Circular dichroism spectra of human hemoglobin reveal a reversible structural transition at body temperature

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.

The Effect of Substrate Temperature on the Evaporative Behaviour and Desiccation Patterns of Foetal Bovine Serum Drops

The drying of bio-fluid drops results in the formation of complex patterns, which are morphologically and topographically affected by environmental conditions including temperature. We examine the effect of substrate temperatures between 20 °C and 40 °C, on the evaporative dynamics and dried deposits of foetal bovine serum (FBS) drops. The deposits consist of four zones: a peripheral protein ring, a zone of protein structures, a protein gel, and a central crystalline zone. We investigate the link between the evaporative behaviour, final deposit volume, and cracking. Drops dried at higher substrate temperatures in the range of 20 °C to 35 °C produce deposits of lower final volume. We attribute this to a lower water content and a more brittle gel in the deposits formed at higher temperatures. However, the average deposit volume is higher for drops dried at 40 °C compared to drops dried at 35 °C, indicating protein denaturation. Focusing on the protein ring, we show that the ring volume decreases with increasing temperature from 20 °C to 35 °C, whereas the number of cracks increases due to faster water evaporation. Interestingly, for deposits of drops dried at 40 °C, the ring volume increases, but the number of cracks also increases, suggesting an interplay between water evaporation and increasing strain in the deposits due to protein denaturation.

Sphingomyelin-induced structural modification of native human hemoglobin and its chemically and thermally disrupted secondary structure: A photophysical exploration

Sphingomyelin-induced structural modification of Human Hemoglobin (Hb) has been investigated in its native and unfolded conformers that are partially denatured in presence of ∼ 4 M urea, completely denatured in ∼ 8 M urea and thermally disrupted (at ∼ 65 °C) state. The absorption studies unveil ground state complexation between Hb and SM. From steady-state fluorescence and quenching studies alteration of the micro-environments around Trp residues of Hb in above mentioned different cases has been determined. Moreover, lesser exposure of Trp residues to SM in thermally disrupted Hb can be accounted for the exceptionally interesting outcomes in other experiments. The alterations in the time-resolved decay profiles of native Hb, partially and totally chemically denatured as well as thermally disrupted Hb with gradual addition of SM also affirm the amendment of the proteinous micro-environment surrounding Trp residues in a view of FRET between Trp residues and heme group. Wavelength-sensitive emission spectral studies reveal that the protein shows red edge effect in its different conformations in presence and absence of SM. Interestingly, the wavelength-responsive time-resolved study at a constant excitation wavelength demonstrates that with addition of lipid the increment of the average fluorescence lifetime signifies a considerable modulation of solvation dynamics of the fluorescent Trp residues in their excited state being greatest in case of thermally disrupted Hb. Nevertheless, the loss of α-helicity of Hb at its various conformers with addition of SM has been portrayed thoroughly by means of far-UV CD spectral studies in a view of disruption of secondary structure of the protein.

The thermal and storage stability of bovine haemoglobin by ultraviolet-visible and circular dichroism spectroscopies

The effects of temperature, pH and long-term storage on the secondary structure and conformation changes of bovine haemoglobin (bHb) were studied using circular dichroism (CD) and ultraviolet--visible (UV–vis) spectroscopies. Neural network software was used to deconvolute the CD data to obtain the fractional content of the five secondary structures. The storage stability of bHb solutions in pH 6, 7 and 8 buffers was significantly higher at 4 °C than at 23 °C for the first 3 days. A complete denaturation of bHb was observed after 40 days irrespective of storage temperature or pH. The bHb solutions were also exposed to heating and cooling cycles between 25 and 65 °C and structural changes were followed by UV–vis and CD spectroscopies. These experiments demonstrated that α-helix content of bHb decreased steadily with the increasing temperature above 35 °C at all pH values. The loss in α-helicity and gain in random coil conformations was pH-dependent and the greatest under alkaline conditions. Furthermore, there was minimal recovery of the secondary structure content upon cooling to 25 °C. The use of bHb as a model drug is very common and this study elucidates the significance of storage and processing conditions on its stability.

Thermal-induced force release in oxyhemoglobin

Oxygen is released to living tissues via conformational changes of hemoglobin from R-state (oxyhemoglobin) to T-state (desoxyhemoglobin). The detailed mechanism of this process is not yet fully understood. We have carried out micromechanical experiments on oxyhemoglobin crystals to determine the behavior of the Young’s modulus and the internal friction for temperatures between 20 °C and 70 °C. We have found that around 49 °C oxyhemoglobin crystal samples undergo a sudden and strong increase of their Young’s modulus, accompanied by a sudden decrease of the internal friction. This sudden mechanical change (and the ensuing force release) takes place in a partially unfolded state and precedes the full denaturation transition at higher temperatures. After this transformation, the hemoglobin crystals have the same mechanical properties as their initial state at room temperatures. We conjecture that it can be relevant for explaining the oxygen-releasing function of native oxyhemoglobin when the temperature is increased, e.g. due to active sport. The effect is specific for the quaternary structure of hemoglobin, and is absent for myoglobin with only one peptide sequence.

