Question
Can you explain why bringing an aqueous solution of a globular protein into a shear
field, shear rate 10^4 s^1 , would not cause its unfolding, whereas adsorption onto the
air surface would? Assume a protein molecule of 5 656 5nm^3 in size.
Answer
Water has a viscosity of 1 mPa?s and a shear rate of 10^4 s^1 , thus would cause a
shear stress of 10 PaðN?m^2 Þ. The stress acts on a surface of area 5 6 5nm^2 , exerting
a maximum tensile force of 10 625? 10 ^18 ¼ 25? 10 ^17 N on a protein molecule. The
force acts over a distance of 5 nm. For every rotation of the molecule, an amount of
mechanical energy of 5? 10 ^9625? 10 ^17 ¼ 1 : 25? 10 ^24 JðN?mÞ is acting on it,
which equals about 0.0003 timeskBT. We have seen that the conformational stability
of a globular protein is about 25 kJ?mol^1 or more, corresponding to about 10 times
kBTfor a single molecule. This is more than 3? 104 the mechanical energy applied to
the molecule during a rotation in the shear field.
We have also seen that the change in free energy upon adsorption of a protein
on the water surface amounts to about 0:03 J?m^2. This corresponds to a free energy
of 0: 03625? 10 ^18 ¼ 7 : 5? 10 ^19 J per adsorbed molecule, or about 185 timeskBT.
Roughly that amount of energy would be available to act on a protein molecule on
adsorption, and this is much larger than the value of the conformational stability.
7.2.3 Denaturation Kinetics
This section is essentially restricted to heat denaturation of globular
proteins. This is a very important subject for food technologists. It is the
basis of the inactivation of enzymes. Since microorganisms depend for their
metabolism on enzymes (including those that act as transport regulators in
the cell membrane), killing of microbes is governed by protein denaturation.
Aggregation of globular proteins often occurs after denaturation, as in heat-
set protein gelation. Knowledge of the kinetics of these processes is essential,
especially for optimizing the time–temperature combination for eliminating
undesired microorganisms and enzymes, while minimizing quality loss due
to heat treatment. The reader is advised to study Chapter 4 first, Sections
4.3.3 and 4.4 in particular. There it is explained that the rate of a reaction
does not depend on the difference in free energy between the states before
and after, i.e., in the present case onDN?UG, but on the activation free
energyDG{.