Coupling of disulfide bond and distal histidine dissociation in human ferrous cytoglobin regulates ligand binding

Hell's Gate Globin-I from Methylacidiphilum infernorum Displays a Unique Temperature-Independent pH Sensing Mechanism Utililized a Lipid-Induced Conformational Change

Hell's Gate globin-I (HGb-I) is a thermally stable globin from the aerobic methanotroph Methylacidiphilium infernorum. Here we report that HGb-I interacts with lipids stoichiometrically to induce structural changes in the heme pocket, changing the heme iron distal ligation coordination from hexacoordinate to pentacoordinate. Such changes in heme geometry have only been previously reported for cytochrome c and cytoglobin, linked to apoptosis regulation and enhanced lipid peroxidation activity, respectively. However, unlike cytoglobin and cytochrome c, the heme iron of HGb-I is altered by lipids in ferrous as well as ferric oxidation states. The apparent affinity for lipids in this thermally stable globin is highly pH-dependent but essentially temperature-independent within the range of 20-60 °C. We propose a mechanism to explain these observations, in which lipid binding and stability of the distal endogenous ligand are juxtaposed as a function of temperature. Additionally, we propose that these coupled equilibria may constitute a mechanism through which this acidophilic thermophile senses the pH of its environment.

The circularly permuted globin domain of androglobin exhibits atypical heme stabilization and nitric oxide interaction

In the decade since the discovery of androglobin, a multi-domain hemoglobin of metazoans associated with ciliogenesis and spermatogenesis, there has been little advance in the knowledge of the biochemical and structural properties of this unusual member of the hemoglobin superfamily. Using a method for aligning remote homologues, coupled with molecular modelling and molecular dynamics, we have identified a novel structural alignment to other hemoglobins. This has led to the first stable recombinant expression and characterization of the circularly permuted globin domain. Exceptional for eukaryotic globins is that a tyrosine takes the place of the highly conserved phenylalanine in the CD1 position, a critical point in stabilizing the heme. A disulfide bond, similar to that found in neuroglobin, forms a closed loop around the heme pocket, taking the place of androglobin's missing CD loop and further supporting the heme pocket structure. Highly unusual in the globin superfamily is that the heme iron binds nitric oxide as a five-coordinate complex similar to other heme proteins that have nitric oxide storage functions. With rapid autoxidation and high nitrite reductase activity, the globin appears to be more tailored toward nitric oxide homeostasis or buffering. The use of our multi-template profile alignment method to yield the first biochemical characterisation of the circularly permuted globin domain of androglobin expands our knowledge of the fundamental functioning of this elusive protein and provides a pathway to better define the link between the biochemical traits of androglobin with proposed physiological functions.

Molecular analysis of the human cytoglobin mRNA isoforms

Multiple functions have been proposed for the ubiquitously expressed vertebrate globin cytoglobin (Cygb), including nitric oxide (NO) metabolism, lipid peroxidation/signalling, superoxide dismutase activity, reactive oxygen/nitrogen species (RONS) scavenging, regulation of blood pressure, antifibrosis, and both tumour suppressor and oncogenic effects. Since alternative splicing can expand the biological roles of a gene, we investigated whether this mechanism contributes to the functional diversity of Cygb. By mining of cDNA data and molecular analysis, we identified five alternative mRNA isoforms for the human CYGB gene (V-1 to V-5). Comprehensive RNA-seq analyses of public datasets from human tissues and cells confirmed that the canonical CYGB V-1 isoform is the primary CYGB transcript in the majority of analysed datasets. Interestingly, we revealed that isoform V-3 represented the predominant CYGB variant in hepatoblastoma (HB) cell lines and in the majority of analysed normal and HB liver tissues. CYGB V-3 mRNA is transcribed from an alternate upstream promoter and hypothetically encodes a N-terminally truncated CYGB protein, which is not recognized by some antibodies used in published studies. Little to no transcriptional evidence was found for the other CYGB isoforms. Comparative transcriptomics and flow cytometry on CYGB+/+ and gene-edited CYGB-/- HepG2 HB cells did not unveil a knockout phenotype and, thus, a potential function for CYGB V-3. Our study reveals that the CYGB gene is transcriptionally more complex than previously described as it expresses alternative mRNA isoforms of unknown function. Additional experimental data are needed to clarify the biological meaning of those alternative CYGB transcripts.