Investigation on the conformation change of hemoglobin immobilized on MPA-modified electrode by electrochemical method

The conformation change of bovine hemoglobin (Hb) during the unfolding process induced by urea and acid was investigated by an electrochemical method. Hb unfolding induced by urea of different concentrations was realized by bonding Hb onto a 3-mercaptopropionic acid (MPA) modified gold electrode. The difference in unfolding percentage showed that the Hb unfolding induced by urea was a two-step, three-state transition process, while the unfolding induced by acid was a two-state transition process. The results obtained by the electrochemical method coincided closely with those obtained by UV-vis spectroscopy and fluorescence spectroscopy. Some thermodynamic parameters during the conformational change were also calculated to study the intermediate state during the Hb unfolding process. The present work may lead to an easy and effective way to study metalloproteins unfolding, and holds great promise for the design of novel sensitive biosensors.

Effects of spermine NONOate and ATP on protein aggregation: light scattering evidences

BACKGROUND AND OBJECTIVE

Regulating protein function in the cell by small molecules, provide a rapid, reversible and tunable tool of metabolic control. However, due to its complexity the issue is poorly studied so far. The effects of small solutes on protein behavior can be studied by examining changes of protein secondary structure, in its hydrodynamic radius as well as its thermal aggregation. The study aim was to investigate effects of adenosine-5'-triphosphate (ATP), spermine NONOate (NO donor) as well as sodium/potassium ions on thermal aggregation of albumin and hemoglobin. To follow aggregation of the proteins, their diffusion coefficients were measured by quasi-elastic light scattering (QELS) at constant pH (7.4) in the presence of solutes over a temperature range from 25°C to 80°C.

RESULTS AND DISCUSSION

1) Spermine NONOate persistently decreased the hemoglobin aggregation temperature Tairrespectively of the Na+/K+ environment, 2) ATP alone had no effect on the protein's thermal stability but it facilitated protein's destabilization in the presence of spermine NONOate and 3) mutual effects of ATP and NO were strongly influenced by particular buffer ionic compositions.

CONCLUSION

The ATP effect on protein aggregation was ambiguous: ATP alone had no effect on the protein's thermal stability but it facilitated protein's destabilization in the presence of nitric oxide. The magnitude and direction of the observed effects strongly depended on concentrations of K+ and Na+ in the solution.

Effects of spermine NONOate and ATP on the thermal stability of hemoglobin

Background

Minor changes in protein structure induced by small organic and inorganic molecules can result in significant metabolic effects. The effects can be even more profound if the molecular players are chemically active and present in the cell in considerable amounts. The aim of our study was to investigate effects of a nitric oxide donor (spermine NONOate), ATP and sodium/potassium environment on the dynamics of thermal unfolding of human hemoglobin (Hb). The effect of these molecules was examined by means of circular dichroism spectrometry (CD) in the temperature range between 25°C and 70°C. The alpha-helical content of buffered hemoglobin samples (0.1 mg/ml) was estimated via ellipticity change measurements at a heating rate of 1°C/min.

Results

Major results were: 1) spermine NONOate persistently decreased the hemoglobin unfolding temperature _Tuirrespectively of the ^{Na +} /^{K +} environment, 2) ATP instead increased the unfolding temperature by 3°C in both sodium-based and potassium-based buffers and 3) mutual effects of ATP and NO were strongly influenced by particular buffer ionic compositions. Moreover, the presence of potassium facilitated a partial unfolding of alpha-helical structures even at room temperature.

Conclusion

The obtained data might shed more light on molecular mechanisms and biophysics involved in the regulation of protein activity by small solutes in the cell.

