Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

112 Panel 3–1: The 20 Amino Acids Found in Proteins
THE AMINO ACID
The general formula of an amino acid is
amino
group
H 2 N
H
C
COOH
α-carbon atom
carboxyl
group
OPTICAL ISOMERS
H
The α-carbon atom is asymmetric, which
allows for two mirror images (or stereo-)
isomers, L and D.
H
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
side-chain group
L
NH 3
+ COO –
C α
R
COO –
C α
R
NH 3
+
D
R
Proteins consist exclusively of L-amino acids.
PEPTIDE BONDS
Amino acids are commonly joined together by an amide linkage,
called a peptide bond.
Peptide bond: The four atoms in each gray box form a rigid
planar unit. There is no rotation around the C–N bond.
H
H
N
H
C
R
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.
The sequence of this tripeptide
is histidine-cysteine-valine.
H
H
N
R
C
H
amino- or
N-terminus
C
O
OH
+ H 3 N
H
C
C
HN
HC
CH 2
CH
O
C
NH +
H 2 O
N
H
SH
CH 2
C
C
O
H
H
H
N
N
H
C
CH
H
C
R
CH 3 CH 3
O
C
COO –
N
H
R
C
H
C
O
OH
carboxyl- or
C-terminus
These two single bonds allow rotation, so that long chains of
amino acids are very flexible.
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.
H CH 2
CH 2
CH 2
NH
C
+ H 2 N NH 2
H CH 2
C
HN CH
HC
NH +
These nitrogens have a
relatively weak affinity for an
H + and are only partly positive
at neutral pH.