OH bond. The rR spectra of Cld-CO as a function of pH reveal two forms of the complex, one in which there is minimal interaction of distal residues with the carbonyl oxygen and another, acidic form in which the oxygen is under the influence of positive charge. Recent crystallographic data reveal arginine 183 as the lone H-bond-donating residue in the distal pocket. It is likely that this Arg is the strong, positively charged H-bond donor implicated by vibrational data to interact with exogenous axial heme ligands. The same Arg in its neutral (pK(a) approximately 6.5) form also appears to act as the active-site base in binding reactions of protonated ligands, such as HCN, to ferric Cld. The steady-state profile for the rate of chlorite decomposition is characterized by these same pK(a) values. The five-coordinate high-spin acidic Cld is more active than the alkaline hydroxide-bound form. The acid form decomposes chlorite most efficiently when the distal Arg is protonated/cationic (maximum k(cat) = 2.0(+/-0.6) x 10(5) s(-1), k(cat)/K(M) = 3.2(+/-0.4) x 10(7) M(-1) s(-1), pH 5.2, 4 degrees C) and to a somewhat lesser extent when it acts as a H-bond donor to the axial hydroxide ligand under alkaline conditions.

Heme oxygenase reveals its strategy for catalyzing three successive oxygenation reactions

Heme oxygenase (HO) is an enzyme that catalyzes the regiospecific conversion of heme to biliverdin IXalpha, CO, and free iron. In mammals, HO has a variety of physiological functions, including heme catabolism, iron homeostasis, antioxidant defense, cellular signaling, and O(2) sensing. The enzyme is also found in plants (producing light-harvesting pigments) and in some pathogenic bacteria, where it acquires iron from the host heme. The HO-catalyzed heme conversion proceeds through three successive oxygenations, a process that has attracted considerable attention because of its reaction mechanism and physiological importance. The HO reaction is unique in that all three O(2) activations are affected by the substrate itself. The first step is the regiospecific self-hydroxylation of the porphyrin alpha-meso carbon atom. The resulting alpha-meso-hydroxyheme reacts in the second step with another O(2) to yield verdoheme and CO. The third O(2) activation, by verdoheme, cleaves its porphyrin macrocycle to release biliverdin and free ferrous iron. In this Account, we provide an overview of our current understanding of the structural and biochemical properties of the complex self-oxidation reactions in HO catalysis. The first meso-hydroxylation is of particular interest because of its distinct contrast with O(2) activation by cytochrome P450. Although most heme enzymes oxidize exogenous substrates by high-valent oxo intermediates, HO was proposed to utilize the Fe-OOH intermediate for the self-hydroxylation. We have succeeded in preparing and characterizing the Fe-OOH species of HO at low temperature, and an analysis of its reaction, together with mutational and crystallographic studies, reveals that protonation of Fe-OOH by a distal water molecule is critical in promoting the unique self-hydroxylation. The second oxygenation is a rapid, spontaneous auto-oxidation of the reactive alpha-meso-hydroxyheme; its mechanism remains elusive, but the HO enzyme has been shown not to play a critical role in it. Until recently, the means of the third O(2) activation had remained unclear as well, but we have recently untangled its mechanistic outline. Reaction analysis of the verdoheme-HO complex strongly suggests the Fe-OOH species as a key intermediate of the ring-opening reaction. This mechanism is very similar to that of the first meso-hydroxylation, including the critical roles of the distal water molecule. A comprehensive study of the three oxygenations of HO highlights the rational design of the enzyme architecture and its catalytic mechanism. Elucidation of the last oxygenation step has enabled a kinetic analysis of the rate-determining step, making it possible to discuss the HO reaction mechanism in relation to its physiological functions.

Coupling of the distal hydrogen bond network to the exogenous ligand in substrate-bound, resting state human heme oxygenase

Mammalian heme oxygenase (HO) possesses catalytically implicated distal ordered water molecules within an extended H-bond network, with one of the ordered water molecules (#1) providing a bridge between the iron-coordinated ligand and the catalytically critical Asp140, that, in turn, serves as an acceptor for the Tyr58 OH H-bond. The degree of H-bonding by the ligated water molecule and the coupling of this water molecule to the H-bond network are of current interest and are herein investigated by (1)H NMR. Two-dimensional NMR allowed sufficient assignments to provide both the H-bond strength and hyperfine shifts, the latter of which were used to quantify the magnetic anisotropy in both the ferric high-spin aquo and low-spin hydroxo complexes. The anisotropy in the aquo complex indicates that the H-bond donation to water #1 is marginally stronger than in a bacterial HO, while the anisotropy for the hydroxo complex reveals a conventional (d(xz), d(yz))(1) ground state indicative of only moderate to weak H-bond acceptance by the ligated hydroxide. Mapping out the changes of the H-bond strengths in the network during the ligated water --> hydroxide conversion by correcting for the effects of magnetic anisotropy reveals a very substantial change in H-bond strength for Tyr58 OH and lesser effects on nearby H-bonds. The effect of pH on the H-bonding network in human HO is much larger and transmitted much further from the iron than in a pathogenic bacterial HO. The implications for the HO mechanism of the H-bond of Tyr58 to Asp140 are discussed.

