Questions
- The globular proteinb-lactoglobulin generally is a mixture of two genetic
variants, A and B. One difference is that A has Asp at position 64, whereas B has
Gly. How could this affect the solubility profile as given in Figure 7.12? - Figure 7.1 shows that b-lactoglobulin and b-casein have an equal
proportion of hydrophobic residues. Nevertheless, for a not too small ionic strength,
b-lactoglobulin is well soluble at its isoelectric pH, whereasb-casein is not at all.
How is this to be explained? - Figure 7.15 gives examples of the association ofb-casein in aqueous
solutions at various temperature and ionic strength, as determined by light
scattering. Can you qualitatively explain the shape of the curves and the differences
caused by the variables applied?
Answers
- The substitution of Asp for Gly means an additional positive charge (at
neutral pH). This would cause a somewhat lower isoelectric pH, according to Figure
6.7b, by about 0.1 pH unit. The average charge of the mixture would thus hardly
vary over this range, making the ‘‘peak’’ of minimum solubility somewhat broader. - At the isoelectric pH, the solubility of a protein is essentially determined
by the hydrophobicity of its surface.b-lactoglobulin is a globular protein, which
implies that its strongly hydrophobic residues are largely buried in the core or
FIGURE7.15 Association ofb-caseinate (expressed as the average number of
molecules in an aggregate) as a function of protein concentration at various
temperatures (indicated, 8 C) at pH 7.0. Ionic strength 0.2 molar (full lines) or 0.05
molar (broken line).