Organic Chemistry

Section 24.8 Catalysis in Biological Reactions 1015

The following reaction, in which catalyzes the hydrolysis of the ester, is an ex-
ample of intramolecular metal-ion catalysis:

The metal ion complexes with an oxygen and a nitrogen of the reactant, as well as with
a molecule of water. The metal ion increases the rate of the reaction by positioning the
water molecule and increasing its nucleophilicity by converting it to metal-bound
hydroxide.

24.8 Catalysis in Biological Reactions

Essentially all organic reactions that occur in biological systems require a catalyst.
Most biological catalysts are enzymes, which are globular proteins (Section 23.11).
Each biological reaction is catalyzed by a different enzyme. Enzymes are extraordi-
narily good catalysts—they can increase the rate of an intermolecular reaction by as
much as In contrast, rate enhancements achieved by nonbiological catalysts in
intermolecular reactions are seldom greater than
The reactant of an enzyme-catalyzed reaction is called a substrate. The enzyme
has a pocket or cleft known as an active site. The substrate specifically fits and
binds to the active site, and all the bond-making and bond-breaking steps of the
reaction occur while the substrate is at that site. Enzymes differ from nonbiological
catalysts in that they are specific for the reactant whose reaction they catalyze
(Section 5.20). Not all enzymes have the same degree of specificity. Some are spe-
cific for a single compound and will not tolerate even the slightest variation in
structure, whereas some catalyze the reaction of an entire family of compounds
with related structures. The specificity of an enzyme for its substrate is an example
of the phenomenon known as molecular recognition—the ability of one molecule
to recognize another.
The specificity of an enzyme results from its conformation and the particular
amino acid side chains that make up the active site. For example, a negatively
charged side chain of an amino acid at the active site of an enzyme can associate
with a positively charged group on the substrate, a hydrogen-bond donor on the
enzyme can associate with a hydrogen-bond acceptor on the substrate, and hydro-
phobic groups on the enzyme associate with hydrophobic groups on the substrate.
The specificity of an enzyme for its substrate has been described by the lock-and-
key model. In the lock-and-key model, the substrate is said to fit the enzyme much
as a key fits a lock.

10,000-fold.

1016.

Ni OH 2

+ H+

C OCH 3

HN N

OH

O

OH

C OCH 3

HN N

OH

O

−

Ni

+ +

+ H 2 O + CH 3 OH

C OCH 3

HN N

OH

O

Ni^2 +

COH

HN N

OH

O

Ni^2 +

AU: OK as changed?