Page 281 - Introduction to chemical reaction engineering and kinetics

262 Chapter 10: Biochemical Reactions: Enzyme Kinetics
Enzymes are commonly grouped according to the type of reaction catalyzed. Six
classes of enzymes have been identified:
(1) oxidoreductases, which catalyze various types of oxidation-reduction reactions;
(2) transferases, which catalyze the transfer of functional groups, such as aldehydic
or acyl groups;
(3) hydrolases, which catalyze hydrolysis reactions;
(4) isomerases, which catalyze isomerization;
(5) ligases, which, with ATP (adenosine triphosphate) as a cofactor,2 lead to the for-
mation of bonds between carbon and other atoms, including carbon, oxygen, ni-
trogen, and sulfur; and
(6) lyases, which catalyze the addition of chemical groups onto double bonds.
Examples of some common enzyme-catalyzed reactions are as follows:

(1) hydrolysis of urea with the enzyme (E) urease

(NH,),CO + H,O 5 CO2 + 2 NH, (A)

(the specific nature of the catalytic action is indicated by the fact that urease has
no effect on the rate of hydrolysis of substituted ureas, e.g., methyl urea);
(2) hydrolysis of sucrose with invertase to form glucose and fructose

(B)

(3) hydrolysis of starch with amylase to form glucose (this is a key step in the con-
version of corn to fuel-grade ethanol)

m
K,H,,O,), + nHzo ’ nC6H1206
(this may also be carried out by acid-catalyzed hydrolysis with HCl, but at a
higher T and probably with a lower yield);
(4) decomposition of hydrogen peroxide in aqueous solution with catalase (which
contains iron), an example of oxidation-reduction catalyzed by an enzyme;

2H,O, 5 2H,O + 0, 03

(5) treatment of myocardial infarction with aldolase;
(6) treatment of Parkinson’s disease with L-DOPA, produced via tyrosinase (Pialis
et al., 1996):

L-tyrosine + 0, 5 dihydroxyphenylalanine (L-DOPA) + H,O

As a model reaction, we represent an enzyme-catalyzed reaction by

S+E+P+E (10.1-1)

where S is a substrate (reactant), and P is a product.

*A cofactor is a nonprotein compound that combines with an inactive enzyme to generate a complex that is
catalytically active. Metal ions are common cofactors for enzymatic processes. A cofactor may be consumed in
the reaction, but may be regenerated by a second reaction unrelated to the enzymatic process.