Download PANEL 3–1 The 20 Amino Acids Found in Proteins THE AMINO

PANEL 3–1 The 20 Amino Acids Found in Proteins
THE AMINO ACID
OPTICAL ISOMERS
The α-carbon atom is asymmetric, which
allows for two mirror image (or stereo-)
isomers, L and D.
The general formula of an amino acid is
H
amino
group H2N
C
R
α-carbon atom
carboxyl
COOH group
COO–
NH3+
COO–
NH3+
side-chain group
L
R is commonly one of 20 different side chains.
At pH 7 both the amino and carboxyl groups
are ionized.
H
+
H3N C COO
D
Cα
R
H
R
FAMILIES OF
AMINO ACIDS
Cα
R
H
Proteins consist exclusively of L-amino acids.
BASIC SIDE CHAINS
The common amino acids
are grouped according to
whether their side chains
are
acidic
basic
uncharged polar
nonpolar
lysine
arginine
histidine
(Lys, or K)
(Arg, or R)
(His, or H)
H
O
N
C
C
H
CH2
H
O
N
C
C
H
CH2
CH2
O
N
C
C
H
CH2
C
CH2
This group is
very basic
because its
positive charge
is stabilized by
resonance.
CH2
These 20 amino acids
are given both three-letter
and one-letter abbreviations.
H
CH2
+
NH3
Thus: alanine = Ala = A
HN
CH2
These nitrogens have a
relatively weak affinity for an
H+ and are only partly positive
at neutral pH.
C
NH2
2N
NH+
HC
NH
+H
CH
PEPTIDE BONDS
Amino acids are commonly joined together by an amide linkage,
called a peptide bond.
H
H
N
H
C
R
O
C
N
OH
H
H2O
R
H
C
H
Peptide bond: The four atoms in each gray box form a rigid
planar unit. There is no rotation around the C–N bond.
O
H
C
N
OH
H
H
O
C
C
R
R
N
C
H
H
O
C
OH
SH
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.
amino- or
N-terminus
+H N
3
H
O
C
C
CH2
HC
N
H
C
C
O
H
H
N
C
CH
NH+
carboxyl- or
C-terminus
COO–
CH
CH3
C
HN
CH2
CH3
These two single bonds allow rotation, so that long chains of
amino acids are very flexible.