Page 208 - Essentials of physical chemistry

170 Essentials of Physical Chemistry

FIGURE 8.5 Prof. Walter J. Kauzmann (1916–2009) was an American physical chemist whose research
spanned thermodynamics (Kauzmann’s paradox of supercooled liquids), quantum chemistry (1957 text), and
biochemistry (the hydrophobic effect in enzymes). He was the Chair of Chemistry at Princeton University from
1964 to 1968 and the Chair of the Department of Biochemistry from 1980 to 1981. He is probably best known
for his work on the thermodynamics and optical activity of proteins. (From Princeton University Department of
Chemistry. With permission.)

enzyme reactions in the sense that a substrate molecule may enter an active site but wander out
before a reaction occurs. Since enzymes are very efficient, the in-out part of the equilibrium may
favor fewer unreacted exits, but the treatment includes that possibility.
Enzymes can be very difficult to isolate and purify but they are usually so efficient that one can
purchase milligram quantities of purified enzyme from biochemical companies for several hundred
dollars per milligram but then only use microgram quantities for a given experiment. It is important
to know that only a small amount of enzyme is necessary to carry out reactions. This leads to a quite
different type of laboratory requiring refrigerator storage and even a large walk-in room, which is
refrigerated to isolate enzymes and carry out reaction studies.
We have left this discussion until after description of the Eyring transition-state theory because
there are many similarities. The main difference is that usually the active site pocket of an enzyme
has two main geometrical attributes. First, there is the ‘‘lock and key’’ analogy, which notes that
usually the entrance to the active site is stereospecific to a particular substrate molecule or a class
of molecules. We see that in Figure 8.6 with catechol oxidase for substituted phenols. Much has
been made of the lock-and-key concept in pharmaceutical research since that is how substrate
specificity is achieved and many medicinal drug molecules are designed with a specific shape and