Thermal fluctuations of haemoglobin from different species: adaptation to temperature via conformational dynamics

Thermodynamic stability, configurational motions and internal forces of haemoglobin (Hb) of three endotherms (platypus, Ornithorhynchus anatinus; domestic chicken, Gallus gallus domesticus and human, Homo sapiens) and an ectotherm (salt water crocodile, Crocodylus porosus) were investigated using circular dichroism, incoherent elastic neutron scattering and coarse-grained Brownian dynamics simulations. The experimental results from Hb solutions revealed a direct correlation between protein resilience, melting temperature and average body temperature of the different species on the 0.1 ns time scale. Molecular forces appeared to be adapted to permit conformational fluctuations with a root mean square displacement close to 1.2 Å at the corresponding average body temperature of the endotherms. Strong forces within crocodile Hb maintain the amplitudes of motion within a narrow limit over the entire temperature range in which the animal lives. In fully hydrated powder samples of human and chicken, Hb mean square displacements and effective force constants on the 1 ns time scale showed no differences over the whole temperature range from 10 to 300 K, in contrast to the solution case. A complementary result of the study, therefore, is that one hydration layer is not sufficient to activate all conformational fluctuations of Hb in the pico- to nanosecond time scale which might be relevant for biological function. Coarse-grained Brownian dynamics simulations permitted to explore residue-specific effects. They indicated that temperature sensing of human and chicken Hb occurs mainly at residues lining internal cavities in the β-subunits.

Macromolecular dynamics in red blood cells investigated using neutron spectroscopy

We present neutron scattering measurements on the dynamics of haemoglobin (Hb) in human red blood cells (RBCs) in vivo. Global and internal Hb dynamics were measured in the ps to ns time and Å length scales using quasi-elastic neutron backscattering spectroscopy. We observed the cross over from global Hb short-time to long-time self-diffusion. Both short- and long-time diffusion coefficients agree quantitatively with predicted values from the hydrodynamic theory of non-charged hard-sphere suspensions when a bound water fraction of around 0.23 gram H(2)O per gram Hb is taken into account. The higher amount of water in the cells facilitates internal protein fluctuations in the ps time scale when compared with fully hydrated Hb powder. Slower internal dynamics of Hb in RBCs in the ns time range were found to be rather similar to results obtained with fully hydrated protein powders, solutions and Escherichia coli cells.

Dynamics in Biological Systems as seen by QENS

Quasielastic incoherent neutron scattering is a well suited and established experimental method to study protein and water dynamics in the picosecond to nanosecond time- and Ångstrom length-scale. Using deuterium labelling either protein or water motions can be selected and brought into focus. Protein and cell water dynamics were separately studied in red blood cells. A consistent picture of cytoplasmic water and protein dynamics in whole cells is emerging from recent experimental results.

From powder to solution: hydration dependence of human hemoglobin dynamics correlated to body temperature

A transition in hemoglobin (Hb), involving partial unfolding and aggregation, has been shown previously by various biophysical methods. The correlation between the transition temperature and body temperature for Hb from different species, suggested that it might be significant for biological function. To focus on such biologically relevant human Hb dynamics, we studied the protein internal picosecond motions as a response to hydration, by elastic and quasielastic neutron scattering. Rates of fast diffusive motions were found to be significantly enhanced with increasing hydration from fully hydrated powder to concentrated Hb solution. In concentrated protein solution, the data showed that amino acid side chains can explore larger volumes above body temperature than expected from normal temperature dependence. The body temperature transition in protein dynamics was absent in fully hydrated powder, indicating that picosecond protein dynamics responsible for the transition is activated only at a sufficient level of hydration. A collateral result from the study is that fully hydrated protein powder samples do not accurately describe all aspects of protein picosecond dynamics that might be necessary for biological function.

Hemoglobin senses body temperature

When aspirating human red blood cells (RBCs) into 1.3 mum pipettes (DeltaP = -2.3 kPa), a transition from blocking the pipette below a critical temperature T(c) = 36.3 +/- 0.3 degrees C to passing it above the T(c) occurred (micropipette passage transition). With a 1.1 mum pipette no passage was seen which enabled RBC volume measurements also above T(c). With increasing temperature RBCs lost volume significantly faster below than above a T(c) = 36.4 +/- 0.7 (volume transition). Colloid osmotic pressure (COP) measurements of RBCs in autologous plasma (25 degrees C < or = T < or = 39.5 degrees C) showed a T (c) at 37.1 +/- 0.2 degrees C above which the COP rapidly decreased (COP transition). In NMR T(1)-relaxation time measurements, the T(1) of RBCs in autologous plasma changed from a linear (r = 0.99) increment below T(c) = 37 +/- 1 degrees C at a rate of 0.023 s/K into zero slope above T(c) (RBC T(1) transition).

IN CONCLUSION

An amorphous hemoglobin-water gel formed in the spherical trail, the residual partial sphere of the aspirated RBC. At T(c), a sudden fluidization of the gel occurs. All changes mentioned above happen at a distinct T(c) close to body temperature. The T(c) is moved +0.8 degrees C to higher temperatures when a D(2)O buffer is used. We suggest a mechanism similar to a "glass transition" or a "colloidal phase transition". At T(c), the stabilizing Hb bound water molecules reach a threshold number enabling a partial Hb unfolding. Thus, Hb senses body temperature which must be inscribed in the primary structure of hemoglobin and possibly other proteins.

