Myoglobin strikes back

Myoglobin in Brown Adipose Tissue: A Multifaceted Player in Thermogenesis

Brown adipose tissue (BAT) plays an important role in energy homeostasis by generating heat from chemical energy via uncoupled oxidative phosphorylation. Besides its high mitochondrial content and its exclusive expression of the uncoupling protein 1, another key feature of BAT is the high expression of myoglobin (MB), a heme-containing protein that typically binds oxygen, thereby facilitating the diffusion of the gas from cell membranes to mitochondria of muscle cells. In addition, MB also modulates nitric oxide (NO•) pools and can bind C16 and C18 fatty acids, which indicates a role in lipid metabolism. Recent studies in humans and mice implicated MB present in BAT in the regulation of lipid droplet morphology and fatty acid shuttling and composition, as well as mitochondrial oxidative metabolism. These functions suggest that MB plays an essential role in BAT energy metabolism and thermogenesis. In this review, we will discuss in detail the possible physiological roles played by MB in BAT thermogenesis along with the potential underlying molecular mechanisms and focus on the question of how BAT-MB expression is regulated and, in turn, how this globin regulates mitochondrial, lipid, and NO• metabolism. Finally, we present potential MB-mediated approaches to augment energy metabolism, which ultimately could help tackle different metabolic disorders.

Kinetic mechanisms for O2 binding to myoglobins and hemoglobins

Antonini and Brunori's 1971 book "Hemoglobin and Myoglobin in Their Reactions with Ligands" was a truly remarkable publication that summarized almost 100 years of research on O₂ binding to these globins. Over the ensuing 50 years, ultra-fast laser photolysis techniques, high-resolution and time resolved X-ray crystallography, molecular dynamics simulations, and libraries of recombinant myoglobin (Mb) and hemoglobin (Hb) variants have provided structural interpretations of O₂ binding to these proteins. The resultant mechanisms provide quantitative descriptions of the stereochemical factors that govern overall affinity, including proximal and distal steric restrictions that affect iron reactivity and favorable positive electrostatic interactions that preferentially stabilize bound O₂. The pathway for O₂ uptake and release by Mb and subunits of Hb has been mapped by screening libraries of site-directed mutants in laser photolysis experiments. O₂ enters mammalian Mb and the α and β subunits of human HbA through a channel created by upward and outward rotation of the distal His at the E7 helical position, is non-covalently captured in the interior of the distal cavity, and then internally forms a bond with the heme Fe(II) atom. O₂ dissociation is governed by disruption of hydrogen bonding interactions with His (E7), breakage of the Fe(II)-O₂ bond, and then competition between rebinding and escape through the E7-gate. The structural features that govern the rates of both the individual steps and overall reactions have been determined and provide the framework for: (1) defining the physiological functions of specific globins and their evolution; (2) understanding the clinical features of hemoglobinopathies; and (3) designing safer and more efficient acellular hemoglobin-based oxygen carriers (HBOCs) for transfusion therapy, organ preservation, and other commercially relevant O₂ transport and storage processes.

Ca2+ as activator of pseudoperoxidase activity of pigeon, Eurasian woodcock and chicken myoglobins: New features for meat preservation studies

Myoglobin (Mb), hemeprotein that binds dioxygen in muscle, affects meat colour. Moreover, in presence of peroxides, metMb is a potent oxidant involved in oxidative rancidity in meat. Here, following pigeon Mb purification and primary structure mass spectroscopy characterization, we determined its autoxidation rate and pseudoperoxidase activity with respect to chicken and E. woodcock Mbs. The three Mbs exhibit different autoxidation rates (0.153-h^-1 pigeon, 0.194-h^-1 chicken and 0.220-h^-1 E. woodcock Mbs) and similar specificity constant (9.86x10³ M^-1s^-1 pigeon, 8.81x10³ M^-1s^-1 chicken and 9.90x10³ M^-1s^-1 E. woodcock Mbs), considering their pseudoperoxidase activity. Moreover, for the first time, we detected an increase in pseudoperoxidase activity in presence of Ca²⁺, particularly at pH 5.8. NMR and CD data indicate that the nonspecific Ca²⁺ binding induces small local structural rearrangements that in turn slightly reduce pigeon Mb thermal stability. However, considering Ca²⁺ concentration variations before and post-mortem, this finding must be considered for meat preservation.

