Protoglobin: structure and ligand-binding properties

Directed Evolution of Protoglobin Optimizes the Enzyme Electric Field

To unravel why computational design fails in creating viable enzymes, while directed evolution (DE) succeeds, our research delves into the laboratory evolution of protoglobin. DE has adapted this protein to efficiently catalyze carbene transfer reactions. We show that the previously proposed enhanced substrate access and binding alone cannot account for increased yields during DE. The 3D electric field in the entire active site is tracked through protein dynamics, clustered using the affinity propagation algorithm, and subjected to principal component analysis. This analysis reveals notable changes in the electric field with DE, where distinct field topologies influence transition state energetics and mechanism. A chemically meaningful field component emerges and takes the lead during DE and facilitates crossing the barrier to carbene transfer. Our findings underscore intrinsic electric field dynamic's influence on enzyme function, the ability of the field to switch mechanisms within the same protein, and the crucial role of the field in enzyme design.

Enzymatic Nitrogen Incorporation Using Hydroxylamine

Hydroxylamine-derived reagents have enabled versatile nitrene transfer reactions for introducing nitrogen-containing functionalities in small-molecule catalysis, as well as biocatalysis. These reagents, however, result in a poor atom economy and stoichiometric organic waste. Activating hydroxylamine (NH2OH) for nitrene transfer offers a low-cost and sustainable route to amine synthesis, since water is the sole byproduct. Despite its presence in nature, hydroxylamine is not known to be used for enzymatic nitrogen incorporation in biosynthesis. Here, we report an engineered heme enzyme that can utilize hydroxylammonium chloride, an inexpensive commodity chemical, for nitrene transfer. Directed evolution of Pyrobaculum arsenaticum protoglobin generated efficient enzymes for benzylic C-H primary amination and styrene aminohydroxylation. Mechanistic studies supported a stepwise radical pathway involving rate-limiting hydrogen atom transfer. This unprecedented activity is a useful addition to the "nitrene transferase" repertoire and hints at possible future discovery of natural enzymes that use hydroxylamine for amination chemistry.

A novel single sensor hemoglobin domain from the thermophilic cyanobacteria Thermosynechococcus elongatus BP-1 exhibits higher pH but lower thermal stability compared to globins from mesophilic organisms

Thermosynechococcus elongatus-BP1 belongs to the class of photoautotrophic cyanobacterial organisms. The presence of chlorophyll a, carotenoids, and phycocyanobilin are the characteristics that categorize T. elongatus as a photosynthetic organism. Here, we report the structural and spectroscopic characteristics of novel hemoglobin (Hb) Synel Hb from T.elongatus, synonymous with Thermosynechococcus vestitus BP-1. The X-ray crystal structure (2.15 Å) of Synel Hb suggests the presence of a globin domain with a pre-A helix similar to the sensor domain (S) family of Hbs. The rich hydrophobic core accommodates heme in a penta-coordinated state and readily binds an extraneous ligand(imidazole). The absorption and circular dichroic spectral analysis of Synel Hb reiteratedthat the heme is in Fe^III+ state with a predominantly α-helical structure similar to myoglobin. Synel Hb displays higher resistance to structural perturbations induced via external stresses like pH and guanidium hydrochloride, which is comparable to Synechocystis Hb. However, Synel Hb exhibited lower thermal stability compared to mesophilic hemoglobins. Overall, the data is suggestive of the structural sturdiness of Synel Hb, which probably corroborates its origin in extreme thermophilic conditions. The stable globin provides scope for further investigation and may lead to new insights with scope for engineering stability in hemoglobin-based oxygen carriers.