Hemoglobin dynamics in red blood cells: correlation to body temperature

A transition in hemoglobin behavior at close to body temperature has been discovered recently by micropipette aspiration experiments on single red blood cells (RBCs) and circular dichroism spectroscopy on hemoglobin solutions. The transition temperature was directly correlated to the body temperatures of a variety of species. In an exploration of the molecular basis for the transition, we present neutron scattering measurements of the temperature dependence of hemoglobin dynamics in whole human RBCs in vivo. The data reveal a change in the geometry of internal protein motions at 36.9 degrees C, at human body temperature. Above that temperature, amino acid side-chain motions occupy larger volumes than expected from normal temperature dependence, indicating partial unfolding of the protein. Global protein diffusion in RBCs was also measured and the findings compared favorably with theoretical predictions for short-time self-diffusion of noncharged hard-sphere colloids. The results demonstrated that changes in molecular dynamics in the picosecond time range and angstrom length scale might well be connected to a macroscopic effect on whole RBCs that occurs at body temperature.

Molecular processes in biological thermosensation

Since thermal gradients are almost everywhere, thermosensation could represent one of the oldest sensory transduction processes that evolved in organisms. There are many examples of temperature changes affecting the physiology of living cells. Almost all classes of biological macromolecules in a cell (nucleic acids, lipids, proteins) can present a target of the temperature-related stimuli. This review discusses some features of different classes of temperature-sensing molecules as well as molecular and biological processes that involve thermosensation. Biochemical, structural, and thermodynamic approaches are applied in the paper to organize the existing knowledge on molecular mechanisms of thermosensation. Special attention is paid to the fact that thermosensitive function cannot be assigned to any particular functional group or spatial structure but is rather of universal nature. For instance, the complex of thermodynamic, structural, and functional features of hemoglobin family proteins suggests their possible accessory role as “molecular thermometers”.

Structural transition temperature of hemoglobins correlates with species' body temperature

Human red blood cells (RBCs) exhibit sudden changes in their biophysical properties at body temperature (T (B)). RBCs were seen to undergo a spontaneous transition from blockage to passage at T (C) = 36.4 +/- 0.3 degrees C, when the temperature dependency of RBC-passages through 1.3 mum narrow micropipettes was observed. Moreover, concentrated hemoglobin solutions (45 g/dl) showed a viscosity breakdown between 36 and 37 degrees C. With human hemoglobin, a structural transition was observed at T (B) as circular dichroism (CD) experiments revealed. This leads to the assumption that a species' body temperature occupies a unique position on the temperature scale and may even be imprinted in the structure of certain proteins. In this study, it was investigated whether hemoglobins of species with a T (B) different from those of human show temperature transitions and whether those were also linked to the species' T (B). The main conclusion was drawn from dynamic light scattering (DLS) and CD experiments. It was observed that such structural temperature transitions did occur in hemoglobins from all studied species and were correlated linearly (slope 0.81, r = 0.95) with the species' body temperature. We presumed that alpha-helices of hemoglobin were able to unfold more readily around T (B). alpha-helical unfolding would initiate molecular aggregation causing RBC passage and viscosity breakdown as mentioned above. Thus, structural molecular changes of hemoglobin could determine biophysical effects visible on a macroscopic scale. It is hypothesized that the species' body temperature was imprinted into the structure of hemoglobins.

Body temperature-related structural transitions of monotremal and human hemoglobin

In this study, temperature-related structural changes were investigated in human, duck-billed platypus (Ornithorhynchus anatinus, body temperature T(b) = 31-33 degrees C), and echidna (Tachyglossus aculeatus, body temperature T(b) = 32-33 degrees C) hemoglobin using circular dichroism spectroscopy and dynamic light scattering. The average hydrodynamic radius (R(h)) and fractional (normalized) change in the ellipticity (F(obs)) at 222 +/- 2 nm of hemoglobin were measured. The temperature was varied stepwise from 25 degrees C to 45 degrees C. The existence of a structural transition of human hemoglobin at the critical temperature T(c) between 36-37 degrees C was previously shown by micropipette aspiration experiments, viscosimetry, and circular dichroism spectroscopy. Based on light-scattering measurements, this study proves the onset of molecular aggregation at T(c). In two different monotremal hemoglobins (echidna and platypus), the critical transition temperatures were found between 32-33 degrees C, which are close to the species' body temperature T(b). The data suggest that the correlation of the structural transition's critical temperature T(c) and the species' body temperature T(b) is not mere coincidence but, instead, is a more widespread structural phenomenon possibly including many other proteins.