pH effect upon HbGp oligomeric stability: characterization of the dissociated species by AUC and DLS studies

Heat-induced agglomeration of water-soluble cod proteins toward gelled structures

Cod proteins (CPs) have potential applications in designing desirable gel-based products, and this study aimed to unravel their heat-induced aggregation pattern and further probe the roles in protein gels. SDS-PAGE analysis indicated that high-precipitation-coefficient aggregates (HPCAs) of CPs aggregates were composed of considerable polymers of myosin heavy chains and actin, and their low-precipitation-coefficient aggregates (LPCAs) contained myosin light chains and tropomyosin. Studies from correlation analysis between the structure and aggregation kinetics revealed that the generation of β-sheet and SS bonds were responsible for their spontaneous thermal aggregation induced by heating temperature and protein concentration, respectively. Additionally, as protein denaturation ratio increased, more and larger HPCAs were formed, which was evidenced driving the network formation of protein gels and resulting in higher storage modulus (G') values. These novel findings may be applicable to other animal proteins for better tailoring the manufacturing of muscle gel-based products.

Interactional, Functional and Biological Properties of Lactone Sophorolipid (LSL) and Collagen Oligopeptides (COP) in Aqueous Solution

For the mixed aqueous solution of LSL and COP, the interaction mode and mechanism have been comprehensively studied using multispectral methods including fluorescence spectrum, ultraviolet-visible adsorption spectrum (UV-Vis), and circular dichroism spectrum (CD). Then its surface activity, particle size, foaming, emulsifying, viscosity, and antibacterial properties are evaluated in detail by surface tension measurement (ST), dynamic light scattering (DLS), oscillametric method, spectrophotometer, ubbelohde viscometer and zone of inhibition separately. Compared with the single LSL or COP aqueous solution, the mixed system shows different performance optimizations in different aspects. The surface activity and foaming properties are mainly attributed to LSL, and the viscosity is attributed to COP. Fluorescence spectroscopy results show that the fluorescence distribution of COP has significant changes by the LSL addition and a static quenching mechanism is proved. The results of UV-Vis and CD spectra also show the changing conformation of COP by the LSL addition. The data of thermodynamic parameters prove that the combination of LSL and COP is a spontaneous exothermic process and is an enthalpy-driven process. The interaction mechanism between LSL and COP is very helpful for the application and development of the mixed mild biosurfactant-protein system used in the cosmetic and food industries.

Biorecognition of hydrogen peroxide using a novel electrochemical platform designed with Glossoscolex paulistus giant hemoglobin

The giant extracellular hemoglobin of the annelid Glossoscolex paulistus (HbGp; 3.6 MDa) is a valuable and underexplored supramolecular hemoprotein system for the biorecognition of reactive oxygen species. In this work, an efficient and simple electrochemical platform was designed for analyzing H₂O₂, using HbGp covalently immobilized on Nafion®-modified glassy carbon electrode, named as HbGp/Nafion/GCE. Voltammetric and spectroscopic studies revealed the importance of prior modification of the electrodic support with the conducting polymer to obtain satisfactory hemoglobin electroactivity, as well as a biocompatible microenvironment for its immobilization. In terms of biological activity, it was observed a greater reactivity of the biomolecule in acidic medium, enabling the detection of the analyte by a quasi-reversible mechanism, whose kinetics was limited by analyte diffusion. In the presence of H₂O₂, the native structure of hemoglobin (oxy-HbGp (Fe²⁺)) oxidizes to ferryl-HbGp (Fe⁴⁺) and this redox reaction can be monitored on HbGp/Nafion/GCE with a detection limit of 8.5 × 10^‒7  mol L^-1. In addition to high sensitivity, the electrochemical biosensor also provided reproducible, consistent, and accurate measurements. The electroanalytical method showed an appropriate performance to quantify different levels of H₂O₂ in milk samples, proving the potential of HbGp/Nafion/GCE for this purpose.

