pH and heat-induced structural changes of bovine apo-α-lactalbumin

Influence of divalent metal cations on α-lactalbumin fibril formation

The effect of binding of divalent metal cations (Ca2+, Cu2+, Mg2+, Mn2+, Zn2+) on the kinetics of fibril formation of bovine α-lactalbumin at acidic conditions is considered. The kinetic parameters of the process were determined using a thioflavin T fluorescence assay. The DSC thermograms of bovine α-lactalbumin in the presence and absence of cations were recorded. The duration of the lag period correlates with the changes in the thermal stability of the molten globule of the protein in the presence of cations. The final thioflavin T fluorescence intensity after formation of the mature fibrils decreases under the influence of calcium ions which strongly bind to the monomeric protein, and increases in solutions containing copper and especially zinc. These ions seem to accelerate secondary nucleation processes and change the fibril morphology, which was confirmed by atomic force microscopy imaging.

Complexation of ellagic acid with α-lactalbumin and its antioxidant property

Ellagic acid possesses numerous bioactivities such as antioxidant activity and anti-inflammatory effect. In this work, the binding interaction between ellagic acid and α-lactalbumin was investigated by multi-spectroscopy and the results suggested that ellagic acid could change the conformation of α-lactalbumin. Chromatographic analysis proved the interaction of α-lactalbumin with ellagic acid taken place in less than 30 min and this interaction was stable. Computer simulations showed that both aromatic clusters Ⅰ and Ⅱ of α-lactalbumin were active sites for ellagic acid. Interestingly, both the results of molecular docking and molecular dynamics simulations suggested that ellagic acid tended to bind to aromatic cluster Ⅱ rather than aromatic cluster Ⅰ. Moreover, α-lactalbumin could enhance the antioxidant property of ellagic acid, indicating that the solubility of ellagic acid might be improved by combining α-lactalbumin. Overall, this work suggested that α-lactalbumin exhibited binding affinity for ellagic acid and enhanced its antioxidant property.

Molecular Analytical Assessment of Thermally Precipitated α-Lactalbumin after Resolubilization

Selective thermal precipitation followed by a mechanical separation step is a well described method for fractionation of the main whey proteins, α-lactalbumin (α-la) and β-lactoglobulin (β-lg). By choosing appropriate environmental conditions the thermal precipitation of either α-la or β-lg can be induced. Whereas β-lg irreversibly aggregates, the precipitated α-la can be resolubilized by a subsequent adjustment of the solution’s pH and the ionic composition. This study reports on the analytical characterization of resolubilized α-la compared to its native counterpart as a reference in order to assess whether the resolubilized α-la can be considered close to ‘native’. Turbidity and quantification by RP-HPLC of the resolubilized α-la solutions were used as a measure of solubility in aqueous environment. RP-HPLC was also applied to determine the elution time as a measure for protein’s hydrophobicity. DSC measurement was performed to determine the denaturation peak temperature of resolubilized α-la. FTIR spectroscopy provided insights in the secondary structure. The refolding of α-la achieved best results using pH 8.0 and a 3-fold stoichiometric amount of Ca²⁺ per α-la molecule. The results showed that the mechanism of aggregation induced by gentle thermal treatment under acidic conditions with subsequent mechanical separation is reversible to a certain extent, however, the exact native conformation was not restored.

Effect of pH on pulsed light inactivation of polyphenol oxidase

The inactivation of diverse food enzymes by pulsed light (PL) has been described before, including the inactivation of polyphenol oxidase (PPO) (at pH 6.5). Since the pH affects the conformation of enzymes, it may influence the inactivation of enzymes by PL. The aim of this work was to evaluate the effect of pH on the kinetics of the PL-inactivation and associated structural changes of a case enzyme. To this, PPO was treated by PL at different pHs (4.0-6.5) and its inactivation kinetics and changes in its structure were evaluated by spectrophotometric and spectrofluorometric methods. The inactivation proceeded faster at low pH and was highly correlated with the decrease in peak intrinsic fluorescence intensity. Phase diagrams and parameter A evolution indicated the absence of intermediate unfolded states during the course of the inactivation. No protein aggregation was detected by turbidimetry. Results indicate that although a low pH favors the PL-inactivation of PPO, the mechanism of inactivation is pH-independent. Beyond the specific outcome for PPO, the results are evidence of a general pH-independence in the mechanism of enzyme inactivation by PL in the pH range 4.0-6.5 and acidification can be a strategy to decrease treatment times during PL processing.