Formation of Myoglobin Corona at Polymer Microparticles

Adsorption of myoglobin molecules at negatively charged polystyrene microparticles was studied using the dynamic light scattering (DLS), electrophoresis (LDV) and the solution depletion method involving atomic force microscopy (AFM). The measurements were carried out at pH 3.5 and NaCl concentration of 10−2 and 0.15 M. Initially, the stability of myoglobin solutions and the particle suspensions as a function of pH were determined. Afterward, the formation of myoglobin molecule corona was investigated via the direct electrophoretic mobility measurements, which were converted to the zeta potential. The experimental results were quantitatively interpreted in terms of the general electrokinetic model. This approach yielded the myoglobin corona coverage under in situ conditions. The maximum hard corona coverage was determined using the AFM concentration depletion method. It was equal to 0.9 mg m−2 for the NaCl concentration in the range 0.01 to 0.15 M and pH 3.5. The electrokinetic properties of the corona were investigated using the electrophoretic mobility measurements for a broad pH range. The obtained results confirmed that thorough physicochemical characteristics of myoglobin molecules can be acquired using nM amounts of the protein. It was also argued that this method can be used for performing electrokinetic characteristics of other proteins such as the SARS-Cov-2 spike protein exhibiting, analogously to myoglobin, a positive charge at acidic pHs.

Mechanism of Myoglobin Molecule Adsorption on Silica: QCM, OWLS and AFM Investigations

Adsorption kinetics of myoglobin on silica was investigated using the quartz crystal microbalance (QCM) and the optical waveguide light-mode spectroscopy (OWLS). Measurements were carried out for the NaCl concentration of 0.01 M and 0.15 M. A quantitative analysis of the kinetic adsorption and desorption runs acquired from QCM allowed to determine the maximum coverage of irreversibly bound myoglobin molecules. At a pH of 3.5-4 this was equal to 0.60 mg m^-2 and 1.3 mg m^-2 for a NaCl concentration of 0.01 M and 0.15 M, respectively, which agrees with the OWLS measurements. The latter value corresponds to the closely packed monolayer of molecules predicted from the random sequential adsorption approach. The fraction of reversibly bound protein molecules and their biding energy were also determined. It is observed that at larger pHs, the myoglobin adsorption kinetics was much slower. This behavior was attributed to the vanishing net charge that decreased the binding energy of molecules with the substrate. These results can be exploited to develop procedures for preparing myoglobin layers at silica substrates of well-controlled coverage useful for biosensing purposes.

Adsorption kinetic of myoglobin on mica and silica - Role of electrostatic interactions

Adsorption kinetics of myoglobin molecules on mica and silica was studied using the atomic force microscopy (AFM), the colloid enhancement and the quartz microbalance (QCM) methods. Measurements were carried out for the NaCl concentration of 0.01 and 0.15 M as a function of pH comprising pH 7.4 stabilized by the PBS buffer. The electrophoretic mobility measurements enabled to derive the molecules zeta potential as a function of pH. The isoelectric point appearing at pH 5, is lower than that predicted from the theoretical calculations of the nominal dissociation charge. The AFM investigations confirmed that myoglobin molecules irreversibly adsorb at pH 3.5 yielding well-defined layers of single molecules. These layers were characterized using the colloid enhancement method involving polymer microparticles for pH range 3-9. The microparticle deposition kinetics was adequately interpreted in terms of a hybrid random sequential adsorption model. It is confirmed that the myoglobin layers exhibit a negligible zeta potential at pH equal to 5 in accordance with the electrophoretic mobility measurements. Analogous adsorption kinetic measurements were performed for the silica substrate using QCM and AFM. It is observed that myoglobin molecules irreversibly adsorb at pH 3.5 forming stable layers of single molecules. On the other hand, its adsorption kinetics at larger pHs was much slower exhibiting a poorly defined maximum coverage. This was attributed to aggregation of the myoglobin solutions due to their vanishing charge. The kinetic QCM runs were adequately interpreted in terms of a theoretical model combining the Smoluchowski aggregation theory with the convective diffusion mass transfer theory.