Insights into the function of cytoglobin

Since its discovery in 2001, the function of cytoglobin has remained elusive. Through extensive in vitro and in vivo research, a range of potential physiological and pathological mechanisms has emerged for this multifunctional member of the hemoglobin family. Currently, over 200 research publications have examined different aspects of cytoglobin structure, redox chemistry and potential roles in cell signalling pathways. This research is wide ranging, but common themes have emerged throughout the research. This review examines the current structural, biochemical and in vivo knowledge of cytoglobin published over the past two decades. Radical scavenging, nitric oxide homeostasis, lipid binding and oxidation and the role of an intramolecular disulfide bond on the redox chemistry are examined, together with aspects and roles for Cygb in cancer progression and liver fibrosis.

Nitrite Reductase Activity of Ferrous Nitrobindins: A Comparative Study

Nitrobindins (Nbs) are all-β-barrel heme proteins spanning from bacteria to Homo sapiens. They inactivate reactive nitrogen species by sequestering NO, converting NO to HNO2, and promoting peroxynitrite isomerization to NO3−. Here, the nitrite reductase activity of Nb(II) from Mycobacterium tuberculosis (Mt-Nb(II)), Arabidopsis thaliana (At-Nb(II)), Danio rerio (Dr-Nb(II)), and Homo sapiens (Hs-Nb(II)) is reported. This activity is crucial for the in vivo production of NO, and thus for the regulation of blood pressure, being of the utmost importance for the blood supply to poorly oxygenated tissues, such as the eye retina. At pH 7.3 and 20.0 °C, the values of the second-order rate constants (i.e., kon) for the reduction of NO2− to NO and the concomitant formation of nitrosylated Mt-Nb(II), At-Nb(II), Dr-Nb(II), and Hs-Nb(II) (Nb(II)-NO) were 7.6 M−1 s−1, 9.3 M−1 s−1, 1.4 × 101 M−1 s−1, and 5.8 M−1 s−1, respectively. The values of kon increased linearly with decreasing pH, thus indicating that the NO2−-based conversion of Nb(II) to Nb(II)-NO requires the involvement of one proton. These results represent the first evidence for the NO2 reductase activity of Nbs(II), strongly supporting the view that Nbs are involved in NO metabolism. Interestingly, the nitrite reductase reactivity of all-β-barrel Nbs and of all-α-helical globins (e.g., myoglobin) was very similar despite the very different three-dimensional fold; however, differences between all-α-helical globins and all-β-barrel Nbs suggest that nitrite reductase activity appears to be controlled by distal steric barriers, even though a more complex regulatory mechanism can be also envisaged.

Modulating Nitric Oxide Dioxygenase and Nitrite Reductase of Cytoglobin through Point Mutations

Cytoglobin is a hexacoordinate hemoglobin with physiological roles that are not clearly understood. Previously proposed physiological functions include nitric oxide regulation, oxygen sensing, or/and protection against oxidative stress under hypoxic/ischemic conditions. Like many globins, cytoglobin rapidly consumes nitric oxide under normoxic conditions. Under hypoxia, cytoglobin generates nitric oxide, which is strongly modulated by the oxidation state of the cysteines. This gives a plausible role for this biochemistry in controlling nitric oxide homeostasis. Mutations to control specific properties of hemoglobin and myoglobin, including nitric oxide binding/scavenging and the nitrite reductase activity of various globins, have been reported. We have mapped these key mutations onto cytoglobin, which represents the E7 distal ligand, B2/E9 disulfide, and B10 heme pocket residues, and examined the nitric oxide binding, nitric oxide dioxygenase activity, and nitrite reductase activity. The Leu46Trp mutation decreases the nitric oxide dioxygenase activity > 10,000-fold over wild type, an effect 1000 times greater than similar mutations with other globins. By understanding how particular mutations can affect specific reactivities, these mutations may be used to target specific cytoglobin activities in cell or animal models to help understand the precise role(s) of cytoglobin under physiological and pathophysiological conditions.

