Enzymes - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 327

R 4

Substrate

C

R 3

R 1

Site 3

R 2

Site 1

Site 2

Active sites of an enzyme

FIGURE 8.2 Representation of the three-point interaction of substrate with enzyme.

others are more exacting in their substrate requirements and are said to be absolutely specific.
The product formed from a particular enzyme and substrate is also unique. Enzymes are able
to distinguish between stereochemical forms and only one isomer of a particular substrate
may be catalyzed to react. Surprisingly, enzyme reactions may yield stereospecific products
from substrates that possess no asymmetric carbon atoms, as long as one carbon is prochiral.
This chirality is a result of at least three-point interaction between substrate and enzyme in
the active site of the enzyme. In Fig. 8.2 , sites 1, 2, and 3 are binding sites on the enzyme.
When two of the R groups on the substrate are identical, the molecule has a prochiral center
and a chiral center can result from the enzymatic reaction, as the substrate can only “fit” into
the active site in one configuration if the site has binding selectivity for three of the R-group
substituents. If the substrate has four different R groups, then chirality can be preserved in
the reaction as a result of the multipoint attachment.

8.1. HOW ENZYMES WORK

Enzymes lower the activation energy of the reaction catalyzed by binding the substrate and
forming an enzyme-substrate complex. Enzymes do not affect the free-energy change or the
equilibrium constant. Figure 8.3 illustrates the action of an enzyme from the activation-energy
point of view. For example, the activation energy for the decomposition of hydrogen peroxide
varies depending on the type of catalysis. The activation energy E of the uncatalyzed reaction
at 20 C is 75 kJ per mole (kJ/mol), whereas the E values for chemically catalyzed (by colloidal
platinum) and enzymatically catalyzed (catalase) decomposition are 54 and 29 kJ/mol,
respectively. That is, catalase accelerates the rate of reaction by a factor of about 10. One