Physical Chemistry of Foods

energy. The great advantage of the Eyring theory is the introduction of the

activation entropy, which may be considerable in many cases. Consider, for

instance, the heat denaturation of a protein, the essential step of which is a

change from the native (globular) conformation to an unfolded state. This

may lead to its inactivation if the protein is an enzyme or other biologically

active agent. An example is given in Figure 4.5 for the enzyme alkaline

phosphatase (EC 3.1.3.1). The activation enthalpy equals about

450 kJ?mol
^1 ; applying Eq. (4.11), while taking only this contribution to

DG{into account, would lead to a presumed rate constant of about 10
^55 s
^1

at 75C, whereas in fact the reaction proceeds fairly fast at that temperature.

The activation entropy is, however, large and positive, presumably owing to

the unfolding of the protein leading to a greatly increased conformational

FIGURE4.5 Examples of the molar enthalpyðHÞ, entropyðSÞtimes temperature
ðTÞ, and free energyðGÞof the enzyme alkaline phosphatase in the native (N) and
denatured (D) state; the intermediate state refers to the ‘‘activated complex.’’ Results
at 340K, derived from kinetic data on inactivation of the enzyme.