Designing Artificial Metalloenzymes by Tuning of the Environment beyond the Primary Coordination Sphere

Metalloenzymes catalyze a variety of reactions using a limited number of natural amino acids and metallocofactors. Therefore, the environment beyond the primary coordination sphere must play an important role in both conferring and tuning their phenomenal catalytic properties, enabling active sites with otherwise similar primary coordination environments to perform a diverse array of biological functions. However, since the interactions beyond the primary coordination sphere are numerous and weak, it has been difficult to pinpoint structural features responsible for the tuning of activities of native enzymes. Designing artificial metalloenzymes (ArMs) offers an excellent basis to elucidate the roles of these interactions and to further develop practical biological catalysts. In this review, we highlight how the secondary coordination spheres of ArMs influence metal binding and catalysis, with particular focus on the use of native protein scaffolds as templates for the design of ArMs by either rational design aided by computational modeling, directed evolution, or a combination of both approaches. In describing successes in designing heme, nonheme Fe, and Cu metalloenzymes, heteronuclear metalloenzymes containing heme, and those ArMs containing other metal centers (including those with non-native metal ions and metallocofactors), we have summarized insights gained on how careful controls of the interactions in the secondary coordination sphere, including hydrophobic and hydrogen bonding interactions, allow the generation and tuning of these respective systems to approach, rival, and, in a few cases, exceed those of native enzymes. We have also provided an outlook on the remaining challenges in the field and future directions that will allow for a deeper understanding of the secondary coordination sphere a deeper understanding of the secondary coordintion sphere to be gained, and in turn to guide the design of a broader and more efficient variety of ArMs.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Endogenous Hemoprotein-Dependent Signaling Pathways of Nitric Oxide and Nitrite

Interdisciplinary research at the interface of chemistry, physiology, and biomedicine have uncovered pivotal roles of nitric oxide (NO) as a signaling molecule that regulates vascular tone, platelet aggregation, and other pathways relevant to human health and disease. Heme is central to physiological NO signaling, serving as the active site for canonical NO biosynthesis in nitric oxide synthase (NOS) enzymes and as the highly selective NO binding site in the soluble guanylyl cyclase receptor. Outside of the primary NOS-dependent biosynthetic pathway, other hemoproteins, including hemoglobin and myoglobin, generate NO via the reduction of nitrite. This auxiliary hemoprotein reaction unlocks a "second axis" of NO signaling in which nitrite serves as a stable NO reservoir. In this Forum Article, we highlight these NO-dependent physiological pathways and examine complex chemical and biochemical reactions that govern NO and nitrite signaling in vivo. We focus on hemoprotein-dependent reaction pathways that generate and consume NO in the presence of nitrite and consider intermediate nitrogen oxides, including NO2, N2O3, and S-nitrosothiols, that may facilitate nitrite-based signaling in blood vessels and tissues. We also discuss emergent therapeutic strategies that leverage our understanding of these key reaction pathways to target NO signaling and treat a wide range of diseases.

