Download Amino Acids Found in Proteins

Amino Acids Found in Proteins - Part 1
©1998 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter
. http://www.essentialcellbiology.com
Published by Garland Publishing, a member of the Taylor & Francis Group.
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
CH2
+
NH3
Thus: alanine = Ala = A
THE AMINO ACID
C
R
CH2
C
HC
NH
NH2
2N
CH
NH+
These nitrogens have a
relatively weak affinity for an
H+ and are only partly positive
at neutral pH.
C
+H
The a -carbon atom is asymmetric, which
allows for two mirror image (or stereo-)
isomers, L and D.
The general formula of an amino acid is
amino
group H2N
C
H
HN
CH2
OPTICAL ISOMERS
H
O
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
N
a -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
Ca
D
Ca
R
H
R
H
Proteins consist exclusively of L-amino acids.
R
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.