Organic Chemistry

986 CHAPTER 23 Amino Acids, Peptides, and Proteins

Table 23.3 Specificity of Peptide or Protein Cleavage

Reagent Specificity

Chemical reagents

Edman’s reagent removes the N-terminal amino acid

Cyanogen bromide hydrolyzes on the C-side of Met

Exopeptidases*

Carboxypeptidase A removes the C-terminal amino acid (not Arg or Lys)

Carboxypeptidase B removes the C-terminal amino acid (only Arg or Lys)

Endopeptidases*

Trypsin hydrolyzes on the C-side of Arg and Lys

Chymotrypsin hydrolyzes on the C-side of amino acids that contain

aromatic six-membered rings (Phe, Tyr, Trp)

Elastase hydrolyzes on the C-side of small amino acids

(Gly and Ala)

*Cleavage will not occur if Pro is on either side of the bond to be hydrolyzed.

that catalyze the hydrolysis of only the specific peptide bonds listed in Table 23.3.

Trypsin, for example, catalyzes the hydrolysis of the peptide bond on the C-side of

only arginine or lysine residues.

Thus, trypsin will catalyze the hydrolysis of three peptide bonds in the following

peptide, creating a hexapeptide, a dipeptide, and two tripeptides.

Chymotrypsin catalyzes the hydrolysis of the peptide bond on the C-side of amino

acids that contain aromatic six-membered rings (Phe, Tyr, Trp).

Elastase catalyzes the hydrolysis of peptide bonds on the C-side of small amino acids

(Gly, Ala). Chymotrypsin and elastase are much less specific than trypsin. (An expla-

nation for the specificity of these enzymes is given in Section 24.9.)

cleavage by elastase

Ala-Lys-Phe-Gly-Asp-Trp-Ser-Arg-Met-Val-Arg-Tyr-Leu-His

cleavage by chymotrypsin

Ala-Lys-Phe-Gly-Asp-Trp-Ser-Arg-Met-Val-Arg-Tyr-Leu-His

cleavage by trypsin

Ala-Lys-Phe-Gly-Asp-Trp-Ser-Arg-Met-Val-Arg-Tyr-Leu-His

CH 2 R′

CH 2

CH 2

CH 2

+NH

3

CH 2

CH 2

NH

C

NH 2

NH 2

R

NHCHC

OOO

R′′

NHCHC

O

CH 2

NHCHC

O

R′′′

NHCHC

O

NHCHC NHCHC

+

C-side of lysine C-side of arginine

3-D Molecules:

Carboxypeptidase A;

Chymotrypsin

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