Redox sensor properties of human cytoglobin allosterically regulate heme pocket reactivity

Cytoglobin is a conserved hemoprotein ubiquitously expressed in mammalian tissues, which conducts electron transfer reactions with proposed signaling functions in nitric oxide (NO) and lipid metabolism. Cytoglobin has an E7 distal histidine (His81), which unlike related globins such as myoglobin and hemoglobin, is in equilibrium between a bound, hexacoordinate state and an unbound, pentacoordinate state. The His81 binding equilibrium appears to be allosterically modulated by the presence of an intramolecular disulfide between two cysteines (Cys38 and Cys83). The formation of this disulfide bridge regulates nitrite reductase activity and lipid binding. Herein, we attempt to clarify the effects of defined thiol oxidation states on small molecule binding of cytoglobin heme, using cyanide binding to probe the ferric state. Cyanide binding kinetics to wild-type cytoglobin reveal at least two kinetically distinct subpopulations, depending on thiol oxidation states. Experiments with covalent thiol modification by NEM, glutathione, and amino acid substitutions (C38S, C83S and H81A), indicate that subpopulations ranging from fully reduced thiols, single thiol oxidation, and intramolecular disulfide formation determine heme binding properties by modulating the histidine-heme affinity and ligand binding. The redox modulation of ligand binding is sensitive to physiological levels of hydrogen peroxide, with a functional midpoint redox potential for the native cytoglobin intramolecular disulfide bond of -189 ± 4 mV, a value within the boundaries of intracellular redox potentials. These results support the hypothesis that Cys38 and Cys83 on cytoglobin serve as sensitive redox sensors that modulate the cytoglobin distal heme pocket reactivity and ligand binding.

Structural and functional properties of Antarctic fish cytoglobins-1: Cold-reactivity in multi-ligand reactions

While the functions of the recently discovered cytoglobin, ubiquitously expressed in vertebrate tissues, remain uncertain, Antarctic fish provide unparalleled models to study novel protein traits that may arise from cold adaptation. We report here the spectral, ligand-binding and enzymatic properties (peroxynitrite isomerization, nitrite-reductase activity) of cytoglobin-1 from two Antarctic fish, Chaenocephalus aceratus and Dissostichus mawsoni, and present the crystal structure of D. mawsoni cytoglobin-1. The Antarctic cytoglobins-1 display high O₂ affinity, scarcely compatible with an O₂-supply role, a slow rate constant for nitrite-reductase activity, and do not catalyze peroxynitrite isomerization. Compared with mesophilic orthologues, the cold-adapted cytoglobins favor binding of exogenous ligands to the hexa-coordinated bis-histidyl species, a trait related to their higher rate constant for distal-His/heme-Fe dissociation relative to human cytoglobin. At the light of a remarkable 3D-structure conservation, the observed differences in ligand-binding kinetics may reflect Antarctic fish cytoglobin-1 specific features in the dynamics of the heme distal region and of protein matrix cavities, suggesting adaptation to functional requirements posed by the cold environment. Taken together, the biochemical and biophysical data presented suggest that in Antarctic fish, as in humans, cytoglobin-1 unlikely plays a role in O₂ transport, rather it may be involved in processes such as NO detoxification.

Regulation of Nitric Oxide Metabolism and Vascular Tone by Cytoglobin

Significance: Cytoglobin (Cygb) was discovered as a new addition to the globin superfamily and subsequently identified to have potent nitric oxide (NO) dioxygenase function. Cygb plays a critical role in the oxygen-dependent regulation of NO levels and vascular tone. Recent Advances: In recent years, the mechanism of the Cygb-mediated NO dioxygenation has been studied in isolated protein, smooth muscle cell, isolated blood vessel, and in vivo animal model systems. Studies in Cygb^-/- mice have demonstrated that Cygb plays a critical role in regulating blood pressure and vascular tone. This review summarizes advances in the knowledge of NO dioxygenation/metabolism regulated by Cygb. Advances in measurement of NO diffusion dynamics across blood vessels and kinetic modeling of Cygb-mediated NO dioxygenation are summarized. The oxygen-dependent regulation of NO degradation by Cygb is also reviewed along with how Cygb paradoxically generates NO from nitrite under anaerobic conditions. The important role of Cygb in the regulation of vascular function and disease is reviewed. Critical Issues: Cygb is a more potent NO dioxygenase (NOD) than previously known globins with structural differences in heme coordination and environment, conferring it with a higher rate of reduction and more rapid process of NO dioxygenation with unique oxygen dependence. Various cellular reducing systems regenerate the catalytic oxyferrous Cygb species, supporting a high rate of NO dioxygenation. Future Directions: There remains a critical need to further characterize the factors and processes that modulate Cygb-mediated NOD function, and to develop pharmacological or other approaches to modulate Cygb function and expression.

