Page 421 - Elements of Chemical Reaction Engineering Ebook

392 Nonelementary Reaction Kinetics Chap. 7

inhibition of any particular enzyme involved in a primary metabolic sequence
will render the entire sequence inoperative, resulting in either serious damage
or death of the organism. For example, the inhibition of a single enzyme, cyto-
chrome oxidase, by cyanide will cause the aerobic oxidation process to stop;
death occurs in a very few minutes. There are also beneficial inhibitors such as
the ones used in the treatment of leukemia and other neoplasic diseases.
The three most common types of reversible inhibition occurring in enzy-
matic reactions are competitive, uncompetitive, and noncompetitive. (See Prob-
lem P7-12B) The enzyme molecule is analogous to the heterogeneous catalytic
surface in that it contains active sites. When competitive inhibition occurs, the
substrate and inhibitor are usually similar molecules that compete for the same
site on the enzyme. Uncompetitive inhibition occurs when the inhibitor deacti-
vates the enzyme-substrate complex, usually by attaching itself to both the sub-
strate and enzyme molecules of the complex. Noncompetitive inhibition occurs
with enzymes containing at least two different types of sites. The inhibitor
attaches to only one type of site and the substrate only to the other. Derivation of
thz rate laws for these three types of inhibition is shown on the CD-ROM.
7.4.5 Multiple Enzyme and Substrate Systems

In the preceding section we discussed how the addition of a second sub-
strate, I, to enzyme-catalyzed reactions could deactivate the enzyme and
greatly inhibit the reaction. In the present section we look not only at systems
in which the addition of a second substrate is necessary to activate the enzyme,
but also other multiple-enzyme and multiple-substrate systems in which cyclic
regeneration of the activated enzyme occurs.

Enzyme Regeneration. The first example considered is the oxidation of glu-
cose (S,) with the aid of the enzyme glucose oxidase [represented as either
G.O. or (E,)] to give 6-gluc,onolactone (P):

(glucose
glucose + G.O. e G.O.) e @-lactone G.0.H2)
a -lactone + G.0.H2
6
In this reaction, the reduced form of glucose oxidase (G.0.H2), which will be
represented by E,, cannot catalyze further reactions until it is oxidized back to
E,. This oxidation is usually carried out by adding molecular oxygen to the
system so that glucose oxidase, E,, is regenerated. Hydrogen peroxide is also
produced in this oxidation regeneration step:
G.O.H2 + 02 *
G.O. + H202
Overall, the reaction is written

glucose
glucose + 02 ’ H202 + 6-gluconolactone
In biochemistry texts, reactions of this type involving regeneration are
usually written in the form