Page 282 - Introduction to chemical reaction engineering and kinetics

10.1 Enzyme Catalysis 263

Since enzymes are proteins, which are polymers of amino acids, they have relatively
large molar masses (> 15,000 g mol-l), and are in the colloidal range of size. Hence,
enzyme catalysis may be considered to come between homogeneous catalysis, involving
molecular dispersions (gas or liquid), and heterogeneous catalysis, involving particles
(solid particles or liquid droplets). For the purpose of developing a kinetics model or
rate law, the approach involving formation of an intermediate complex may be used,
as in molecular catalysis, or that involving sites on the catalyst, as in the Langmuir-
Hinshelwood model in surface catalysis. We use the former in Section 10.2, but see
problem 10-2 for the latter.

10.1.2 Experimental Aspects

Factors that affect enzyme activity, that is, the rates of enzyme-catalyzed reactions, in-
clude:
(1) the nature of the enzyme (E), including the presence of inhibitors and coen-
zymes;
(2) the nature of the substrate;
(3) the concentrations of enzyme and substrate;
(4) temperature;
(5) pH; and
(6) external factors such as irradiation (photo, sonic or ionizing) and shear stress.
Some of the main experimental observations with respect to concentration effects,
point (3) above, are:

(1) The rate of reaction, (--I~) or r,, is proportional to the total (initial) enzyme
concentration, cnO.
(2) At low (initial) concentration of substrate, cs, the initial rate, (-Is,), is first-order
with respect to substrate.
(3) At high cs, (-rso) is independent of (initial) cs.
The effect of temperature generally follows the Arrhenius relation, equations 3.1-6
to -8, but the applicable range is relatively small because of low- and high-temperature
effects. Below, say, 0°C enzyme-catalyzed biochemical reactions take place slowly, as
indicated by the use of refrigerators and freezers for food preservation. At sufficiently
high temperatures (usually 45 to 50°C) the enzyme, as a protein with a helical or ran-
dom coil structure, becomes “denatured” by the unwinding of the coil, resulting in loss
of its catalytic activity. It is perhaps not surprising that the catalytic activity of enzymes,
in general, is usually at a maximum with respect to T at about “body” temperature,
37°C. However, some recently developed thermophilic enzymes may remain active up
to 1OOT.
The effect of extreme pH values can be similar to that of T in denaturing the enzyme.
This is related to the nature of enzymatic proteins as polyvalent acids and bases, with
acid and basic groups (hydrophilic) concentrated on the outside of the protein.
Mechanical forces such as shear and surface tension affect enzyme activity by dis-
turbing the shape of the enzyme molecule. Since the shape of the active site of the
enzyme is specifically “engineered” to correspond to the shape of the substrate, even
small changes in structure may drastically affect enzyme activity. Consequently, fluid
flow rates, stirrer speeds, and foaming must be carefully controlled in order to ensure
that an enzyme’s productivity is maintained.
For these reasons, in the experimental study of the kinetics of enzyme-catalyzed re-
actions, T, shear and pH are carefully controlled, the last by use of buffered solutions.
In the development, examples, and problems to follow, we assume that both T and pH