Protoglobin-Catalyzed Formation of cis-Trifluoromethyl-Substituted Cyclopropanes by Carbene Transfer

Trifluoromethyl-substituted cyclopropanes (CF3 -CPAs) constitute an important class of compounds for drug discovery. While several methods have been developed for synthesis of trans-CF3 -CPAs, stereoselective production of corresponding cis-diastereomers remains a formidable challenge. We report a biocatalyst for diastereo- and enantio-selective synthesis of cis-CF3 -CPAs with activity on a variety of alkenes. We found that an engineered protoglobin from Aeropyrnum pernix (ApePgb) can catalyze this unusual reaction at preparative scale with low-to-excellent yield (6-55 %) and enantioselectivity (17-99 % ee), depending on the substrate. Computational studies revealed that the steric environment in the active site of the protoglobin forced iron-carbenoid and substrates to adopt a pro-cis near-attack conformation. This work demonstrates the capability of enzyme catalysts to tackle challenging chemistry problems and provides a powerful means to expand the structural diversity of CF3 -CPAs for drug discovery.

Structural and (Pseudo-)Enzymatic Properties of Neuroglobin: Its Possible Role in Neuroprotection

Neuroglobin (Ngb), the third member of the globin family, was discovered in human and murine brains in 2000. This monomeric globin is structurally similar to myoglobin (Mb) and hemoglobin (Hb) α and β subunits, but it hosts a bis-histidyl six-coordinated heme-Fe atom. Therefore, the heme-based reactivity of Ngb is modulated by the dissociation of the distal HisE7-heme-Fe bond, which reflects in turn the redox state of the cell. The high Ngb levels (~100–200 μM) present in the retinal ganglion cell layer and in the optic nerve facilitate the O₂ buffer and delivery. In contrast, the very low levels of Ngb (~1 μM) in most tissues and organs support (pseudo-)enzymatic properties including NO/O₂ metabolism, peroxynitrite and free radical scavenging, nitrite, hydroxylamine, hydrogen sulfide reduction, and the nitration of aromatic compounds. Here, structural and (pseudo-)enzymatic properties of Ngb, which are at the root of tissue and organ protection, are reviewed, envisaging a possible role in the protection from neuronal degeneration of the retina and the optic nerve.

Structural modeling of a novel membrane-bound globin-coupled sensor in Geobacter sulfurreducens

Globin-coupled sensors (GCS) usually consist of three domains: a sensor/globin, a linker, and a transmitter domain. The globin domain (GD), activated by ligand binding and/or redox change, induces an intramolecular signal transduction resulting in a response of the transmitter domain. Depending on the nature of the transmitter domain, GCSs can have different activities and functions, including adenylate and di-guanylate cyclase, histidine kinase activity, aerotaxis and/or oxygen sensing function. The gram-negative delta-proteobacterium Geobacter sulfurreducens expresses a protein with a GD covalently linked to a four transmembrane domain, classified, by sequence similarity, as GCS (GsGCS). While its GD is fully characterized, not so its transmembrane domain, which is rarely found in the globin superfamily. In the present work, GsGCS was characterized spectroscopically and by native ion mobility-mass spectrometry in combination with cryo-electron microscopy. Although lacking high resolution, the oligomeric state and the electron density map were valuable for further rational modeling of the full-length GsGCS structure. This model demonstrates that GsGCS forms a transmembrane domain-driven tetramer with minimal contact between the GDs and with the heme groups oriented outward. This organization makes an intramolecular signal transduction less likely. Our results, including the auto-oxidation rate and redox potential, suggest a potential role for GsGCS as redox sensor or in a membrane-bound e-/H+ transfer. As such, GsGCS might act as a player in connecting energy production to the oxidation of organic compounds and metal reduction. Database searches indicate that GDs linked to a four or seven helices transmembrane domain occur more frequently than expected.