Molecular Dynamics Simulation and Kinetic Study of Fluoride Binding to V21C/V66C Myoglobin with a Cytoglobin-like Disulfide Bond

Protein design is able to create artificial proteins with advanced functions, and computer simulation plays a key role in guiding the rational design. In the absence of structural evidence for cytoglobin (Cgb) with an intramolecular disulfide bond, we recently designed a de novo disulfide bond in myoglobin (Mb) based on structural alignment (i.e., V21C/V66C Mb double mutant). To provide deep insight into the regulation role of the Cys21-Cys66 disulfide bond, we herein perform molecular dynamics (MD) simulation of the fluoride–protein complex by using a fluoride ion as a probe, which reveals detailed interactions of the fluoride ion in the heme distal pocket, involving both the distal His64 and water molecules. Moreover, we determined the kinetic parameters of fluoride binding to the double mutant. The results agree with the MD simulation and show that the formation of the Cys21-Cys66 disulfide bond facilitates both fluoride binding to and dissociating from the heme iron. Therefore, the combination of theoretical and experimental studies provides valuable information for understanding the structure and function of heme proteins, as regulated by a disulfide bond. This study is thus able to guide the rational design of artificial proteins with tunable functions in the future.

Emerging perspectives on cytoglobin, beyond NO dioxygenase and peroxidase

Cytoglobin is an evolutionary ancient hemoglobin with poor functional annotation. Rather than constrained to penta coordination, cytoglobin's heme iron may exist either as a penta or hexacoordinated arrangement when exposed to different intracellular environments. Two cysteine residues at the surface of the protein form an intramolecular disulfide bond that regulates iron coordination, ligand binding, and peroxidase activity. Overall, biochemical results do not support a role for cytoglobin as a direct antioxidant enzyme that scavenges hydrogen peroxide because the rate of the reaction of cytoglobin with hydrogen peroxide is several orders of magnitude slower than metal and thiol-based peroxidases. Thus, alternative substrates such as fatty acids have been suggested and regulation of nitric oxide bioavailability through nitric oxide dioxygenase and nitrite reductase activities has received experimental support. Cytoglobin is broadly expressed in connective, muscle, and nervous tissues. Rational for differential cellular distribution is poorly understood but inducibility in response to hypoxia is one of the most established features of cytoglobin expression with regulation through the transcription factor hypoxia-inducible factor (HIF). Phenotypic characterization of cytoglobin deletion in the mouse have indicated broad changes that include a heightened inflammatory response and fibrosis, increase tumor burden, cardiovascular dysfunction, and hallmarks of senescence. Some of these changes might be reversed upon inhibition of nitric oxide synthase. However, subcellular and molecular interactions have been seldom characterized. In addition, specific molecular mechanisms of action are still lacking. We speculate that cytoglobin functionality will extend beyond nitric oxide handling and will have to encompass indirect regulatory antioxidant and redox sensing functions.

Engineering tyrosine residues into hemoglobin enhances heme reduction, decreases oxidative stress and increases vascular retention of a hemoglobin based blood substitute

