74
Chapter 3
To function, an enzyme must come into contact with
reactants, which are called
substrates
in the case of enzyme-
mediated reactions. The substrate becomes bound to the
enzyme, forming an enzyme-substrate complex, which then
breaks down to release products and enzyme. The reaction
between enzyme and substrate can be written:
S
+
E
34
ES
34
P
+
E
Substrate Enzyme
Enzyme-
Product
Enzyme
substrate
complex
At the end of the reaction, the enzyme is free to undergo the
same reaction with additional substrate molecules. The overall
effect is to accelerate the conversion of substrate into product,
with the enzyme acting as a catalyst. Note that an enzyme
increases both the forward and reverse rates of a reaction and
thus does not change the chemical equilibrium that is fi nally
reached.
The interaction between substrate and enzyme has all
the characteristics described previously for the binding of
a ligand to a binding site on a protein—specifi city, affi nity,
competition, and saturation. The region of the enzyme the
substrate binds to is known as the enzyme’s
active site
(a term
equivalent to “binding site”). The shape of the enzyme in the
region of the active site provides the basis for the enzyme’s
chemical specifi city. Two models have been proposed to
describe the interaction of an enzyme with its substrate(s).
In one, the enzyme and substrate(s) fi t together in a “lock-
and-key” confi guration. In another model, the substrate itself
induces a shape change in the active site of the enzyme, which
results in a highly specifi c binding interaction (“induced fi t
model”) (
Figure 3–33
).
There are approximately 4000 different enzymes in
a typical cell, each capable of catalyzing a different chemical
reaction. Enzymes are generally named by adding the suffi x
-
ase
to the name of either the substrate or the type of reac-
tion the enzyme catalyzes. For example, the reaction in which
carbonic acid is broken down into carbon dioxide and water is
catalyzed by the enzyme
carbonic anhydrase.
The catalytic activity of an enzyme can be extremely
large. For example, a single molecule of carbonic anhydrase
can catalyze the conversion of about 100,000 substrate mol-
ecules to products in one second! The major characteristics of
enzymes are listed in
Table 3–7
.
Cofactors
Many enzymes are inactive in the absence of small amounts of
other substances known as
cofactors.
In some cases, the cofac-
tor is a trace metal, such as magnesium, iron, zinc, or copper.
Binding of one of the metals to an enzyme alters the enzyme’s
conformation so that it can interact with the substrate (this is
a form of allosteric modulation). Because only a few enzyme
molecules need be present to catalyze the conversion of large
amounts of substrate to product, very small quantities of these
trace metals are suffi cient to maintain enzymatic activity.
In other cases, the cofactor is an organic molecule that
directly participates as one of the substrates in the reaction, in
which case the cofactor is termed a
coenzyme.
Enzymes that
require coenzymes catalyze reactions in which a few atoms
(for example, hydrogen, acetyl, or methyl groups) are either
removed from or added to a substrate. For example:
Enzyme
R—2 H + Coenzyme
⎯⎯⎯→
R + Coenzyme—2 H
What distinguishes a coenzyme from an ordinary sub-
strate is the fate of the coenzyme. In our example, the two
hydrogen atoms that transfer to the coenzyme can then be
transferred from the coenzyme to another substrate with the
aid of a second enzyme. This second reaction converts the
coenzyme back to its original form so that it becomes available
to accept two more hydrogen atoms. A single coenzyme mol-
ecule can act over and over again to transfer molecular frag-
ments from one reaction to another. Thus, as with metallic
cofactors, only small quantities of coenzymes are necessary to
maintain the enzymatic reactions in which they participate.
Coenzymes are derived from several members of a spe-
cial class of nutrients known as
vitamins.
For example, the
coenzymes
NAD
+
(nicotinamide adenine dinucleotide) and
FAD
(fl avine adenine dinucleotide) are derived from the B-
vitamins niacin and ribofl avin, respectively. As we will see,
they play major roles in energy metabolism by transferring
hydrogen from one substrate to another.
Enzyme-substrate
complex
+
Enzyme
+
Enzyme
Enzyme
Enzyme
Enzyme-substrate
complex
(a)
Lock-and-key model
(b)
Induced-fit model
Product
Product
Substrates
Substrates
Active site
Active site
Figure 3–33
Binding of substrate to the active site of an enzyme catalyzes the formation of products.
From M. S. Silberberg,
Chemistry:The Molecular Nature of Matter and Change
3d ed., p. 701. The McGraw-Hill Companies, Inc., New York, NY, 2003.
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