Casein micelles from bovine Milk exerts Neuroprotection by stalling metabolic complications linked to oxidative brain injury

Caseins are the most abundant milk proteins in mammalian species and are assembled in supra-macromolecular structures called micelles. In this study, the microstructural properties, particle size, and elemental composition of isolated casein from bovine milk and its therapeutic effect on oxidative and cholinergic activities linked to dementia in oxidative brain injury were investigated. SEM analysis of the isolated casein micelles from skimmed fresh bovine milk revealed spherical colloid aggregates, while TEM analysis revealed dispersed spherical particles with a mean size of 63.15 ± 4.77 nm. SEM-EDX analysis revealed clusters of carbon, oxygen, sulfur, copper, sodium, magnesium, potassium, iron, and selenium. Treatment of oxidative brain injury with the isolated casein micelles led to elevated levels of GSH, SOD, catalase, ENTPDase, 5'NTPase activities, while concomitantly suppressing MDA, cholesterol, HDL-c levels, acetylcholinesterase and lipase activities. Treatment with the isolated casein micelles led to complete depletion of oxidative generated lipid metabolites, while deactivating oxidative-activated lipid metabolic pathways. These results indicate the microstructural properties, particle size, elemental composition, and antioxidant neuroprotective effect of casein micelles from bovine milk. Thus, demonstrating the nutraceutical properties of milk in the management of oxidative induced cognitive impairment.

Denaturation Behavior and Kinetics of Single- and Multi-Component Protein Systems at Extrusion-Like Conditions

In this study, the influence of defined extrusion-like treatment conditions on the denaturation behavior and kinetics of single- and multi-component protein model systems at a protein concentration of 70% (w/w) was investigated. α-Lactalbumin (αLA) and β-Lactoglobulin (βLG), and whey protein isolate (WPI) were selected as single- and multi-component protein model systems, respectively. To apply defined extrusion-like conditions, treatment temperatures in the range of 60 and 100 °C, shear rates from 0.06 to 50 s⁻1, and treatment times up to 90 s were chosen. While an aggregation onset temperature was determined at approximately 73 °C for WPI systems at a shear rate of 0.06 s⁻1, two significantly different onset temperatures were determined when the shear rate was increased to 25 and 50 s⁻1. These two different onset temperatures could be related to the main fractions present in whey protein (βLG and αLA), suggesting shear-induced phase separation. Application of additional mechanical treatment resulted in an increase in reaction rates for all the investigated systems. Denaturation was found to follow 2.262 and 1.865 order kinetics for αLA and WPI, respectively. The reaction order of WPI might have resulted from a combination of a lower reaction order in the unsheared system (i.e., fractional first order) and higher reaction order for sheared systems, probably due to phase separation, leading to isolated behavior of each fraction at the local level (i.e., fractional second order).

Potential allergenicity assessment after bovine apo-α-lactalbumin binding to calcium ion

Bovine α-lactalbumin (α-LA) is recognized as a major milk allergen. Generally, α-LA in the natural state combines with a calcium ion, however, some studies have shown that calcium ions can binding the other metal binding sites in α-LA as well. In our study, the optimal condition of calcium ion binding to α-LA and the change of structure and allergenicity were explored. By optimizing the conditions, the maximum calcium binding amounts of apo-α-LA were obtained in a ratio of 1:4. The structure of α-LA after removal of calcium obviously changed by the spectroscopic detection. For the digestive stability, there was no obvious change in three forms of α-LA. While the allergenic properties were characterized by IgG/IgE inhibition ELISA and the human basophil KU812 degranulation assay. The results showed that IgG and IgE binding decreased, and the degranulation capacity of basophils weakened. Based on these results, calcium binding to apo-α-LA can reduce the potential allergenic properties. PRACTICAL APPLICATIONS: By optimizing the conditions, bovine apo-α-LA can obtain the most calcium binding amount. And calcium binding to apo-α-LA can reduce the potential allergenic properties. Compared with α-LA in the natural state, calcium binding to α-LA not only can reduce the allergenic properties, but also play a role in calcium supplementation. It might be used to guide the development of hypoallergenic α-LA and provide a method to reduce the potential allergenic properties of α-LA.