Myoglobin molecule charging in electrolyte solutions

The compensated charge of myoglobin molecule in electrolyte solution is considerably smaller than the nominal charge.

Dynamic multiple-scattering treatment of X-ray absorption: Parameterization of a new molecular dynamics force field for myoglobin

We present a detailed analysis of the X-ray absorption near-edge structure (XANES) data on the Fe K-edge of CO Myoglobin based on a combined procedure of Molecular Dynamics (MD) calculations and MXAN (Minuit XANes) data analysis that we call D-MXAN. The ability of performing quantitative XANES data analysis allows us to refine classical force field MD parameters, thus obtaining a reliable tool for the atomic investigation of this important model system for biological macromolecules. The iterative procedure here applied corrects the greatest part of the structural discrepancy between classical MD sampling and experimental determinations. Our procedure, moreover, is able to discriminate between different heme conformational basins visited during the MD simulation, thus demonstrating the necessity of a sampling on the order of tens of nanoseconds, even for an application such X-ray absorption spectroscopy data analysis.

Insight into the phylogenetic relationship and structural features of vertebrate myoglobin family

Myoglobin (Mb) is studied to clarify the structure-function relationships in protein science. In this work, we report the results of a comparative analysis of amino acid sequences from 298 vertebrate Mbs. Forty-one high conserved residues were identified and seven of them were invariants [E18, G25, F43, V68, L72, H93 (proximal histidine) and H97]. E18 is the only invariant amino acid residue located out of the heme-pocket and Xe-cavities playing a role in interaction between the A and E-helices. A comparative analysis of several parameters related to amino acid composition shows an increase of average mass, accessible surface area and volume per residue from Actinopterygii to Mammalia and Aves. This may be due to an increased number of bulky residues reducing the non-specific cavities volume and thus improving the oxygen flow between the heme site and the outside of the protein. Finally, the phylogenetic analyses of Mb in vertebrates are consistent with an evolution that runs with the diversification of the species, but in which several episodes of gene duplication and lost have occurred, less frequently in the ancestors of great taxons, cartilaginous fishes and non-avian reptiles, most frequently in ray-finned fishes and mammals, and very frequently in birds.

Direct observation of subpicosecond vibrational dynamics in photoexcited myoglobin

Determining the initial pathway for ultrafast energy redistribution within biomolecules is a challenge, and haem proteins, for which energy can be deposited locally in the haem moiety using short light pulses, are suitable model systems to address this issue. However, data acquired using existing experimental techniques that fail to combine sufficient structural sensitivity with adequate time resolution have resulted in alternative hypotheses concerning the interplay between energy flow among highly excited vibrational levels and potential concomitant electronic processes. By developing a femtosecond-stimulated Raman set-up, endowed with the necessary tunability to take advantage of different resonance conditions, here we visualize the temporal evolution of energy redistribution over different vibrational modes in myoglobin. We establish that the vibrational energy initially stored in the highly excited Franck-Condon manifold is transferred with different timescales into low- and high-frequency modes, prior to slow dissipation through the protein. These findings demonstrate that a newly proposed mechanism involving the population dynamics of specific vibrational modes settles the controversy on the existence of transient electronic intermediates.