Hemoglobin (Hb)-based oxygen carriers (HBOC) are modified extracellular proteins, designed to replace or augment the oxygen-carrying capacity of erythrocytes. However, clinical results have generally been disappointing due to adverse side effects, in part linked to the intrinsic oxidative toxicity of Hb. Previously a redox-active tyrosine residue was engineered into the Hb β subunit (βF41Y) to facilitate electron transfer between endogenous antioxidants such as ascorbate and the oxidative ferryl heme species, converting the highly oxidizing ferryl species into the less reactive ferric (met) form. We inserted different single tyrosine mutations into the α and β subunits of Hb to determine if this effect of βF41Y was unique. Every mutation that was inserted within electron transfer range of the protein surface and the heme increased the rate of ferryl reduction. However, surprisingly, three of the mutations (βT84Y, αL91Y and βF85Y) also increased the rate of ascorbate reduction of ferric(met) Hb to ferrous(oxy) Hb. The rate enhancement was most evident at ascorbate concentrations equivalent to that found in plasma (< 100 μM), suggesting that it might be of benefit in decreasing oxidative stress in vivo. The most promising mutant (βT84Y) was stable with no increase in autoxidation or heme loss. A decrease in membrane damage following Hb addition to HEK cells correlated with the ability of βT84Y to maintain the protein in its oxygenated form. When PEGylated and injected into mice, βT84Y was shown to have an increased vascular half time compared to wild type PEGylated Hb. βT84Y represents a new class of mutations with the ability to enhance reduction of both ferryl and ferric Hb, and thus has potential to decrease adverse side effects as one component of a final HBOC product.

Nitrite and nitrate chemical biology and signalling

Inorganic nitrate (NO3 - ), nitrite (NO2 - ) and NO are nitrogenous species with a diverse and interconnected chemical biology. The formation of NO from nitrate and nitrite via a reductive 'nitrate-nitrite-NO' pathway and resulting in vasodilation is now an established complementary route to traditional NOS-derived vasodilation. Nitrate, found in our diet and abundant in mammalian tissues and circulation, is activated via reduction to nitrite predominantly by our commensal oral microbiome. The subsequent in vivo reduction of nitrite, a stable vascular reserve of NO, is facilitated by a number of haem-containing and molybdenum-cofactor proteins. NO generation from nitrite is enhanced during physiological and pathological hypoxia and in disease states involving ischaemia-reperfusion injury. As such, modulation of these NO vascular repositories via exogenously supplied nitrite and nitrate has been evaluated as a therapeutic approach in a number of diseases. Ultimately, the chemical biology of nitrate and nitrite is governed by local concentrations, reaction equilibrium constants, and the generation of transient intermediates, with kinetic rate constants modulated at differing physiological pH values and oxygen tensions. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

Ultrafast photochemistry of the bc1 complex

We present a full investigation of ultrafast light-induced events in the membraneous cytochrome bc₁ complex by transient absorption spectroscopy. This energy-transducing complex harbors four redox-active components per monomer: heme c₁, two 6-coordinate b-hemes and a [2Fe-2S] cluster. Using excitation of these components in different ratios under various excitation conditions, probing in the full visible range and under three well-defined redox conditions, we demonstrate that for all ferrous hemes of the complex photodissociation of axial ligands takes place and that they rebind in 5-7 ps, as in other 6-coordinate heme proteins, including cytoglobin, which is included as a reference in this study. By contrast, the signals are not consistent with photooxidation of the b hemes. This conclusion contrasts with a recent assessment based on a more limited data set. The binding kinetics of internal and external ligands are indicative of a rigid heme environment, consistent with the electron transfer function. We also report, for the first time, photoactivity of the very weakly absorbing iron-sulfur center. This yields the unexpected perspective of studying photochemistry, initiated by excitation of iron-sulfur clusters, in a range of protein complexes.

Regulation of both the structure and function by a de novo designed disulfide bond: a case study of heme proteins in myoglobin

The V21C/V66C/F46S myoglobin mutant, with a de novo designed intramolecular disulfide bond resembling that in cytoglobin without structural evidence, exhibits a dehalogenation activity exceeding that of a native dehaloperoxidase.

