Organic Chemistry

Section 23.7 Peptide Bonds and Disulfide Bonds 973

Biological organisms can also convert acids into amino acids, but because

and metal catalysts are not available to the cell, they do so by a different mechanism

(Section 25.6.)

a. What amino acid is obtained from the reductive amination of each of the following

metabolic intermediates in the cell?

b. What amino acids are obtained from the same metabolic intermediates when they are

synthesized in the laboratory?

23.7 Peptide Bonds and Disulfide Bonds

Peptide bonds and disulfide bonds are the only covalent bonds that hold amino acid

residues together in a peptide or a protein.

Peptide Bonds

The amide bonds that link amino acid residues are called peptide bonds. By conven-

tion, peptides and proteins are written with the free amino group (the N-terminal

amino acid) on the left and the free carboxyl group (the C-terminal amino acid) on

the right.

When the identities of the amino acids in a peptide are known but their sequence is

not known, the amino acids are written separated by commas. When the sequence of

amino acids is known, the amino acids are written separated by hyphens. In the fol-

lowing pentapeptide shown on the right, valine is the N-terminal amino acid and histi-

dine is the C-terminal amino acid. The amino acids are numbered starting with the

N-terminal end. The glutamate residue is referred to as Glu 4 because it is the fourth

amino acid from the N-terminal end. In naming the peptide, adjective names (ending

in “yl”) are used for all the amino acids except the C-terminal amino acid. Thus, this

pentapeptide is named valylcysteylalanylglutamylhistidine.

A peptide bond has about 40% double-bond character because of electron delo-

calization. Steric hindrance causes the trans configuration to be more stable than the

Glu, Cys, His, Val, Ala Val-Cys-Ala-Glu-His

the pentapeptide contains the indicated

amino acids, but their sequence is not known

the amino acids in the pentapeptide

have the indicated sequence

O

H 3 NCHC

R

O

NHCHC

R′

O

NHCHCO− +

+

2 H 2 O

R′′

O

H 3 NCHCO− ++

RR′

+

O

H 3 NCHCO−

+

O

H 3 NCHCO−

+

R′′

a tripeptide

peptide bonds

the N-terminal amino acid the C-terminal amino acid

O O

CH 3 CCOH

OOO

HOCCH 2 C COH

OOO

HOCCH 2 CH 2 C COH

pyruvic acid oxaloacetic acid -ketoglutaric acid

a-keto H 2

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