Download THE AMINO ACID OPTICAL ISOMERS FAMILIES OF AMINO ACIDS

128
PANEL 3–1: The 20 Amino Acids Found in Proteins
THE AMINO ACID
The a-carbon atom is asymmetric, which
allows for two mirror image (or stereo-)
isomers, L and D.
OPTICAL ISOMERS
The general formula of an amino acid is
a-carbon atom
H
amino
group H2N
C
carboxyl
H
COOH group
R
H
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
R
COO–
NH3+
Ca
Ca
R
R
D
Proteins consist exclusively of L-amino acids.
FAMILIES OF
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
CH2
These 20 amino acids
are given both three-letter
and one-letter abbreviations.
H
CH2
+
NH3
Thus: alanine = Ala = A
This group is
very basic
because its
positive charge
is stabilized by
resonance.
HN
CH2
HC
NH
NH+
These nitrogens have a
relatively weak affinity for an
H+ and are only partly positive
at neutral pH.
C
+H N
2
CH
NH2
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.
THE SHAPE AND STRUCTURE OF PROTEINS
129
ACIDIC SIDE CHAINS
NONPOLAR SIDE CHAINS
alanine
valine
aspartic acid
glutamic acid
(Ala, or A)
(Val, or V)
(Asp, or D)
(Glu, or E)
H
O
C
H
O
N
C
C
H
CH2
H
O
N
C
N
C
C
H
CH3
H
CH2
O
N
C
C
H
CH
CH3
CH3
CH2
C
O–
O
H
leucine
C
O–
O
(Leu, or L)
H
O
N
C
C
H
CH2
CH3
O
N
C
C
H
CH
CH3
CH
UNCHARGED POLAR SIDE CHAINS
H
CH3
CH2
CH3
asparagine
glutamine
proline
phenylalanine
(Asn, or N)
(Gln, or Q)
(Pro, or P)
(Phe, or F)
H
O
N
C
C
H
CH2
H
O
N
C
C
H
CH2
NH2
O
C
C
CH2
CH2
CH2
C
O
N
H
H
O
N
C
C
H
CH2
CH2
(actually an
imino acid)
C
O
NH2
Although the amide N is not charged at
neutral pH, it is polar.
methionine
tryptophan
(Met, or M)
(Trp, or W)
H
O
N
C
C
H
CH2
H
O
N
C
C
H
CH2
CH2
S
serine
threonine
tyrosine
(Ser, or S)
(Thr, or T)
(Tyr, or Y)
H
O
N
C
C
H
CH2
OH
H
O
N
C
C
H
CH
CH3
CH3
N
H
H
O
glycine
cysteine
N
C
C
(Gly, or G)
(Cys, or C)
H
CH2
OH
H
O
N
C
C
H
H
OH
The –OH group is polar.
H
O
N
C
C
H
CH2
SH
Disulfide bonds can form between two cysteine side chains
in proteins.
CH2
S
S
CH2