Initial biophysical characterization of Amynthas gracilis giant extracellular hemoglobin (HbAg)

The aim of the present work was the biophysical characterization of the Amynthas gracilis hemoglobin (HbAg). The oxy-HbAg optical absorption data, with Soret and Q bands centered at 415, 540 and 575 nm, were stable and unchanged at pH 7.0. An increase in pH promotes decrease in the intensity in the optical absorption bands, suggesting an oligomeric dissociation and partial oxidation. Identical stability at pH 7.0 was observed in DLS results that presented a hydrodynamic diameter of 28 nm, characteristic of the whole oligomer. DLS shows that HbAg undergoes oligomeric dissociation and an aggregation/denaturation process that corroborates spectroscopic data. Our results showed that the monomer d presents four isoforms with molecular mass (MM) ranging from 16,244 to 16,855 Da; the trimer subunit presents two isoforms, (abc)₁ and (abc)₂, with MM of 51,415 ± 20 Da and 51,610 ± 14 Da, respectively, and a less intense species, at 67,793 Da, assigned to the tetramer abcd. Monomeric chains a, obtained from reduction of the disulfide-bonded trimer abc, present four isoforms with MM 17,015 Da, 17,061 Da, 17,138 Da and 17,259 Da. DLS and LSI revealed an isoeletric point (pI) of oxy-HbAg of 6.0 ± 0.3 and 5.5, respectively. Data analysis by IEF-SDS-PAGE revealed that the pI of oxy-HbAg is 6.11, correlating with DLS and LSI data. These studies indicate that oxy-HbAg is very stable, at pH 7.0, and has differing properties from orthologous giant hemoglobins.

Characterization of the apo-form of extracellular hemoglobin of Glossoscolex paulistus (HbGp) and its stability in the presence of urea

The structural study of small heme-containing proteins, such as myoglobin, in the apo-form lacking heme has been extensively described, but the characterization and stability of the giant Glossoscolex paulistus hemoglobin (HbGp), in the absence of heme groups, has not been studied. Spectroscopic data show efficient extraction of the heme groups from the hemoglobin, with relatively small secondary and tertiary structural changes in apo-HbGp noticed compared to oxy-HbGp. Electrophoresis shows a partial precipitation of the trimer abc (significantly lower intensity of the corresponding band in the gel), due to extraction of heme groups, and the predominance of the intense monomeric d band, as well as of two linker bands. AUC and DLS data agree with SDS-PAGE in showing that the apo-HbGp undergoes dissociation into the d and abc subunits. Subunits d and abc are characterized by sedimentation coefficients and percentage contributions of 2.0 and 3.0 S and 76 and 24%, respectively. DLS data suggest that the apo-HbGp is unstable, and two populations are present in solution: one with a diameter around 6.0 nm, identified with the dissociated species, and a second one with diameter 100-180 nm, due to aggregated protein. Finally, the presence of urea promotes the exposure of the fluorescent probes, extrinsic ANS and intrinsic protein tryptophans to the aqueous solvent due to the unfolding process. An understanding of the effect of heme extraction on the stability of hemoproteins is important for biotechnological approaches such as the introduction of non-native prosthetic groups and development of artificial enzymes with designed properties.

Oligomeric stability of Glossoscolex paulistus hemoglobin as a function of the storage time

Glossoscolex paulistus hemoglobin structure is composed of 144 globin chains and 36 polypeptide chains lacking the heme group, with a total molecular mass of 3600 kDa. The current study focuses on the oxy-HbGp oligomeric stability, as a function of the storage time, at pH 7.0, using dynamic light scattering, analytical ultracentrifugation (AUC), optical absorption and size exclusion chromatography (SEC). HbGp stored in Tris-HCl buffer, pH 7.0, at 4 °C, for two years remains in the native form, while 4-6 years HbGp stocks present typical hemichrome species absorption spectra. AUC and SEC analyses show that the contribution of HbGp-subunits, such as, dodecamer (abcd)₃, tetramer abcd, trimer abc and monomer d, increases with the protein aging due to the lower stability of the HbGp with the time. The dissociation and the oxidation of the iron noted for the older protein solutions indicate that HbGp storage for periods of time longer than two years changes its ability to carry oxygen. Despite the reduction of HbGp stability and oxygen carrying capacity with aging, the protein stability is still larger as compared to mammalian hemoglobins. Thus, the extracellular hemoglobins are quite stable and resistant to the auto-oxidation process, making them of interest for biotechnological applications.