Change in the structural and functional properties of goat milk protein due to pH and heat

This study was carried out to investigate the effects of pH and heat on the structure and function of milk proteins by comparing goat milk treated under different pH and temperature conditions. The results showed that pH had a significant effect on the thermal stability of goat milk proteins, and the proteins were least thermally stable at pH 7.7. Except for the pH 6.9 goat milk, the surface hydrophobicities of the milk proteins at various pH values reached their maxima at 85°C. The particle size, zeta potential, and content of regular secondary structure also decreased significantly at 85°C, and the turbidity of milk proteins under alkaline pH conditions was lower than that under acidic conditions. It was concluded that alkaline conditions resulted in better emulsion stability and oil-holding capacity, and acidic conditions offered better foaming ability, foam stability, and water-holding capacity for goat milk protein during heat processing. It can also be seen that 85°C was the key temperature for milk proteins after changing the pH of the milk. This paper provides a theoretical basis for optimizing the processing conditions for goat milk and the applications of goat milk proteins.

Purification and Characterization of a Cadmium-Binding Protein from Lentinula edodes

Many organisms possess the ability to produce metal-binding proteins to absorb cadmium. Metallothioneins, an important family of cysteine-rich metal-binding proteins, have been isolated and well characterized. However, Lentinula edodes may have a different type of cadmium-binding protein that contains fewer cysteine residues. In the present study, we purified a cadmium-binding protein from L. edodes (LECBP) by gel filtration and anion exchange chromatography and then identified LECBP by LC-MS/MS. We found LECBP to be a novel cadmium-binding protein, which contained 220 amino acid residues but no cysteine residue. LECBP had a high binding affinity for Cd(II) with a K_d value of 97.3 μM. The percentages of α-helix, β-sheet, β-turn, and random coil in LECBP were 15.7%, 39.4%, 8.0%, and 37.1%, respectively. In addition, high temperatures and an acidic environment influenced the conformation of LECBP. Our results will thus provide a new perspective to understand the mechanism of cadmium accumulation in L. edodes.

A biophysical study on the mechanism of interactions of DOX or PTX with α-lactalbumin as a delivery carrier

Doxorubicin and paclitaxel, two hydrophobic chemotherapeutic agents, are used in cancer therapies. Presence of hydrophobic patches and a flexible fold could probably make α-Lactalbumin a suitable carrier for hydrophobic drugs. In the present study, a variety of thermodynamic, spectroscopic, computational, and cellular techniques were applied to assess α-lactalbumin potential as a carrier for doxorubicin and paclitaxel. According to isothermal titration calorimetry data, the interaction between α-lactalbumin and doxorubicin or paclitaxel is spontaneous and the K (M^-1) value for the interaction of α-lactalbumin and paclitaxel is higher than that for doxorubicin. Differential scanning calorimetry and anisotropy results indicated formation of α-lactalbumin complexes with doxorubicin or paclitaxel. Furthermore, molecular docking and dynamic studies revealed that TRPs are not involved in α-Lac's interaction with Doxorubicin while TRP 60 interacts with paclitaxel. Based on Pace analysis to determine protein thermal stability, doxorubicin and paclitaxel induced higher and lower thermal stability in α-lactalbumin, respectively. Besides, fluorescence lifetime measurements reflected that the interaction between α-lactalbumin with doxorubicin or paclitaxel was of static nature. Therefore, the authors hypothesized that α-lactalbumin could serve as a carrier for doxorubicin and paclitaxel by reducing cytotoxicity and apoptosis which was demonstrated during our in vitro cell studies.

Exploring the process-structure-function relationship of horseradish peroxidase through investigation of pH- and heat induced conformational changes

Given the importance of peroxidase as an indicator for the preservation of vegetables by heat treatment, the present study is focused on enzyme behavior under different pH and temperature conditions, in terms of process-structure-function relationships. Thus, the process-structure-function relationship of peroxidase was investigated by combining fluorescence spectroscopy, in silico prediction methods and inactivation kinetic studies. The fluorescence spectra indicated that at optimum pH value, the Trp(117) residue is not located in the hydrophobic core of the protein. Significant blue- and red-shifts were obtained at different pH values, whereas the heat-treatment did not cause significant changes in Trp and Tyr environment. The ANS and quenching experiments demonstrated a more flexible conformation at lower pH and respectively at higher temperature. On the other hand molecular dynamics simulations at different temperatures highlighted that the secondary structure appeared better preserved against temperature, whereas the tertiary structure around the heme was more affected. Temperature dependent changes in the hydrogen bonding and ion paring involving amino acids from the heme-binding region (His(170) and Asp(247)) might trigger miss-coordination of the heme iron atom by His(170) residue and further enzyme activity loss.

pH-induced structural changes of tyrosinase from Agaricus bisporus using fluorescence and in silico methods

BACKGROUND

Tyrosinases are involved in enzymatic browning reactions in damaged fruits during post-harvest handling and processing. The overall structure of tyrosinase from Agaricus bisporus mushrooms at different pH values was monitored using fluorescence spectroscopy and molecular dynamics simulations.