Sulfmyoglobin Conformational Change: A Role in the Decrease of Oxy-Myoglobin Functionality

This work is focused at understanding the interaction of H2S with Myoglobin (Mb), in particular the Sulfmyoglobin (SMb) product, whose physiological role is controversial and not well understood. The scattering curves, Guinier, Kratky, Porod and P(r) plots were analyzed for oxy-Mb and oxy-Hemoglobin I (oxyHbI) in the absence and presence of H2S, using Small and Wide Angle X-ray Scattering (SAXS/WAXS) technique. Three dimensional models were also generated from the SAXS/WAXS data. The results show that SMb formation, produced by oxyMb and H2S interaction, induces a change in the protein conformation where its envelope has a very small cleft and the protein is more flexible, less rigid and compact. Based on the direct relationship between Mb's structural conformation and its functionality, we suggest that the conformational change observed upon SMb formation plays a contribution to the protein decrease in O2 affinity and, therefore, on its functionality.

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.

Benchmarking Rapid TLES Simulations of Gas Diffusion in Proteins: Mapping O2 Migration and Escape in Myoglobin as a Case Study

Standard molecular dynamics (MD) simulations of gas diffusion consume considerable computational time and resources even for small proteins. To combat this, temperature-controlled locally enhanced sampling (TLES) examines multiple diffusion trajectories per simulation by accommodating multiple noninteracting copies of a gas molecule that diffuse independently, while the protein and water molecules experience an average interaction from all copies. Furthermore, gas migration within a protein matrix can be accelerated without altering protein dynamics by increasing the effective temperature of the TLES copies. These features of TLES enable rapid simulations of gas diffusion within a protein matrix at significantly reduced (∼98%) computational cost. However, the results of TLES and standard MD simulations have not been systematically compared, which limits the adoption of the TLES approach. We address this drawback here by benchmarking TLES against standard MD in the simulation of O2 diffusion in myoglobin (Mb) as a case study since this model system has been extensively characterized. We find that 2 ns TLES and 108 ns standard simulations map the same network of diffusion tunnels in Mb and uncover the same docking sites, barriers, and escape portals. We further discuss the influence of simulation time as well as the number of independent simulations on the O2 population density within the diffusion tunnels and on the sampling of Mb's conformational space as revealed by principal component analysis. Overall, our comprehensive benchmarking reveals that TLES is an appropriate and robust tool for the rapid mapping of gas diffusion in proteins when the kinetic data provided by standard MD are not required. Furthermore, TLES provides explicit ligand diffusion pathways, unlike most rapid methods.

Exploring the mechanisms of the reductase activity of neuroglobin by site-directed mutagenesis of the heme distal pocket

Neuroglobin (Ngb) is a six-coordinate globin that can catalyze the reduction of nitrite to nitric oxide. Although this reaction is common to heme proteins, the molecular interactions in the heme pocket that regulate this reaction are largely unknown. We have shown that the H64L Ngb mutation increases the rate of nitrite reduction by 2000-fold compared to that of wild-type Ngb [Tiso, M., et al. (2011) J. Biol. Chem. 286, 18277–18289]. Here we explore the effect of distal heme pocket mutations on nitrite reduction. For this purpose, we have generated mutations of Ngb residues Phe28(B10), His64(E7), and Val68(E11). Our results indicate a dichotomy in the reactivity of deoxy five- and six-coordinate globins toward nitrite. In hemoglobin and myoglobin, there is a correlation between faster rates and more negative potentials. However, in Ngb, reaction rates are apparently related to the distal pocket volume, and redox potential shows a poor relationship with the rate constants. This suggests a relationship between the nitrite reduction rate and heme accessibility in Ngb, particularly marked for His64(E7) mutants. In five-coordinate globins, His(E7) facilitates nitrite reduction, likely through proton donation. Conversely, in Ngb, the reduction mechanism does not rely on the delivery of a proton from the histidine side chain, as His64 mutants show the fastest reduction rates. In fact, the rate observed for H64A Ngb (1120 M^–1 s^–1) is to the best of our knowledge the fastest reported for a heme nitrite reductase. These differences may be related to a differential stabilization of the iron–nitrite complexes in five- and six-coordinate globins.