Interaction of Glossoscolex paulistus extracellular hemoglobin with hydrogen peroxide: Formation and decay of ferryl-HbGp

The giant extracellular hemoglobin from earthworm Glossoscolex paulistus (HbGp) reacts with hydrogen peroxide, displaying peroxidase activity in the presence of guaiacol. The formation of ferryl-HbGp (compound II) from the peroxidase cycle was studied in the present work. The hypervalent ferryl-HbGp species was formed directly by the reaction of oxy-HbGp and hydrogen peroxide. The oxy-HbGp heme groups (144) under different excess of H₂O₂, relative to heme, showed an influence in the total amount of ferryl-HbGp at the end of the reaction. The ferryl-HbGp was formed with second order rate constant of 27.1±0.5M^-1s^-1, at pH7.0 and 25°C. The increase of the pH value to 8.0 induces both faster formation and decay of ferryl-HbGp, together with oligomeric dissociation induced by the presence of H₂O₂, as observed by DLS. This effect of dissociation increases the heme exposure and decreases the ferryl-HbGp stability, affecting the rate constant as a parallel reaction. At pH7.0, high excess of H₂O₂, above 1:5 oxy-HbGp heme: H₂O₂, produces the aggregation of the protein. Our results show for the first time, for an extracellular giant hemoglobin, the possible effects of oxidative stress induced by hydrogen peroxide.

Glutaraldehyde cross-linking increases the stability of Lumbricus terrestris erythrocruorin

Since donated red blood cells must be constantly refrigerated, they are not available in remote areas and battlefields. We have previously shown that the hemoglobin of the earthworm Lumbricus terrestris (LtEc) is an effective and safe substitute for donated blood that is stable enough to be stored for long periods at the relatively high temperatures that may be encountered in remote areas. The goal of this study was to further increase the thermal stability of LtEc by covalently cross-linking LtEc with glutaraldehyde (gLtEc). Our results show that the melting temperatures of the gLtEc samples steadily increase as the molar ratio of glutaraldehyde to heme increases (from T_m  = 57°C for native LtEc up to T_m  = 68°C at a ratio of 128:1). In addition, while native LtEc is susceptible to subunit dissociation at alkaline pH (8-10), cross-linking with glutaraldehyde completely prevents dissociation of gLtEc at pH 10. Increasing the molar ratio of glutaraldehyde:heme also significantly increased the oxygen affinity of gLtEc, but this effect was decreased by cross-linking gLtEc in the deoxygenated T state. Finally, while gLtEc samples cross-linked at low G:H ratios (e.g., 2:1) exhibited slight increases in oxidation rate in Tris buffer, no significant difference in oxidation rate was observed between native LtEc and the gLtEc samples in Ringer's Solution, which contains antioxidants. Overall, cross-linking LtEc with glutaraldehyde significantly increases its thermal and structural stability without any loss of function, making gLtEc an attractive blood substitute for deployment in remote areas and battlefields. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 34:521-528, 2018.

Direct comparison of oligochaete erythrocruorins as potential blood substitutes

While many blood substitutes are based on mammalian hemoglobins (e.g., human hemoglobin, HbA), the naturally extracellular hemoglobins of invertebrates (a.k.a. erythrocruorins, Ecs) are intriguing alternative oxygen carriers. Specifically, the erythrocruorin of Lumbricus terrestris has been shown to effectively deliver oxygen in mice and rats without the negative side effects observed with HbA. In this study, the properties of six oligochaete Ecs (Lumbricus terrestris, Eisenia hortensis, Eisenia fetida, Eisenia veneta, Eudrilus eugeniae, and Amynthas gracilis) were compared in vitro to identify the most promising blood substitute candidate(s). Several metrics were used to compare the Ecs, including their oxidation rates, dissociation at physiological pH, thermal stability, and oxygen transport characteristics. Overall, the Ecs of Lumbricus terrestris (LtEc) and Eisenia fetida (EfEc) were identified as promising candidates, since they demonstrated high thermal and oligomeric stability, while also exhibiting relatively low oxidation rates. Interestingly, the O₂ affinity of LtEc (P₅₀ = 26.25 mmHg at 37 °C) was also observed to be uniquely lower than EfEc and all of the other Ecs (P₅₀ = 9.29–13.62 mmHg). Subsequent alignment of the primary sequences of LtEc and EfEc revealed several significant amino acid substitutions within the D subunit interfaces that may be responsible for this significant change in O₂ affinity. Nonetheless, these results show that LtEc and EfEc are promising potential blood substitutes that are resistant to oxidation and denaturation, but additional experiments will need to be conducted to determine their safety, efficacy, and the effects of their disparate oxygen affinities in vivo.