NO Scavenging through Reductive Nitrosylation of Ferric Mycobacterium tuberculosis and Homo sapiens Nitrobindins

Ferric nitrobindins (Nbs) selectively bind NO and catalyze the conversion of peroxynitrite to nitrate. In this study, we show that NO scavenging occurs through the reductive nitrosylation of ferric Mycobacterium tuberculosis and Homo sapiens nitrobindins (Mt-Nb(III) and Hs-Nb(III), respectively). The conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-Nb(II)-NO, respectively, is a monophasic process, suggesting that over the explored NO concentration range (between 2.5 × 10⁻⁵ and 1.0 × 10⁻³ M), NO binding is lost in the mixing time (i.e., ^NOk_on ≥ 1.0 × 10⁶ M⁻¹ s⁻¹). The pseudo-first-order rate constant for the reductive nitrosylation of Mt-Nb(III) and Hs-Nb(III) (i.e., k) is not linearly dependent on the NO concentration but tends to level off, with a rate-limiting step (i.e., k_lim) whose values increase linearly with [OH⁻]. This indicates that the conversion of Mt-Nb(III) and Hs-Nb(III) to Mt-Nb(II)-NO and Hs-Nb(II)-NO, respectively, is limited by the OH⁻-based catalysis. From the dependence of k_lim on [OH⁻], the values of the second-order rate constant k_OH− for the reductive nitrosylation of Mt-Nb(III)-NO and Hs-Nb(III)-NO were obtained (4.9 (±0.5) × 10³ M⁻¹ s⁻¹ and 6.9 (±0.8) × 10³ M⁻¹ s⁻¹, respectively). This process leads to the inactivation of two NO molecules: one being converted to HNO₂ and another being tightly bound to the ferrous heme-Fe(II) atom.

Copper detoxification machinery of the brain-eating amoeba Naegleria fowleri involves copper-translocating ATPase and the antioxidant system

Copper is a trace metal that is necessary for all organisms but toxic when present in excess. Different mechanisms to avoid copper toxicity have been reported to date in pathogenic organisms such as Cryptococcus neoformans and Candida albicans. However, little if anything is known about pathogenic protozoans despite their importance in human and veterinary medicine. Naegleria fowleri is a free-living amoeba that occurs naturally in warm fresh water and can cause a rapid and deadly brain infection called primary amoebic meningoencephalitis (PAM). Here, we describe the mechanisms employed by N. fowleri to tolerate high copper concentrations, which include various strategies such as copper efflux mediated by a copper-translocating ATPase and upregulation of the expression of antioxidant enzymes and obscure hemerythrin-like and protoglobin-like proteins. The combination of different mechanisms efficiently protects the cell and ensures its high copper tolerance, which can be advantageous both in the natural environment and in the host. Nevertheless, we demonstrate that copper ionophores are potent antiamoebic agents; thus, copper metabolism may be considered a therapeutic target.

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N. fowleri employs the combination of copper efflux and antioxidant system to ensure a high copper tolerance.

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Copper efflux in N. fowleri is mediated by a copper-translocating P-type ATPase.

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Copper ionophores have amoebicidal effect against N. fowleri and thus may be potentially used as antiamoebic agents.

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Iron-binding proteins hemerythrin and protoglobin are highly upregulated in N. fowleri under copper overload.

Dynamics based clustering of globin family members

A methodology to cluster proteins based on their dynamics’ similarity is presented. For each pair of proteins from a dataset, the structures are superimposed, and the Anisotropic Network Model modes of motions are calculated. The twelve slowest modes from each protein are matched using a local mode alignment algorithm based on the local sequence alignment algorithm of Smith–Waterman. The dynamical similarity distance matrix is calculated based on the top scoring matches of each pair and the proteins are clustered using a hierarchical clustering algorithm. The utility of this method is exemplified on a dataset of protein chains from the globin family and a dataset of tetrameric hemoglobins. The results demonstrate the effect of the quaternary structure of globin members on their intrinsic dynamics and show good ability to distinguish between different states of hemoglobin, revealing the dynamical relations between them.