Roles of N- and C-terminal domains in the ligand-binding properties of cytoglobin

Cytoglobin (Cygb) is a member of the hexacoordinated globin protein family and is expressed ubiquitously in rat and human tissues. Although Cygb is reportedly upregulated under hypoxic conditions both in vivo and in vitro, suggesting a physiological function to protect cells under hypoxic/ischemic conditions by scavenging reactive oxygen species or by signal transduction, the mechanisms associated with this function have not been fully elucidated. Recent studies comparing Cygbs among several species suggest that mammalian Cygbs show a distinctly longer C-terminal domain potentially involved in unique physiological functions. In this study, we prepared human Cygb mutants (ΔC, ΔN, and ΔNC) with either one or both terminal domains truncated and investigated the enzymatic functions and structural features by spectroscopic methods. Evaluation of the superoxide-scavenging activity between Cygb variants showed that the ΔC and ΔNC mutants exhibited slightly higher activity involving superoxide scavenging as compared with wild-type Cygb. Subsequent experiments involving ligand titration, flash photolysis, and resonance Raman spectroscopic studies suggested that the truncation of the C- and N-terminal domains resulted in less effective to dissociation constants and binding rates for carbon monoxide, respectively. Furthermore, structural stability was assessed by guanidine hydrochloride and revealed that the C-terminal domain might play a vital role in improving structure, whereas the N-terminal domain did not exert a similar effect. These findings indicated that long terminal domains could be important not only in regulating enzymatic activity but also for structural stability, and that the domains might be relevant to other hypothesized physiological functions for Cygb.

Oxygen binding and nitric oxide dioxygenase activity of cytoglobin are altered to different extents by cysteine modification

Cytoglobin (Cygb), like other members of the globin family, is a nitric oxide (NO) dioxygenase, metabolizing NO in an oxygen (O₂)‐dependent manner. We examined the effect of modification of cysteine sulfhydryl groups of Cygb on its O₂ binding and NO dioxygenase activity. The two cysteine sulfhydryls of Cygb were modified to form either an intramolecular disulfide bond (Cygb_SS), thioether bonds to N‐ethylmaleimide (NEM; Cygb_SC), or were maintained as free SH groups (Cygb_SH). It was observed that the NO dioxygenase activity of Cygb only slightly changed (~ 25%) while the P₅₀ of O₂ binding to Cygb changed over four‐fold with these modifications. Our results suggest that it is possible to separately regulate one Cygb function (such as O₂ binding) without largely affecting the other Cygb functions (such as its NO dioxygenase activity).

Redox and Peroxidase Activities of the Hemoglobin Superfamily: Relevance to Health and Disease

SIGNIFICANCE

Erythrocyte hemoglobin (Hb) and myocyte myoglobin, although primarily oxygen-carrying proteins, have the capacity to do redox chemistry. Such redox activity in the wider family of globins now appears to have important associations with the mechanisms of cell stress response. In turn, an understanding of such mechanisms in vivo may have a potential in the understanding of cancer therapy resistance and neurodegenerative disorders such as Alzheimer's. Recent Advances: There has been an enhanced understanding of the redox chemistry of the globin superfamily in recent years, leading to advances in development of Hb-based blood substitutes and in hypotheses relating to specific disease mechanisms. Neuroglobin (Ngb) and cytoglobin (Cygb) have been linked to cell protection mechanisms against hypoxia and oxidative stress, with implications in the onset and progression of neurodegenerative diseases for Ngb and cancer for Cygb.

CRITICAL ISSUES

Despite advances in the understanding of redox chemistry of globins, the physiological roles of many of these proteins still remain ambiguous at best. Confusion over potential physiological roles may relate to multifunctional roles for globins, which may be modulated by surface-exposed cysteine pairs in some globins. Such roles may be critical in deciphering the relationships of these globins in human diseases.

FUTURE DIRECTIONS

Further studies are required to connect the considerable knowledge on the mechanisms of globin redox chemistry in vitro with the physiological and pathological roles of globins in vivo. In doing so, new therapies for neurodegenerative disorders and cancer therapy resistance may be targeted. Antioxid. Redox Signal. 26, 763-776.