Ionic surfactants-Glossoscolex paulistus hemoglobin interactions: Characterization of species in the solution

Glossoscolex paulistus hemoglobin (HbGp) is an oligomeric multisubunit protein with molecular mass of 3600kDa. In the current study, the interaction of sodium dodecyl sulfate (SDS) and cetyl trimethylammonium chloride (CTAC) surfactants with the monomer d and the whole oxy-HbGp, at pH 7.0, was investigated. For pure monomer d solution, SDS promotes the dimerization of subunit d, and the monomeric and dimeric forms have sedimentation coefficient values, s_20,w, around 2.1-2.4 S and 2.9-3.2 S, respectively. Analytical ultracentrifugation (AUC) and isothermal titration calorimetry (ITC) data suggest that up to 26 DS^- anions are bound to the monomer. In the presence of CTAC, only the monomeric form is observed in solution for subunit d. For the oxy-HbGp, SDS induces the dissociation into smaller subunits, such as, monomer d, trimer abc, and tetramer abcd, and unfolding without promoting the protein aggregation. On the other hand, lower CTAC concentration promotes protein aggregation, mainly of trimer, while higher concentration induces the unfolding of dissociated species. Our study provides strong evidence that surfactant effects upon the HbGp-subunits are different, and depend on the surfactant: protein concentration ratio and the charges of surfactant headgroups.

The structure of the giant haemoglobin from Glossoscolex paulistus

The sequences of all seven polypeptide chains from the giant haemoglobin of the free-living earthworm Glossoscolex paulistus (HbGp) are reported together with the three-dimensional structure of the 3.6 MDa complex which they form. The refinement of the full particle, which has been solved at 3.2 Å resolution, the highest resolution reported to date for a hexagonal bilayer haemoglobin composed of 12 protomers, is reported. This has allowed a more detailed description of the contacts between subunits which are essential for particle stability. Interpretation of features in the electron-density maps suggests the presence of metal-binding sites (probably Zn(2+) and Ca(2+)) and glycosylation sites, some of which have not been reported previously. The former appear to be important for the integrity of the particle. The crystal structure of the isolated d chain (d-HbGp) at 2.1 Å resolution shows different interchain contacts between d monomers compared with those observed in the full particle. Instead of forming trimers, as seen in the complex, the isolated d chains associate to form dimers across a crystallographic twofold axis. These observations eliminate the possibility that trimers form spontaneously in solution as intermediates during the formation of the dodecameric globin cap and contribute to understanding of the possible ways in which the particle self-assembles.

Guanidine hydrochloride and urea effects upon thermal stability of Glossoscolex paulistus hemoglobin (HbGp)

Glossoscolex paulistus hemoglobin (HbGp) has a molecular mass of 3600kDa. It belongs to the hexagonal bilayer hemoglobin class, which consists of highly cooperative respiratory macromolecules found in mollusks and annelids. The present work focusses on oxy-HbGp thermal stability, in the presence of urea and guanidine hydrochloride (GuHCl), monitored by several techniques. Initially, dynamic light scattering data show that the presence of GuHCl induces the protein oligomeric dissociation, followed by a significant 11-fold increase in the hydrodynamic diameter (DH) values, due to the formation of protein aggregates in solution. In contrast, urea promotes the HbGp oligomeric dissociation, followed by unfolding process at high temperatures, without aggregation. Circular dichroism data show that unfolding critical temperature (Tc) of oxy-HbGp decreases from 57°C, at 0.0 mol/L of the denaturant, to 45°C, in the presence of 3.5 mol/L of urea, suggesting the reduction of HbGp oligomeric stability. Moreover, differential scanning calorimetry results show that at lower GuHCl concentrations, some thermal stabilization of the hemoglobin is observed, whereas at higher concentrations, the reduction of stability takes place. Besides, HbGp is more stable in the presence of urea when compared with the guanidine effect, as deduced from the differences in the concentration range of denaturants.