Suppressor mutations reveal an NtrC-like response regulator, NmpR, for modulation of Type-IV Pili-dependent motility in Myxococcus xanthus

Two-component signaling systems (TCS) regulate bacterial responses to environmental signals through the process of protein phosphorylation. Specifically, sensor histidine kinases (SK) recognize signals and propagate the response via phosphorylation of a cognate response regulator (RR) that functions to initiate transcription of specific genes. Signaling within a single TCS is remarkably specific and cross-talk between TCS is limited. However, regulation of the flow of information through complex signaling networks that include closely related TCS remains largely unknown. Additionally, many bacteria utilize multi-component signaling networks which provide additional genetic and biochemical interactions that must be regulated for signaling fidelity, input and output specificity, and phosphorylation kinetics. Here we describe the characterization of an NtrC-like RR that participates in regulation of Type-IV pilus-dependent motility of Myxococcus xanthus and is thus named NmpR, NtrC Modulator of Pili Regulator. A complex multi-component signaling system including NmpR was revealed by suppressor mutations that restored motility to cells lacking PilR, an evolutionarily conserved RR required for expression of pilA encoding the major Type-IV pilus monomer found in many bacterial species. The system contains at least four signaling proteins: a SK with a protoglobin sensor domain (NmpU), a hybrid SK (NmpS), a phospho-sink protein (NmpT), and an NtrC-like RR (NmpR). We demonstrate that ΔpilR bypass suppressor mutations affect regulation of the NmpRSTU multi-component system, such that NmpR activation is capable of restoring expression of pilA in the absence of PilR. Our findings indicate that pilus gene expression in M. xanthus is regulated by an extended network of TCS which interact to refine control of pilus function.

Drosophila globin1 is required for maintenance of the integrity of F-actin based cytoskeleton during development

Hemoglobins (Hbs) are evolutionarily conserved small globular proteins with characteristic 3-over-3 α-helical sandwich structure that is typically known as "globin fold". Hbs have been found to be involved in diverse biological functions and the characteristic property of oxygen transportation is relatively a recent adaptation. Drosophila genome possesses three globin genes (glob1, glob2, and glob3) and it was previously reported that adequate expression of glob1 is required for various aspects of development, and also to regulate the cellular level of reactive oxygen species (ROS). The present study illustrates the explicit role of glob1 gene in Drosophila development. We demonstrate a dynamic expression pattern of glob1 in larval tissues which largely concentrate around F-actin rich structures and also co-precipitate. Reduced expression of glob1 leads to developmental abnormalities which appeared to be largely mediated by inappropriately formed F-actin based cytoskeletal structures. Our subsequent analysis in FLP/FRT mediated somatic clones establishes specific role of Drosophila glob1 in maintenance of the integrity of F-actin based cytoskeleton during development. For the first time, we report interaction between Glob1 and actin, and propose a novel role of glob1 in maintenance of F-actin based cytoskeleton in Drosophila.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

Warfarin inhibits allosterically the reductive nitrosylation of ferric human serum heme-albumin

Human serum heme-albumin (HSA-heme-Fe) displays heme-based ligand binding and (pseudo-)enzymatic properties. Here, the effect of the prototypical drug warfarin on kinetics and thermodynamics of NO binding to ferric and ferrous HSA-heme-Fe (HSA-heme-Fe(III) and HSA-heme-Fe(II), respectively) and on the NO-mediated reductive nitrosylation of the heme-Fe atom is reported; data were obtained between pH5.5 and 9.5 at 20.0°C. Since warfarin is a common drug, its effect on the reactivity of HSA-heme-Fe represents a relevant issue in the pharmacological therapy management. The inhibition of NO binding to HSA-heme-Fe(III) and HSA-heme-Fe(II) as well as of the NO-mediated reductive nitrosylation of the heme-Fe(III) atom by warfarin has been ascribed to drug binding to the fatty acid binding site 2 (FA2), shifting allosterically the penta-to-six coordination equilibrium of the heme-Fe atom toward the low reactive species showing the six-coordinated metal center by His146 and Tyr161 residues. These data: (i) support the role of HSA-heme-Fe in trapping NO, (ii) highlight the modulation of the heme-Fe-based reactivity by drugs, and (iii) could be relevant for the modulation of HSA functions by drugs in vivo.