Unusual diheme conformation of the heme-degrading protein from Mycobacterium tuberculosis

Heme degradation plays a pivotal role in the availability of the essential nutrient, iron, in pathogenic bacteria. A previously unannotated protein from Mycobacterium tuberculosis, Rv3592, which shares homology to heme-degrading enzymes, has been identified. Biochemical analyses confirm that Rv3592, which we have termed MhuD (mycobacterial heme utilization, degrader), is able to bind and degrade heme. Interestingly, contrary to previously reported stoichiometry for the Staphylococcus aureus heme degraders, iron-regulated surface determinant (Isd)G and IsdI, MhuD has the ability to bind heme in a 1:2 protein-to-heme ratio, although the MhuD-diheme complex is inactive. Furthermore, the 1.75-A crystal structure of the MhuD-diheme complex reveals two stacked hemes forming extensive contacts with residues in the active site. In particular, the solvent-exposed heme is axially liganded by His75 and is stacked planar upon the solvent-protected heme. The solvent-protected heme is coordinated by a chloride ion, which is, in turn, stabilized by Asn7. Structural comparison between MhuD-diheme and inactive IsdG and IsdI bound to only one highly distorted metalloporphyrin ring reveals that several residues located in alpha-helix 2 and the subsequent loop appear to be responsible for heme stoichiometric differences and suggest open and closed conformations for substrate entry and product exit.

Comparison of apo- and heme-bound crystal structures of a truncated human heme oxygenase-2

Heme oxygenase (HO) catalyzes the first step in the heme degradation pathway. The crystal structures of apo- and heme-bound truncated human HO-2 reveal a primarily alpha-helical architecture similar to that of human HO-1 and other known HOs. Proper orientation of heme in HO-2 is required for the regioselective oxidation of the alpha-mesocarbon. This is accomplished by interactions within the heme binding pocket, which is made up of two helices. The iron coordinating residue, His(45), resides on the proximal helix. The distal helix contains highly conserved glycine residues that allow the helix to flex and interact with the bound heme. Tyr(154), Lys(199), and Arg(203) orient the heme through direct interactions with the heme propionates. The rearrangements of side chains in heme-bound HO-2 compared with apoHO-2 further elucidate HO-2 heme interactions.

The hydrogen-bonding network in heme oxygenase also functions as a modulator of enzyme dynamics: chaotic motions upon disrupting the H-bond network in heme oxygenase from Pseudomonas aeruginosa

Relaxation compensated Carr-Purcell-Meiboom-Gill (rc-CPMG) NMR experiments have been used to investigate micros-ms motions in heme oxygenase from Pseudomonas aeruginosa (pa-HO) in its ferric state, inhibited by CN- (pa-HO-CN) and N3- (pa-HO-N3), and in its ferrous state, inhibited by CO (pa-HO-CO). Comparative analysis of the data from the three forms indicates that the nature of the coordinated distal ligand affects the micros-ms conformational freedom of the polypeptide in regions of the enzyme far removed from the heme iron and distal ligand. Interpretation of the dynamical information in the context of the crystal structure of resting state pa-HO shows that residues involved in the network of structural hydrogen-bonded waters characteristic of HOs undergo micros-ms motions in pa-HO-CN, which was studied as a model of the highly paramagnetic S = 5/2 resting state form. In comparison, similar motions are suppressed in the pa-HO-CO and pa-HO-N3 complexes, which were studied as mimics of the obligatory oxyferrous and ferric hydroperoxide intermediates, respectively, in the catalytic cycle of heme degradation. These findings suggest that in addition to proton delivery to the nascent Fe(III)-OO(-) intermediate during catalysis, the hydrogen-bonding network serves two additional roles: (i) propagate the electronic state (reactive state) in each of the distinct steps of the catalytic cycle to key but remote sections of the polypeptide via small rearrangements in the network of hydrogen bonds and (ii) modulate the conformational freedom of the enzyme, thus allowing it to adapt to the demanding changes in axial coordination state and substrate transformations that take place during the catalytic cycle. This idea was probed by disrupting the hydrogen-bonding network in pa-HO by replacing R80 with L. NMR spectroscopic studies conducted with R80L-pa-HO-N3 and R80L-pa-HO-CO revealed that the mutant exhibits nearly global conformational disorder, which is absent in the equivalent complexes of the wild type enzyme. The "chaotic" disorder in the R80L mutant is likely related to its significantly lower efficiency to hydroxylate heme in the presence of H2O2, relative to the wild type enzyme.

