Chemistry Reference
In-Depth Information
13.2 Peptides and proteins
Although superficially similar, peptides and pro-
teins display a wide variety of biological functions,
and many have marked physiological properties. For
example, they may function as structural molecules
in tissues, as enzymes, as antibodies, or as neu-
rotransmitters. Acting as hormones, they can con-
trol many physiological processes, ranging from gas-
tric acid secretion and carbohydrate metabolism to
growth itself. The toxic components of snake and
spider venoms are usually peptide in nature, as are
some plant toxins. These different activities arise as
a consequence of the sequence of amino acids in
the peptide or protein (the primary structure), the
three-dimensional structure that the molecule then
adopts as a result of this sequence (the secondary and
tertiary structures), and the specific nature of individ-
ual side-chains in the molecule. Many structures have
additional modifications to the basic polyamide sys-
tem shown, and these features may also contribute
significantly to their biological activity.
The tripeptide formed from
L
-alanine,
L
-phenylala-
nine and
L
-serine by two condensation reactions is
alanyl - phenylalanyl - serine, considering each addi-
tional amino acid residue as a substituent on the
previous. This would be more commonly represented
as Ala-Phe-Ser, using the standard three-letter abbre-
viations for amino acids shown in Table 13.1.
amino-
or
N
-terminus
carboxyl-
or
C
-terminus
O
O
O
H
O
OH
H
2
N
H
2
N
N
H
2
N
H
2
N
OH
N
OH
OH
O
H
O
OH
OH
L-Phe
L-Ala
L-Ser
Ala-Phe-Ser
or H-Ala-Phe-Ser-OH
or Ala→Phe→Ser
or AFS
O
H
O
N
-terminus
C
-terminus
H
2
N
N
N
OH
O
H
O
HO
H
Ser−Phe−Ala
By convention, the left-hand amino acid in this
sequence is the one with a free amino group, the
N
-terminus
, and the right-hand amino acid has the
free carboxyl, the
C
-terminus
. Thus, Ser-Phe-Ala
is different from Ala-Phe-Ser, and represents a
quite different molecule. Sometimes, the termini
identities are emphasized by showing H- and -OH;
H- represents the amino group and -OH the
carboxyl group. Some peptides are cyclic, and this
convention can have no significance, so arrows are
incorporated into the sequence to indicate peptide
bonds
acids are commonly used instead of the three
letter abbreviations, thus Ala-Phe-Ser becomes AFS.
Abbreviations
assume
the
L
-configuration
applies
throughout,
and
any
D
-amino
acids
would
be
specifically noted, e.g. Ala-
D
-Phe-Ser.
Glutathione
is an important tripeptide (see Boxes
6.6 and 10.20); but it is a slightly unusual one, in
that it has an amide linkage that involves the
γ
-
carboxyl of glutamic acid rather than a normal amide
bond utilizing the C-1 carboxyl group. To specify
this bonding, the glutathione structure is written as
γ
NH. As sequences
become longer, one-letter abbreviations for amino
in
the
direction
CO
→
-Glu-Cys-Gly.