RESULTS

When the pH value was increased from 6.0 to 9.0, the protein passed through several structural intermediates, including the tetramer, trimer and dimer stages. Changes in the secondary and tertiary structure of tyrosinase at neutral pH were outlined after running molecular dynamics simulations. A detailed check at the single-molecule level by means of molecular modeling tools suggested that the most important contribution to the fluorescence intensity is given by the H subunits with seven Trp and nine Tyr residues exposed to the solvent, whereas the lectin-like folded L subunits have only six Trp and three Tyr residues, of which only Trp(15) , Trp(59) and Trp(93) are partially exposed to the solvent.

CONCLUSIONS

The results indicated that the enzyme was sensitive to pH. The experimental results revealed the unfolding of the native tetrameric enzyme in acidic pH range, causing exposure of the hydrophobic residues.

Probing thermal behaviour of microbial transglutaminase with fluorescence and in silico methods

BACKGROUND

Knowledge of transglutaminase behaviour at thermal treatment allows efficient applications in food processing. The heat-induced conformational changes of microbial transglutaminase were studied by fluorescence spectroscopy and a molecular modelling approach.

RESULTS

The experimental results indicate the unfolding of transglutaminase in a single-phase reaction, at temperatures over 60 °C. The incidence of conformational changes is also supported by the increase of both intrinsic and 1-anilino-8-naphthalene sulfonate fluorescence intensity with temperature. Changes in the secondary and tertiary structure of transglutaminase were outlined after running molecular dynamics simulations at temperatures ranging from 25 °C to 80 °C.

CONCLUSION

The motif's particularities varied with the temperature, suggesting structural rearrangements of the protein, mainly in helices. The largest deviation from the structure equilibrated at 25 °C was observed at 80 °C.

Analysis of the thermally induced structural changes of bovine lactoferrin

Bovine lactoferrin (LF) is subjected to thermal processing during isolation for commercial use and while preparing milk products intended for infant nutrition. The present study is focused on the heat-induced structural changes of LF in buffer solution. Fluorescence spectroscopy, molecular modeling, and enzymatic hydrolysis studies were combined to extensively characterize LF thermal behavior. The temperature-induced changes induced on LF conformation were analyzed through intrinsic and ANS fluorescence parameters (intensity, maximum position, and parameter A value), the phase diagram method, and quenching experiments using acrylamide and iodide. A higher exposure of hydrophobic residues was highlighted through the molecular modeling approach, with a decrease in α-helix content from 23.5% to 21.2% when increasing the temperature from 25 °C to 80 °C. The experimental results demonstrate a more flexible conformation of the protein at higher temperature, thus facilitating the enzymatic hydrolysis by thermolysin.

Effects of macromolecular crowding on the structural stability of human α-lactalbumin

The folding of protein, an important process for protein to fulfill normal functions, takes place in crowded physiological environments. α-Lactalbumin, as a model system for protein-folding studies, has been used extensively because it can form stable molten globule states under a range of conditions. Here we report that the crowding agents Ficoll 70, dextran 70, and polyethylene glycol (PEG) 2000 have different effects on the structural stability of human α-lactalbumin (HLA) represented by the transition to a molten globule state: dextran 70 dramatically enhances the thermal stability of Ca(2+)-depleted HLA (apo-HLA) and Ficoll 70 enhances the thermal stability of apo-HLA to some extent, while PEG 2000 significantly decreases the thermal stability of apo-HLA. Ficoll 70 and dextran 70 have no obvious effects on trypsin degradation of apo-HLA but PEG 2000 accelerates apo-HLA degradation by trypsin and destabilizes the native conformation of apo-HLA. Furthermore, no interaction is observed between apo-HLA and Ficoll 70 or dextran 70, but a weak, non-specific interaction between the apo form of the protein and PEG 2000 is detected, and such a weak, non-specific interaction could overcome the excluded-volume effect of PEG 2000. Our data are consistent with the results of protein stability studies in cells and suggest that stabilizing excluded-volume effects of crowding agents can be ameliorated by non-specific interactions between proteins and crowders.