Crystal structure based design of functional metal/protein hybrids

Preparation of metal/protein hybrids is growing into important topics in the field of bioinorganic chemistry. X-ray crystal structure analyses of them provide direct information on unique interactions of metal cations or metal cofactors to understand and design enzymatic functions. In this mini review, the authors focus on the recent studies on the metal/protein hybrids concerning crystal structure analyses since 2002 and our related works. The precise structural determination promise us to deeply understand coordination chemistry in protein scaffold and shows intriguing suggestions on rational design and application use for biocatalysts, metal drugs and so on.

Comparative analysis of hmuO function and expression in Corynebacterium species

We have constructed defined deletions in the hmuO gene from Corynebacterium diphtheriae and Corynebacterium ulcerans and show that the C. ulcerans hmuO mutation results in a significant reduction in hemoglobin-iron utilization, whereas in C. diphtheriae strains, deletion of hmuO caused no or only partial reduction in the utilization of heme as an iron source. We also show that expression from the C. ulcerans hmuO promoter exhibits minimal regulation by iron and heme whereas transcription from the C. diphtheriae hmuO promoter shows both significant iron repression and heme-dependent activation. These findings indicate that variability in HmuO function and expression exists among Corynebacterium species.

Distinct reaction pathways followed upon reduction of oxy-heme oxygenase and oxy-myoglobin as characterized by Mössbauer spectroscopy

Activation of O(2) by heme-containing monooxygenases generally commences with the common initial steps of reduction to the ferrous heme and binding of O(2) followed by a one-electron reduction of the O(2)-bound heme. Subsequent steps that generate reactive oxygen intermediates diverge and reflect the effects of protein control on the reaction pathway. In this study, Mössbauer and EPR spectroscopies were used to characterize the electronic states and reaction pathways of reactive oxygen intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of oxy-heme oxygenase (HO) and oxy-myoglobin (Mb). The results confirm that one-electron reduction of (Fe(II)-O(2))HO is accompanied by protonation of the bound O(2) to generate a low-spin (Fe(III)-O(2)H(-))HO that undergoes self-hydroxylation to form the alpha-meso-hydroxyhemin-HO product. In contrast, one-electron reduction of (Fe(II)-O(2))Mb yields a low-spin (Fe(III)-O(2)(2-))Mb. Protonation of this intermediate generates (Fe(III)-O(2)H(-))Mb, which then decays to a ferryl complex, (Fe(IV)=O(2-))Mb, that exhibits magnetic properties characteristic of the compound II species generated in the reactions of peroxide with heme peroxidases and with Mb. Generation of reactive high-valent states with ferryl species via hydroperoxo intermediates is believed to be the key oxygen-activation steps involved in the catalytic cycles of P450-type monooxygenases. The Mössbauer data presented here provide direct spectroscopic evidence supporting the idea that ferric-hydroperoxo hemes are indeed the precursors of the reactive ferryl intermediates. The fact that a ferryl intermediate does not accumulate in HO underscores the determining role played by protein structure in controlling the reactivity of reaction intermediates.

1H NMR Study of the influence of hemin vinyl-->methyl substitution on the interaction between the C-terminus and substrate and the "aging" of the heme oxygenase from Neisseria meningitidis: induction of active site structural heterogeneity by a two-fold symmetric hemin

Solution 1H NMR has been used to characterize the active site molecular and electronic structure of the cyanide-inhibited 2,4-dimethyldeuterohemin complex of the heme oxygenase from Neisseria meningitidis (NmHO) with respect to the mode of interaction of the C-terminus with the substrate and the spontaneous "aging" of NmHO that results in the cleavage of the C-terminal Arg208-His209 dipeptide. The structure of the portion involving residues Ala12-Phe192 is found to be essentially identical to that of the protohemin complex in either solution or crystal. However, His207 from the C-terminus is found to interact strongly with the substrate 1CH3, as opposed to the 8CH3 in the protohemin complex. The different mode of interaction of His207 with the alternate substrates is attributed to the 2-vinyl group of protohemin sterically interfering with the optimal orientation of the proximal helix Asp27 carboxylate that serves as acceptor to the strong H-bond by the peptide of His207. The 2,4-dimethyldeuterohemin HO complex "ages" in manner similary to that of protohemin, (Liu, Y., Ma, L.-H., Satterlee, J.D., Zhang, X., Yoshida, T., and La Mar, G. N., (2006) Biochemistry 45, 3875-3886) with mass spectrometry and N-terminal sequencing indicating that the Arg208-His209 dipeptide is cleaved. The 2,4-dimethyldeuterohemin complex of WT HO populates an equilibrium isomer stabilized in low phosphate concentration for which the axial His imidazole ring is rotated by approximately 20 degrees from that in the WT. The His ring reorientation is attributed to Asp24 serving as the H-bond acceptor to the His207 peptide NH, rather than to the His23 ring NdeltaH as in the crystals. The functional implications of the altered C-terminal interaction with substrate modification are discussed.