Chemistry Reference
In-Depth Information
will have a positively charged amino terminal group, and a negatively charged carboxyl terminal group. There
are just 20 amino acids which are routinely found in proteins. This finds its explanation, as we will see later in
the chapter, because these are the amino acids for which there are aminoacyl-tRNA synthetases. These
enzymes can selectively charge each of these 20 amino acids onto their cognate tRNAs. In a small number of
proteins there is a 21st amino acid, selenocysteine, and we will see in Chapter 18 how the codon TGA, which
usually means “stop”, can encode selenocysteine. In
Table 3.1
, the 20 protein amino acids are represented by
TABLE 3.1
Protein Amino Acids
Name of amino acid
Structure of R-group
Properties
p
K of R-group
Alanine
A
Hydrophobic
CH
3
Cysteine
C
Polar, forms disulfide
bridges
8.37
H
2
C
SH
Aspartate
D
Polar, charged
3.90
O
H
2
C
O
-
C
Glutamate
E
Polar, charged
4.07
O
H
2
C
H
2
C
O
-
C
Phenylalanine
F
Hydrophobic
H
2
C
Glycine
G
Highly flexible
H
Histidine
H
Hydrophobic/polar donor/
acceptor of H
+
6.04
NH
H
2
C
N
Isoleucine
I
Hydrophobic, sterically
hindered b-carbon
CH
3
H
2
C
CH
CH
3
Lysine
K
Polar, flexible side chain
10.54
H
2
C
H
2
C
H
2
C
H
2
C
NH
2
Leucine
L
Hydrophobic
CH
3
H
2
C
CH
CH
3
Methionine
M
Hydrophobic
H
2
C
H
2
C
S
CH
3
Asparagine
N
Polar, uncharged
O
H
2
C
C
NH
2
