whereKis the equilibrium value of½N/½UandVis total molar volume of
the system. Since a very high pressure is needed to cause unfolding of a
protein (often much more than 1000 bar), the concomitant volume change
must be small. Hydrophobic bond formation mostly goes along with
DV>0, and high pressure would thus lead to breaking of these bonds,
implying destabilization of the native conformation. On the other hand,
formation of most electrostatic bonds, H-bonds, and van der Waals
interactions involve a negativeDV, and high pressures would be stabilizing.
A full explanation of the effect of pressure on protein stability is lacking.
Figure 7.7c shows that under some conditions, relatively moderate pressures
slightly stabilize (slightly increase the unfolding temperature). Very high
pressures always cause unfolding.
For some proteins, for instance ovalbumin, high pressure may cause
irreversibleaggregation. Whether it occurs may depend on the rate of
pressure increase. Moderately high pressure generally causes dissociation of
most quaternary structures. This is not surprising, since the association is
generally due to hydrophobic interactions.
- Adsorption. Adsorption phenomena are discussed in Sections 10.2
and 10.3. Proteins are surface active, which implies that they lower the
interfacial free energy upon adsorption. For instance, protein adsorption at
an air–water interface lowers surface tension by about 30 mN?m^1 , which
equals 0:03 J?m^2. We will here consider the effect on protein conforma-
tion.
Proteins adsorb onto almost all surfaces, whether air–water, oil–water,
or solid–water. There is only one exception: the adsorbent is a solid that is
hydrophilic and charged, and the protein has a charge of the same sign as
the solid and is a ‘‘hard’’ protein. The latter implies that the protein has a
relatively stable globular conformation, i.e., a fairly highDN?UG. ‘‘Soft’’
proteins also adsorb at hydrophilic solid surfaces, even of the same charge.
Adsorption may thus primarily involve electrostatic attraction, in which
case protein conformation is not greatly affected. However, other solids, oil,
and air provide hydrophobic surfaces, where the main driving force for
adsorption generally is hydrophobic interaction. Since most apolar residues
are buried in the core of a globular protein, adsorption generally involves a
marked change in conformation. This is borne out by results of spectro-
scopic studies, which show a change in secondary and loss of tertiary
structure. DSC applied to an adsorbed protein generally shows a
denaturation peak that is smaller (or even negligible), and that occurs at a
lower temperature as compared to the protein in solution.
Adsorption ofenzymesgenerally leads to loss of enzyme activity,
whether measured in the adsorbed state or after desorption. By and large,
the activity loss is greater under conditions (temperature, pH, etc.) where