Hydroxylamine-induced oxidation of ferrous carbonylated truncated hemoglobins from Mycobacterium tuberculosis and Campylobacter jejuni is limited by carbon monoxide dissociation

Hydroxylamine (HA) is an oxidant of ferrous globins and its action has been reported to be inhibited by CO, even though this mechanism has not been clarified. Here, kinetics of the HA-mediated oxidation of ferrous carbonylated Mycobacterium tuberculosis truncated hemoglobin N and O (Mt-trHbN(II)-CO and Mt-trHbO(II)-CO, respectively) and Campylobacter jejuni truncated hemoglobin P (Cj-trHbP(II)-CO), at pH 7.2 and 20.0 °C, are reported. Mixing Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO solution with the HA solution brings about absorption spectral changes reflecting the disappearance of the ferrous carbonylated derivatives with the concomitant formation of the ferric species. HA oxidizes irreversibly Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO with the 1:2 stoichiometry. The dissociation of CO turns out to be the rate-limiting step for the oxidation of Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO by HA. Values of the second-order rate constant for HA-mediated oxidation of Mt-trHbN(II)-CO, Mt-trHbO(II)-CO, and Cj-trHbP(II)-CO range between 8.8 × 10⁴ and 8.6 × 10⁷ M^-1 s^-1, reflecting different structural features of the heme distal pocket. This study (1) demonstrates that the inhibitory effect of CO is linked to the dissociation of this ligand, giving a functional basis to previous studies, (2) represents the first comparative investigation of the oxidation of ferrous carbonylated bacterial 2/2 globins belonging to the N, O, and P groups by HA, (3) casts light on the correlation between kinetics of HA-mediated oxidation and carbonylation of globins, and (4) focuses on structural determinants modulating the HA-induced oxidation process.

Drosophila glob1 is required for the maintenance of cytoskeletal integrity during oogenesis

BACKGROUND

Hemoglobins (Hbs) are evolutionarily conserved heme-containing metallo-proteins of the Globin protein family that harbour the characteristic "globin fold." Hemoglobins have been functionally diversified during evolution and their usual property of oxygen transport is rather a recent adaptation. Drosophila genome possesses three globin genes (glob1, glob2, and glob3), and we have reported earlier that adequate expression of glob1 is required for various aspects of development, as well as to regulate the cellular level of reactive oxygen species (ROS). The present study illustrates the explicit role of Drosophila globin1 in progression of oogenesis.

RESULTS

We demonstrate a dynamic expression pattern of glob1 in somatic and germ cell derivatives of developing egg chambers during various stages of oogenesis, which largely confines around the F-actin-rich cellular components. Reduced expression of glob1 leads to various types of abnormalities during oogenesis, which were primarily mediated by the inappropriately formed F-actin-based cytoskeleton. Our subsequent analysis in the somatic and germ line clones shows cell autonomous role of glob1 in the maintenance of the integrity of F-actin-based cytoskeleton components in the somatic and germ cell derivatives.

CONCLUSIONS

Our study establishes a novel role of glob1 in maintenance of F-actin-based cytoskeleton during progression of oogenesis in Drosophila. Developmental Dynamics 245:1048-1065, 2016. © 2016 Wiley Periodicals, Inc.

A molecule for all seasons: The heme

If life without heme-Fe were at all possible, it would definitely be different. Indeed this complex and versatile iron-porphyrin macrocycle upon binding to different “globins” yields hemeproteins crucial to sustain a variety of vital functions, generally classified, for convenience, in a limited number of functional families. Over-and-above the array of functions briefly outlined below, the spectacular progress in molecular genetics seen over the last 30 years led to the discovery of many hitherto unknown novel hemeproteins in prokaryotes and eukaryotes. Here, we highlight a few basic aspects of the chemistry of the hemeprotein universe, in particular those that are relevant to the control of heme-Fe reactivity and specialization, as sculpted by a variety of interactions with the protein moiety.

