Essential Cell Biology 5th edition

Panel 2.06a
76
PANEL 2–6 THE 20 AMINO ACIDS FOUND IN PROTEINS
FAMILIES OF
AMINO ACIDS
BASIC SIDE CHAINS
lysine
arginine
histidine
The common amino acids
are grouped according to
whether their side chains
are
(Lys, or K)
H O
N C C
(Arg, or R)
H O
N C C
(His, or H)
H O
N C C
acidic
basic
uncharged polar
nonpolar
These 20 amino acids
are given both three-letter
and one-letter abbreviations.
Thus: alanine = Ala = A
H
CH 2
CH 2
CH 2
CH 2
+
NH 3
This group is
very basic
because its
positive charge
is stabilized by
resonance (see
Panel 2–1).
H CH 2
CH 2
CH 2
NH
C
+ H 2 N NH 2
H CH 2
HN
HC
C
CH
NH +
These nitrogens have a
relatively weak affinity for an
H + and are only partly positive
at neutral pH.
THE AMINO ACID
The general formula of an amino acid is
OPTICAL ISOMERS
The α-carbon atom is asymmetric,
allowing for two mirror-image
(or stereo-) isomers, L and D.
amino
group
H 2 N
H
C
COOH
α-carbon atom
carboxyl
group
H
H
side chain
R is commonly one of 20 different side chains.
At pH 7, both the amino and carboxyl groups
are ionized.
H
+
H 3 N C COO
R
L
NH 3
+ COO –
C α
R
COO –
C α
R
NH 3
+
D
R
Proteins contain exclusively L-amino acids.
PEPTIDE BONDS
In proteins, amino acids are joined together by an
amide linkage, called a peptide bond.
The four atoms involved in each peptide bond form a rigid
planar unit (red box). There is no rotation around the C–N bond.
H
H
N
H
C
R
C
O
OH
H
H
N
R
C
H
C
O
OH
H 2 O
H
H
N
H
C
R
O
C
N
H
R
C
H
C
O
OH
Proteins are long polymers
of amino acids linked by
peptide bonds, and they
are always written with the
N-terminus toward the left.
Peptides are shorter, usually
fewer than 50 amino acids long.
The sequence of this tripeptide
is histidine-cysteine-valine.
amino terminus, or
N-terminus
+ H 3 N
HN
HC
H
C
CH 2
C
CH
NH +
O
C
N
H
SH
CH2
C
C
O
H
N
H
C
CH
CH 3 CH 3
peptide bond
COO –
carboxyl terminus, or
C-terminus
These two single bonds allow rotation, so that long
chains of amino acids are very flexible.