Antarctic fish versus human cytoglobins - The same but yet so different

The cytoglobins of the Antarctic fish Chaenocephalus aceratus and Dissostichus mawsoni have many features in common with human cytoglobin. These cytoglobins are heme proteins in which the ferric and ferrous forms have a characteristic hexacoordination of the heme iron, i.e. axial ligation of two endogenous histidine residues, as confirmed by electron paramagnetic resonance, resonance Raman and optical absorption spectroscopy. The combined spectroscopic analysis revealed only small variations in the heme-pocket structure, in line with the small variations observed for the redox potential. Nevertheless, some striking differences were also discovered. Resonance Raman spectroscopy showed that the stabilization of an exogenous heme ligand, such as CO, occurs differently in human cytoglobin in comparison with Antarctic fish cytoglobins. Furthermore, while it has been extensively reported that human cytoglobin is essentially monomeric and can form an intramolecular disulfide bridge that can influence the ligand binding kinetics, 3D modeling of the Antarctic fish cytoglobins indicates that the cysteine residues are too far apart to form such an intramolecular bridge. Moreover, gel filtration and mass spectrometry reveal the occurrence of non-covalent multimers (up to pentamers) in the Antarctic fish cytoglobins that are formed at low concentrations. Stabilization of these oligomers by disulfide-bridge formation is possible, but not essential. If intermolecular disulfide bridges are formed, they influence the heme-pocket structure, as is shown by EPR measurements.

Peroxidase activation of cytoglobin by anionic phospholipids: Mechanisms and consequences

Cytoglobin (Cygb) is a hexa-coordinated hemoprotein with yet to be defined physiological functions. The iron coordination and spin state of the Cygb heme group are sensitive to oxidation of two cysteine residues (Cys38/Cys83) and/or the binding of free fatty acids. However, the roles of redox vs lipid regulators of Cygb's structural rearrangements in the context of the protein peroxidase competence are not known. Searching for physiologically relevant lipid regulators of Cygb, here we report that anionic phospholipids, particularly phosphatidylinositolphosphates, affect structural organization of the protein and modulate its iron state and peroxidase activity both conjointly and/or independently of cysteine oxidation. Thus, different anionic lipids can operate in cysteine-dependent and cysteine-independent ways as inducers of the peroxidase activity. We establish that Cygb's peroxidase activity can be utilized for the catalysis of peroxidation of anionic phospholipids (including phosphatidylinositolphosphates) yielding mono-oxygenated molecular species. Combined with the computational simulations we propose a bipartite lipid binding model that rationalizes the modes of interactions with phospholipids, the effects on structural re-arrangements and the peroxidase activity of the hemoprotein.

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.

Conformational control of the binding of diatomic gases to cytochrome c'

The cytochromes c' (CYTcp) are found in denitrifying, methanotrophic and photosynthetic bacteria. These proteins are able to form stable adducts with CO and NO but not with O2. The binding of NO to CYTcp currently provides the best structural model for the NO activation mechanism of soluble guanylate cyclase. Ligand binding in CYTcps has been shown to be highly dependent on residues in both the proximal and distal heme pockets. Group 1 CYTcps typically have a phenylalanine residue positioned close to the distal face of heme, while for group 2, this residue is typically leucine. We have structurally, spectroscopically and kinetically characterised the CYTcp from Shewanella frigidimarina (SFCP), a protein that has a distal phenylalanine residue and a lysine in the proximal pocket in place of the more common arginine. Each monomer of the SFCP dimer folds as a 4-alpha-helical bundle in a similar manner to CYTcps previously characterised. SFCP exhibits biphasic binding kinetics for both NO and CO as a result of the high level of steric hindrance from the aromatic side chain of residue Phe 16. The binding of distal ligands is thus controlled by the conformation of the phenylalanine ring. Only a proximal 5-coordinate NO adduct, confirmed by structural data, is observed with no detectable hexacoordinate distal NO adduct.