18
Bioorganic Chemistry
carbon. This makes the carbonyl carbon
more susceptible to nucleophilic attack
and reaction. Enalaprilat is able to make
similar interactions with ACE. Figure 13
shows that the terminal salt bridge, phenyl
ring, and carbonyl hydrogen bond are all
maintained in positions similar to that of
the natural substrate. A major difference
is that the original scissile amide bond has
been replaced with an unreactive amine.
In order to maintain the interaction with
the catalytic zinc, a carboxyl group is po-
sitioned in a similar orientation as the
original carbonyl. This salt bridge is crucial
for tight binding. Enalapril, the prodrug
to this compound, is identical except that
this carboxylate group is esteri±ed, thereby
making salt bridge formation impossible.
It is also a far less potent inhibitor, evi-
dently due to the lack of half of the salt
bridge.
Covalent enzyme inhibitors also must
have some structural similarity to the nat-
ural substrate in order to provide initial
binding and speci±city. An example of a
covalent enzyme inhibitor is aspirin, or
acetylsalicylic acid. It is a nonsteroidal anti-
inflammatory agent. One of the ways by
wh
ichi
texer
tsth
ise
f
fec
tisbycova
len
t
ly
modifying a critical amino group on the en-
zyme cyclooxygenase. Cyclooxygenase is
an enzyme in the synthetic pathway of
prostaglandins, which are key parts of the
inflammatory response. The actual modi-
±cationreactionisanacetylation.
4.2
Enzyme Catalysis
The previous paragraphs describe some of
the fundamental organic chemistry found
in biological systems. We now wish to
demonstrate by example how some of
these principles are applied in enzymes to
achieve catalysis. One of the ways by which
bioorganic chemistry examines this issue
is through the design of small-molecule
mimics of enzyme action. Enzymes are
able to accelerate reactions in a variety of
ways. One of these strategies, transition-
state stabilization, will be discussed later.
Another strategy is nucleophilic cataly-
sis, where a reactive covalent bond is
formed between enzyme and substrate
that is then easily cleaved by another
reactant. Frequently, enzymes take advan-
tage of their ability to provide speci±c
electrostatic interactions that can stabilize
and promote the movements of charge
within a molecule that will encourage its
reaction.
For example, let us consider the reaction
of
ribonuclease
A
with
its
substrate,
a single strand of RNA (Fig. 14). The
reaction that is catalyzed is the cleavage
of the phosphodiester bond to form a
3
0
-phosphorylated species and a 5
0
-free
hydroxyl species. RNA is fairly stable at
neutral pH, although the presence of the
2
0
-hydroxyl makes it less stable than DNA
under similar conditions. The reason this
is so is because the adjacent 2
0
-hydroxyl is
a potential nucleophile that is in an ideal
position to attack the phosphorus atom.
Even though alcohols are not considered
‘‘good’’ nucleophiles, the proximity to
the reactive center increases its reactivity.
Ribonuclease A takes advantage of this
adjacent nucleophile and improves upon
its
nucleophilicity
by
deprotonating
it
with the imidazole base of an active site
histidine. In a sense, the partial minus
charge on the unprotonated imidazole
is
transferred
to
the
2
0
-OH
through
the
transfer
of
a
proton.
The
anion
is a better nucleophile (having greater
negative charge, it will be attracted to
the slightly positive phosphorus atom).
The
result
of
the
2
0
-OH
attack
is
the
formation
of
a
pentacoordinate
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