Cellular Structure, Proteins, and Metabolism
75
Regulation of Enzyme-Mediated
Reactions
The rate of an enzyme-mediated reaction depends on sub-
strate concentration and on the concentration and activity (a
term defi ned later in this section) of the enzyme that catalyzes
the reaction. Body temperature is normally nearly constant,
so changes in temperature do not directly alter the rates of
metabolic reactions. Increases in body temperature can occur
during a fever, however, and around muscle tissue during exer-
cise, and such increases in temperature increase the rates of all
metabolic reactions, including enzyme-catalyzed ones, in the
affected tissues.
Substrate Concentration
Substrate concentration may be altered as a result of factors
that alter the supply of a substrate from outside a cell. For
example, there may be changes in its blood concentration due
to changes in diet or the rate of substrate absorption from the
intestinal tract. Intracellular substrate concentration can also
be altered by cellular reactions that either utilize the substrate,
and thus lower its concentration, or synthesize the substrate,
and thereby increase its concentration.
The rate of an enzyme-mediated reaction increases as
the substrate concentration increases, as illustrated in
Figure
3–34
, until it reaches a maximal rate, which remains con-
stant despite further increases in substrate concentration. The
maximal rate is reached when the enzyme becomes saturated
with substrate—that is, when the active binding site of every
enzyme molecule is occupied by a substrate molecule.
Enzyme Concentration
At any substrate concentration, including saturating concen-
trations, the rate of an enzyme-mediated reaction can be
increased by increasing the enzyme concentration. In most
metabolic reactions, the substrate concentration is much greater
than the concentration of enzyme available to catalyze the
reaction. Therefore, if the number of enzyme molecules is
doubled, twice as many active sites will be available to bind
substrate, and twice as many substrate molecules will be con-
verted to product (
Figure 3–35
). Certain reactions proceed
faster in some cells than in others because more enzyme mol-
ecules are present.
To change the concentration of an enzyme, either the
rate of enzyme synthesis or the rate of enzyme breakdown
must be altered. Because enzymes are proteins, this involves
changing the rates of protein synthesis or breakdown.
Enzyme Activity
In addition to changing the rate of enzyme-mediated reac-
tions by changing the
concentration
of either substrate or
enzyme, the rate can be altered by changing
enzyme activ-
ity.
A change in enzyme activity occurs when either allosteric
or covalent modulation alters the properties of the enzyme’s
active site. Such modulation alters the rate at which the bind-
ing site converts substrate to product, the affi nity of the bind-
ing site for substrate, or both.
Table 3–7
Characteristics of Enzymes
1. An enzyme undergoes no net chemical change as a
consequence of the reaction it catalyzes.
2. The binding of substrate to an enzyme’s active site has all the
characteristics—chemical specifi city, affi nity, competition,
and saturation—of a ligand binding to a protein.
3. An enzyme increases the rate of a chemical reaction but does
not cause a reaction to occur that would not occur in its
absence.
4. Some enzymes increase both the forward and reverse rates
of a chemical reaction and thus do not change the chemical
equilibrium fi nally reached. They only increase the rate at
which equilibrium is achieved.
5. An enzyme lowers the activation energy of a reaction but
does not alter the net amount of energy that is added to or
released by the reactants in the course of the reaction.
Saturation
Enzyme concentration X
Enzyme concentration 2X
Substrate concentration
Reaction rate
Saturation
Substrate concentration
Reaction rate
Figure 3–34
Rate of an enzyme-catalyzed reaction as a function of substrate
concentration.
Figure 3–35
Rate of an enzyme-catalyzed reaction as a function of substrate
concentration at two enzyme concentrations, X and 2X. Enzyme
concentration 2X is twice the enzyme concentration of X,
resulting in a reaction that proceeds twice as fast at any substrate
concentration.
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