Section 24.9 Enzyme-Catalyzed Reactions 1017As we look at some examples of enzyme-catalyzed reactions, notice that the
functional groups on the enzyme side chains are the same functional groups you are
used to seeing in simple organic compounds, and the modes of catalysis used by
enzymes are the same as the modes of catalysis used in organic reactions. The
remarkable catalytic ability of enzymes stems in part from their ability to use
several modes of catalysis in the same reaction. Factors other than those listed can
contribute to the increased rate of enzyme-catalyzed reactions, but not all factors
are employed by every enzyme. We will consider some of these factors when we
discuss individual enzymes. Now let’s look at the mechanisms for five enzyme-
catalyzed reactions.
24.9 Enzyme-Catalyzed Reactions
Mechanism for Carboxypeptidase A
Carboxypeptidase A is an exopeptidase—an enzyme that catalyzes the hydrolysis of
the C-terminal peptide bond in peptides and proteins, releasing the C-terminal amino
acid (Section 23.12).
Carboxypeptidase A is a metalloenzyme—an enzyme that contains a tightly bound
metal ion. The metal ion in carboxypeptidase A is Carboxypeptidase A is one of
several hundred enzymes known to contain zinc. In bovine pancreatic carboxypeptidase
A, is bound to the enzyme at its active site by forming a complex with Glu 72,
His 196, and His 69, as well as with a water molecule (Figure 24.5). (The source of the
enzyme is specified because, although carboxypeptidase A’s from different sources fol-
low the same mechanism, they have slightly different primary structures.)
Several groups at the active site of carboxypeptidase A participate in binding the
substrate in the optimum position for reaction (Figure 24.5). Arg 145 forms two hy-
drogen bonds and Tyr 248 forms one hydrogen bond with the C-terminal carboxyl
group of the substrate. The side chain of the C-terminal amino acid is positioned in a
hydrophobic pocket, which is why carboxypeptidase A is not active if the C-terminal
amino acid is arginine or lysine. Apparently, the long, positively charged side chains of
these amino acid residues (Table 23.1) cannot fit into the nonpolar pocket. The reac-
tion proceeds as follows:
- When the substrate binds to the active site, partially complexes with the oxy-
gen of the carbonyl group of the amide that will be hydrolyzed (Figure 24.5).
polarizes the carbon–oxygen double bond, making the carbonyl carbon more
susceptible to nucleophilic attack and stabilizing the negative charge that develops
on the oxygen atom in the transition state that leads to the tetrahedral intermediate.
Arg 127 also increases the carbonyl group’s electrophilicity and stabilizes the
developing negative charge on the oxygen atom in the transition state. also
complexes with water, thereby making it a better nucleophile. Glu 270 functions as
a general-base catalyst, further increasing water’s nucleophilicity.
Zn^2 +Zn^2 +Zn^2 +Zn^2 +Zn^2 +.NHCHC NHCHCO− + H 2 OORR′′NHCHCR′OONHCHC + H 3 NCHCO−ORR′′NHCHCO−R′OOcarboxypeptidase A+3-D Molecule:
Carboxypeptidase A