AVK (one letter). The ends of a peptide are labelled as the amino end or
amino terminus, and the carboxy end or carboxy terminus.
H 3 N
N
O
H
O
N
H
O
O
NH 3
+
Alanine
Valine
Lysine
Amino end
Carboxy end
Alanylvalyllysine
(Ala-Val-Lys or AVK)
+
Large peptides of biological significance are known by their trivial names;
e.g., insulin is an important peptide composed of 51 amino acid residues.
4.9.1 Fundamental structural features of amino acids
Each amino acid consists of a carbon atom to which is attached a hydrogen
atom, an amino group (À ÀNH 2 ), a carboxyl group (À ÀCOOH) and one of 20
different ‘R’ groups. It is the structure of the R group (side chain)
that determines the identity of an amino acid and its special properties.
The side chain (R group), depending on the functional groups, can be
aliphatic, aromatic, acidic, basic, hydroxylic, sulphur containing or
amidic (containing amide group). However, proline has an unusual ring
structure, where the side chain is bonded at its terminus to the main chain
nitrogen.
C
H 3
NH 2
O
OH
H
C
H 3
NH 3
O
O
H
Alanine
An amino acid
Carboxylic acid group
amino group
Alkyl (R) group
α carbon
The zwitterionic
structure of alanine
+
An amino acid, with an overall charge of zero, can contain within the same
molecule two groups of opposite charge. Molecules containing oppositely
charged groups are known as zwitterions. For amino acids, a zwitterionic
structure is possible because the basic amino group can accept a proton and
the acidic carboxylic group can donate a proton.
4.9.2 Essential amino acids
All living organisms can synthesize amino acids. However, many higher
animals are deficient in their ability to synthesize all of the amino acids they
need for their proteins. Thus, these higher animals require certain amino
4.9 AMINO ACIDS AND PEPTIDES
181
amino terminus, and the carboxy end or carboxy terminus.
H 3 N
N
O
H
O
N
H
O
O
NH 3
+
Alanine
Valine
Lysine
Amino end
Carboxy end
Alanylvalyllysine
(Ala-Val-Lys or AVK)
+
Large peptides of biological significance are known by their trivial names;
e.g., insulin is an important peptide composed of 51 amino acid residues.
4.9.1 Fundamental structural features of amino acids
Each amino acid consists of a carbon atom to which is attached a hydrogen
atom, an amino group (À ÀNH 2 ), a carboxyl group (À ÀCOOH) and one of 20
different ‘R’ groups. It is the structure of the R group (side chain)
that determines the identity of an amino acid and its special properties.
The side chain (R group), depending on the functional groups, can be
aliphatic, aromatic, acidic, basic, hydroxylic, sulphur containing or
amidic (containing amide group). However, proline has an unusual ring
structure, where the side chain is bonded at its terminus to the main chain
nitrogen.
C
H 3
NH 2
O
OH
H
C
H 3
NH 3
O
O
H
Alanine
An amino acid
Carboxylic acid group
amino group
Alkyl (R) group
α carbon
The zwitterionic
structure of alanine
+
An amino acid, with an overall charge of zero, can contain within the same
molecule two groups of opposite charge. Molecules containing oppositely
charged groups are known as zwitterions. For amino acids, a zwitterionic
structure is possible because the basic amino group can accept a proton and
the acidic carboxylic group can donate a proton.
4.9.2 Essential amino acids
All living organisms can synthesize amino acids. However, many higher
animals are deficient in their ability to synthesize all of the amino acids they
need for their proteins. Thus, these higher animals require certain amino
4.9 AMINO ACIDS AND PEPTIDES
181