Insight into the structural and functional features of myoglobin from Hystrix cristata L. and Rangifer tarandus L

Myoglobins (Mbs) from Hystrix cristata L. and Rangifer tarandus L. have been isolated and characterised.

Long-range electron transfer with myoglobin immobilized at Au/mixed-SAM junctions: mechanistic impact of the strong protein confinement

Horse muscle myoglobin (Mb) was tightly immobilized at Au-deposited ~15-Å-thick mixed-type (1:1) alkanethiol SAMs, HS-(CH₂)₁₁-COOH/HS-(CH₂)₁₁-OH, and placed in contact with buffered H₂O or D₂O solutions. Fast-scan cyclic voltammetry (CV) and a Marcus-equation-based analysis were applied to determine unimolecular standard rate constants and reorganization free energies for electron transfer (ET), under variable-temperature (15-55 °C) and -pressure (0.01-150 MPa) conditions. The CV signal was surprisingly stable and reproducible even after multiple temperature and pressure cycles. The data analysis revealed the following values: standard rate constant, 33 s⁻¹ (25 °C, 0.01 MPa, H₂O); reorganization free energy, 0.5 ± 0.1 eV (throughout); activation enthalpy, 12 ± 3 kJ mol⁻¹; activation volume, -3.1 ± 0.2 cm³ mol⁻¹; and pH-dependent solvent kinetic isotope effect (k(H)⁰/k(D)⁰), 0.7-1.4. Furthermore, the values for the rate constant and reorganization free energy are very similar to those previously found for cytochrome c electrostatically immobilized at the monocomponent Au/HS-(CH₂)₁₁-COOH junction. In vivo, Mb apparently forms a natural electrostatic complex with cytochrome b₅ (cyt-b₅) through the "dynamic" (loose) docking pattern, allowing for a slow ET that is intrinsically coupled to the water's removal from the "defective" heme iron (altogether shaping the biological repair mechanism for Mb's "met" form). In contrary, our experiments rather mimic the case of a "simple" (tight) docking of the redesigned (mutant) Mb with cyt-b₅ (Nocek et al. J. Am. Chem. Soc. 2010, 132, 6165-6175). According to our analysis, in this configuration, Mb's distal pocket (linked to the "ligand channel") seems to be arrested within the restricted configuration, allowing the rate-determining reversible ET process to be coupled only to the inner-sphere reorganization (minimal elongation/shortening of an Fe-OH₂ bond) rather than the pronounced detachment (rebinding) of water and, hence, to be much faster.

Structural characterization of carangid fish myoglobins

The primary structures of myoglobin (Mb) from the following five carangid species were determined: yellowtail Seriola quinqueradiata, greater amberjack Seriola dumerili, yellowtail kingfish Seriola lalandi, Japanese horse mackerel Trachurus japonicus, and silver trevally Pseudocaranx dentex. The sequences were of varying composition both in the coding and in the noncoding regions, but all contained the open reading frame of 444 nucleotides encoding 147 amino acids. Amino acid sequence identities of carangid Mbs were in the range of 81-99%. The similarity of the heme pocket and associated heme-binding residues of carangid Mbs were evidence of the conservative nature of Mbs. Similar to the other teleost Mbs, carangid Mbs did not contain a D helix and had mostly conserved A and E helices as well as E-F and G-H inter-helical segments. Hydropathy profiles of carangid Mbs showed species-specific variations where silver trevally Mb exhibited generally higher hydrophobicity. Phylogenetic analysis based on the primary structures was in agreement with conventional morphological taxonomy, establishing close proximity of carangid Mbs with those of cichlid and scombroid, the other members of the Perciformes order.

Femtosecond stimulated Raman spectrometer in the 320-520nm range

Multi-µJ narrow-bandwidth (≈ 10 cm(-1)) picosecond pulses, broadly tunable in the visible-UV range (320-520 nm), are generated by spectral compression of femtosecond pulses emitted by an amplified Ti:sapphire system. Such pulses provide the ideal Raman pump for broadband femtosecond stimulated Raman spectroscopy, as here demonstrated on a heme protein.