Lactalbumin, surface

These assumptions were confirmed by the electrophoresis study of the washed creams. Electrophoresis of purified fat globules is a convenient method to characterize and quantify proteins adsorbed at the oil-water interface [35]. Electrophoretic data indicate that no casein, nor whey proteins, were adsorbed at the surface of raw-milk fat globule. Upon homogenization, caseins adsorbed preferentially at the lipid-water interface. In this case, bound a-lactalbumin accounted for 16% of the total interfacial proteins. Heat treatment also induced the interaction of proteins with the fat globules. The amount of bound proteins (per mg of lipids) for heated raw milk was half that for homogenized milk. 

Fig. 6. Plateau-values, I"P1 /mg m 2, of adsorption isotherms of lysozyme (LSZ), ribonuclease (RNase), a -lactalbumin (aLA), calcium-depleted (X -lactalbumin (aLA(-Ca )) and bovine serum albumin (BSA) on hydrophobic polystyrene (PS) and hydrophilic hematite (a — Fe203) and silica (Si02) surfaces. An indication of the charge density of the surface is given by the zeta-potential, C, and of the proteins by + and signs. Ionic strength 0.05 M T = 25°C. (Derived from Currie et al. 2003).

The effectiveness of surface and interfacial tension depressants can be compared by plots of concentration versus tension. Various dilution studies of milk, skim milk, wheys, and solutions of milk proteins reveal that casein and the proteins of the lactalbumin fraction (/3-lactoglobu-lin, a-lactalbumin, and bovine serum albumin) are powerful depressants, while the proteins of the immunoglobulin fraction are somewhat... 

Removal of calcium ions makes unfolding of a-lactalbumin irreversible. The denaturation temperature of a-lactalbumin decreased 20 °C when calcium ions were removed by a chelator (Bemal and Jelen 1984). Hillier et al (1979) found that an increase in the calcium concentration up to 0.4 mg/ml slowed the heat denaturation of a-lactalbumin, but additional calcium had little effect. There is a slow conformational change at pH 4 as calcium is released from carboxyl groups on the protein surface (Kronman et al 1964). Failure to measure the heat of denaturation for a-lactalbumin at pH 3 shows the protein chain is already unfolded at low pH (de Wit and Klarenbeek 1984). 

Solutions containing mixtures of a-lactalbumin (a-la) and Tween 20 are a classic example of a two component system that displays Type II behavior [21]. A summary of foam stability, film thickness and FITC-a-la surface diffusion is given in Figure 17. Alone, a-la produced less stable foams than 8-lg, and it was necessary to increase the stock protein concentration to 0.5 mg/ml (35.4 / M). 

To further characterize the structural changes of goat a-lactalbumin during unfolding, we examined the probability distributions of the following four structural parameters in each of the nine clusters of the structural ensemble of MD trajectories (1) the fractional native contact (Q) of the entire molecule, (2) the RMSD of C atoms between a pair of structures that belong to the same cluster, (3) the solvent-accessible surface area (SASA) of hydrophobic side chains, and (4) the SASA of hydrophilic side chains [25]. 

Figure 4 The electrostatic distributions of a-lactalbumin (lALC) (a) and chicken egg-white lysozyme (ILZl) (b) calculated by Delphi and presented by Grasp. Both surfaces of calcium-binding protein a-lactalbumin are mostly negatively charged (a) (in red) while the surfaces of noncalcium binding protein lysozyme (b) are mostly positively charged (in blue)...

The lower part of the cleft, where residues E and F bind in lysozyme, is changed both in topology and in the nature of surface groups. While it appears from this model that this part of the cleft could bind saccharides, the precise mode of binding would be expected to differ from that in lysozyme. Residue 52, Asp, is invariant in lysozyme, the equivalent residue in a-lactalbumin being Glu-49. Residue 35 is Glu in lysozyme, but the equivalent residue in a-lactalbumin is variable. However, if residues 32—36 are rearranged to give maximum sequence identity ... 

In contrast to these findings are those by Van Ceunebroeck et al. (1986), who used a l-labeled hydrophobic dye in the study of the apo and Ca(lI)-bound forms of bovine a-lactalbumin. The former protein was more heavily labeled with the dye than the latter, and a larger hydrophobic surface was therefore concluded to be exposed in the absence ofCa(II). 

These findings help to differentiate between possible ways in which a-lactalbumin could influence the production of lactose. One of these, which, on the surface, appears plausible, is that a-lactalbumin might accept iV-acetyllactosamine as a substrate, this product having arisen from the enzymatic action of galactosyltransferase. The ensuing transglycosyl-ation, whereby lactose would be produced, could then account for the effect of a-lactalbumin in the presence of galactosyltransferase. Brew et al. (1968) showed, however, that a-lactalbumin has no affinity for A -acetyllactosamine and thus could not he involved in this reaction. 

Berliner and Kaptein (1981) used another NMR method, induced dynamic nuclear polarization, to investigate the solvent accessibility of Tyr, Trp, and His residues in five species of a-lactalbumin. This method measures the access of the photo-excited flavin dye to the surface-exposed... 

A segment perpendicular to a surface results in an effective steric barrier, while the number of contact points with the interface influences the strength of adsorption. For example, flexible caseins have numerous proline residues, so they have little ordered secondary structure and no intramolecular crosslink. As a result, caseins are able to adopt a number of different conformational states when being adsorbed at the oil-water interface. They are usually adsorbed at the interface in such a way that considerable portions of their structures protruding into the aqueous phase are available (Dickinson, 1992). On the other hand, serum milk proteins, such as p-lg and a-lactalbumin (a-la), bind relatively close to the interface and do not protrude... 

On positively charged polystyrene at pH 7 the rmax values increase in the order lysozyme negative charge at these conditions) in accordance with their net electrostatic attractions (or repulsions) to the surface [17]. On negatively charged polystyrene the relative positions of the plateau values are nearly reversed, but... 

There are probably two mechanisms that can explain these observations. Improvement in the surface functionality of egg white, /3-lactoglobulin, and a-lactalbumin have been noted for these proteins following heating at 80°C for several days at low moisture contents [52,53], The authors hypothesized that heating caused the formation of a molten globulelike structure. Proteins in this state would be par-... 

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