Amino Acids, Peptides,
and Proteins.
Classification of Amino Acids
Fundamentals
While their name implies that amino acids are
compounds that contain an —NH2 group and a
—CO2H group, these groups are actually
present as —NH3+ and —CO2– respectively.
They are classified as a, b, g, etc. amino acids
according the carbon that bears the nitrogen.
Amino Acids
+
NH3
a
CO2–
+
–
H3NCH2CH2CO2
b
+
–
H3NCH2CH2CH2CO2
g
an a-amino acid that is an
intermediate in the biosynthesis
of ethylene
a b-amino acid that is one of
the structural units present in
coenzyme A
a g-amino acid involved in
the transmission of nerve
impulses
The 20 Key Amino Acids
More than 700 amino acids occur naturally, but
20 of them are especially important.
These 20 amino acids are the building blocks of
proteins. All are a-amino acids.
They differ in respect to the group attached to
the a carbon.
These 20 are listed in Table 27.1 (p 1054-1055).
Table 27.1
H
+
H3N
C
O
C
O
–
R
The amino acids obtained by hydrolysis of
proteins differ in respect to R (the side chain).
The properties of the amino acid vary as the
structure of R varies.
Table 27.1
H
Glycine
(Gly or G)
+
H3N
C
O
C
O
–
H
Glycine is the simplest amino acid. It is the only
one in the table that is achiral.
In all of the other amino acids in the table the a
carbon is a stereogenic center.
Table 27.1
H
+
H3N
C
O
C
CH3
Alanine
(Ala or A)
O
–
Table 27.1
H
+
H3N
C
O
C
O
CH(CH3)2
Valine
(Val or V)
–
Table 27.1
H
+
H3N
C
O
C
O
–
CH2CH(CH3)2
Leucine
(Leu or L)
Table 27.1
H
+
H3N
C
O
C
O
–
CH3CHCH2CH3
Isoleucine
(Ile or I)
Table 27.1
H
+
H3N
C
O
C
CH3SCH2CH2
Methionine
(Met or M)
O
–
Table 27.1
H
+
H2N
C
O
C
CH2
H2C
C
H2
Proline
(Pro or P)
O
–
Table 27.1
H
+
H3N
C
O
C
O
CH2
Phenylalanine
(Phe or F)
–
Table 27.1
H
+
H3N
C
O
C
O
–
CH2
Tryptophan
N
H
(Trp or W)
Table 27.1
H
+
H3N
C
O
C
H2NCCH2
O
Asparagine
(Asn or N)
O
–
Table 27.1
H
+
H3N
C
O
C
H2NCCH2CH2
O
Glutamine
(Gln or Q)
O
–
Table 27.1
H
+
H3N
C
O
C
CH2OH
Serine
(Ser or S)
O
–
Table 27.1
H
+
H3N
C
O
C
CH3CHOH
Threonine
(Thr or T)
O
–
Table 27.1
H
+
H3N
–
C
O
C
OCCH2
O
Aspartic Acid
(Asp or D)
O
–
Table 27.1
H
+
H3N
–
C
O
C
O
OCCH2CH2
O
Glutamic Acid
(Glu or E)
–
Table 27.1
H
+
H3N
C
O
C
O
–
CH2
Tyrosine
(Tyr or Y)
OH
Table 27.1
H
+
H3N
C
O
C
CH2SH
Cysteine
(Cys or C)
O
–
Table 27.1
H
+
H3N
C
O
C
O
–
+
CH2CH2CH2CH2NH3
Lysine
(Lys or K)
Table 27.1
H
+
H3N
C
O
C
O
–
CH2CH2CH2NHCNH2
+ NH2
Arginine
(Arg or R)
Table 27.1
H
+
H3N
C
O
C
CH2
N
NH
Histidine
(His or H)
O
–
27.2
Stereochemistry of Amino Acids
Configuration of a-Amino Acids
Glycine is achiral. All of the other amino acids
in proteins have the L-configuration at their a
carbon.
–
CO2
+
H3N
H
R
27.7
Peptides
Peptides
Peptides are compounds in which an amide
bond links the amino group of one a-amino acid
and the carboxyl group of another.
An amide bond of this type is often referred to
as a peptide bond.
Alanine and Glycine
H
+
H3N
C
CH3
H
O
C
–
O
+
H3N
C
H
O
C
–
O
Alanylglycine
H
+
H3N
C
CH3
H
O
C
N
C
H
H
O
C
–
O
Two a-amino acids are joined by a peptide bond
in alanylglycine. It is a dipeptide.
Alanylglycine
H
+
H3N
N-terminus
C
CH3
H
O
C
N
C
H
H
Ala—Gly
AG
O
C
–
O
C-terminus
Alanylglycine and glycylalanine are
constitutional isomers
H
+
H3N
C
C
CH3
H
+
H3N
C
H
H
O
N
C
H
H
H
O
C
N
C
H
CH3
O
C
–
O
Alanylglycine
Ala—Gly
AG
–
O
Glycylalanine
Gly—Ala
GA
O
C
Alanylglycine
H
+
H3N
C
CH3
H
O
C
N
C
H
H
O
C
–
O
The peptide bond is
characterized by a
planar geometry.
Higher Peptides
Peptides are classified according to the number
of amino acids linked together.
dipeptides, tripeptides, tetrapeptides, etc.
Leucine enkephalin is an example of a
pentapeptide.
Leucine Enkephalin
Tyr—Gly—Gly—Phe—Leu
YGGFL
Oxytocin
3
2
4
5
Ile—Gln—Asn
Tyr
1
Cys
N-terminus
C-terminus
Cys—Pro—Leu—GlyNH2
6
S
7
8
9
S
Oxytocin is a cyclic nonapeptide.
Instead of having its amino acids linked in an
extended chain, two cysteine residues are
joined by an S—S bond.
Oxytocin
S—S bond
An S—S bond between two cysteines is
often referred to as a disulfide bridge.