Organic Chemistry

Section 24.9 Enzyme-Catalyzed Reactions 1019

substrate binds to the active site. It has been suggested that the unfavorable

electrostatic interaction between the negatively charged carboxyl group of the

peptide product and the negatively charged Glu 270 residue facilitates the re-

lease of the product from the enzyme.

PROBLEM 15

Which of the following C-terminal peptide bonds would be more readily cleaved by car-

boxypeptidase A?

Explain.

PROBLEM 16

Carboxypeptidase A has esterase activity as well as peptidase activity. In other words, the

compound can hydrolyze ester bonds as well as peptide bonds. When carboxypeptidase A hy-

drolyzes ester bonds, Glu 270 acts as a nucleophilic catalyst instead of a general-base catalyst.

Propose a mechanism for the carboxypeptidase A-catalyzed hydrolysis of an ester bond.

Mechanism for the Serine Proteases
Trypsin, chymotrypsin, and elastase are members of a large group of endopeptidases
known collectively as serine proteases. Recall that an endopeptidase cleaves a peptide bond
that is not at the end of a peptide chain (Section 23.12). They are called proteasesbecause
they catalyze the hydrolysis of protein peptide bonds. They are called serine proteasesbe-
cause they all have a serine residue at the active site that participates in the catalysis.
The various serine proteases have similar primary structures, suggesting that they
are evolutionarily related. They all have the same three catalytic residues at the active
site: an aspartate, a histidine, and a serine. But they have one important difference—
the composition of the pocket at the active site that binds the side chain of the amino
acid residue undergoing hydrolysis (Figure 24.6). This pocket is what gives the serine
proteases their different specificities (Section 23.12).

Ser-Ala-Phe or Ser-Ala-Asp

H

Gly 216 COO

HOCH 2

H

Asp 189

Gly 226

Ser 190

H

Gly 216

H

Gly 226

CHCH 3

Val 216 CH^3

CH 3 CH

OH

Thr 226

trypsin chymotrypsin elastase

Figure 24.6
The binding pockets in trypsin, chymotrypsin, and elastase. The negatively charged aspartate
is shown in red, and the relatively nonpolar amino acids are shown in green. The structures
of the binding pockets explain why trypsin binds long, positively charged amino acids;
chymotrypsin binds flat, nonpolar amino acids; and elastase binds only small amino acids.