Nitrosylation mechanisms of Mycobacterium tuberculosis and Campylobacter jejuni truncated hemoglobins N, O, and P

Truncated hemoglobins (trHbs) are widely distributed in bacteria and plants and have been found in some unicellular eukaryotes. Phylogenetic analysis based on protein sequences shows that trHbs branch into three groups, designated N (or I), O (or II), and P (or III). Most trHbs are involved in the O₂/NO chemistry and/or oxidation/reduction function, permitting the survival of the microorganism in the host. Here, a detailed comparative analysis of kinetics and/or thermodynamics of (i) ferrous Mycobacterium tubertulosis trHbs N and O (Mt-trHbN and Mt-trHbO, respectively), and Campylobacter jejuni trHb (Cj-trHbP) nitrosylation, (ii) nitrite-mediated nitrosylation of ferrous Mt-trHbN, Mt-trHbO, and Cj-trHbP, and (iii) NO-based reductive nitrosylation of ferric Mt-trHbN, Mt-trHbO, and Cj-trHbP is reported. Ferrous and ferric Mt-trHbN and Cj-trHbP display a very high reactivity towards NO; however, the conversion of nitrite to NO is facilitated primarily by ferrous Mt-trHbN. Values of kinetic and/or thermodynamic parameters reflect specific trHb structural features, such as the ligand diffusion pathways to/from the heme, the heme distal pocket structure and polarity, and the ligand stabilization mechanisms. In particular, the high reactivity of Mt-trHbN and Cj-trHbP reflects the great ligand accessibility to the heme center by two protein matrix tunnels and the E7-path, respectively, and the penta-coordination of the heme-Fe atom. In contrast, the heme-Fe atom of Mt-trHbO the ligand accessibility to the heme center of Mt-trHbO needs large conformational readjustments, thus limiting the heme-based reactivity. These results agree with different roles of Mt-trHbN, Mt-trHbO, and Cj-trHbP in vivo.

Nitrite-reductase and peroxynitrite isomerization activities of Methanosarcina acetivorans protoglobin

Within the globin superfamily, protoglobins (Pgb) belong phylogenetically to the same cluster of two-domain globin-coupled sensors and single-domain sensor globins. Multiple functional roles have been postulated for Methanosarcina acetivorans Pgb (Ma-Pgb), since the detoxification of reactive nitrogen and oxygen species might co-exist with enzymatic activity(ies) to facilitate the conversion of CO to methane. Here, the nitrite-reductase and peroxynitrite isomerization activities of the CysE20Ser mutant of Ma-Pgb (Ma-Pgb*) are reported and analyzed in parallel with those of related heme-proteins. Kinetics of nitrite-reductase activity of ferrous Ma-Pgb* (Ma-Pgb*-Fe(II)) is biphasic and values of the second-order rate constant for the reduction of NO₂^– to NO and the concomitant formation of nitrosylated Ma-Pgb*-Fe(II) (Ma-Pgb*-Fe(II)-NO) are k_app1 = 9.6±0.2 M^–1 s^–1 and k_app2 = 1.2±0.1 M^–1 s^–1 (at pH 7.4 and 20°C). The k_app1 and k_app2 values increase by about one order of magnitude for each pH unit decrease, between pH 8.3 and 6.2, indicating that the reaction requires one proton. On the other hand, kinetics of peroxynitrite isomerization catalyzed by ferric Ma-Pgb* (Ma-Pgb*-Fe(III)) is monophasic and values of the second order rate constant for peroxynitrite isomerization by Ma-Pgb*-Fe(III) and of the first order rate constant for the spontaneous conversion of peroxynitrite to nitrate are h_app = 3.8×10⁴ M^–1 s^–1 and h₀ = 2.8×10^–1 s^–1 (at pH 7.4 and 20°C). The pH-dependence of h_on and h₀ values reflects the acid-base equilibrium of peroxynitrite (pK_a = 6.7 and 6.9, respectively; at 20°C), indicating that HOONO is the species that reacts preferentially with the heme-Fe(III) atom. These results highlight the potential role of Pgbs in the biosynthesis and scavenging of reactive nitrogen and oxygen species.