20
chapter
2
Amino Acids
TABLE 2-1
Amino Acid Abbreviations and Nutritional Property
Amino
Abbreviation
acid
Designation
letter
Nutritional
property*
Alanine
Ala
A
non-essential
Arginine
Arg
R
conditionally
essential
Asparagine
Asn
N
non-essential
Aspartic acid
Asp
D
non-essential
Cysteine
Cys
C
non-essential
Glutamic acid
Glu
E
non-essental
Glutamine
Gin
Q
conditionally
essential
Glycine
Gly
G
non-essential
Histidine
His
H
conditionally
essential
Isoleucine
lie
I
essential
Leucine
Leu
L
essential
Lysine
Lys
K
essential
Methionine
Met
M
essential
Phenylalanine
Phe
F
essential
Proline
Pro
P
non-essential
Serine
Ser
S
non-essential
Threonine
Thr
T
essential
Tryptophan
Trp
w
essential
Tyrosine
Tyr
Y
non-essential
Valine
Val
V
essential
*The eight essential amino acids are not synthesized in the body and
have to be supplied in the diet. The conditionally essential amino acids,
although synthesized in the body, may require supplementation during
certain physiological conditions such as pregnancy. The non-essential
amino acids can be synthesized from various metabolites.
Alanine
The side chain of alanine is a hydrophobic methyl
group,
—CH3.
Other amino
acids
may
be
consid-
ered to be chemical derivatives of alanine, with sub-
stituents on the ^-carbon. Alanine and glutamate provide
links between amino acid and carbohydrate metabolism
(Chapter 22).
COO"
I
+ H3N— C— H
a-Carbon /
/3-Carbon '
Alanine
Valine, Leucine, and Isoleucine
These branched-chain aliphatic amino acids contain
bulky nonpolar R-groups and participate in hydrophobic
interactions. All three are essential amino acids. A de-
fect in their catabolism leads to
maple syrup urine disease
(Chapter 17). Isoleucine has asymmetrical centers at both
the
a-
and /3-carbons and four stereoisomers, only one of
which occurs in protein. The bulky side chains tend to as-
sociate in the interior of water-soluble globular proteins.
Thus, the hydrophobic amino acid residues stabilize the
three-dimensional structure of the polymer.
COO"
1
COO"
COO"
I
+ H3N— C— H
i
1
+ H3N— C— H
1
1
+ H3N— C— H
1
1
CH2
1
1
H— C— CH:
1
CH
1
CH
1
CH,
/
\
/ \
c.
1
H3C
CH3 H3C
CH3
CH3
Valine
Leucine
Isoleucine
Phenylalanine
A planar hydrophobic phenyl ring is part of the bulky
R-group of phenylalanine. It is an essential amino acid
whose metabolic conversion to tyrosine is defective in
phenylketonuria
(Chapter 17). Phenylalanine, tyrosine,
and tryptophan are the only a-amino acids that contain
aromatic groups and consequently are the only residues
that absorb ultraviolet (UV) light (Figure 2-4). Trypto-
phan and tyrosine absorb significantly more energy than
phenylalanine at 280 nm, the wavelength generally used
to measure the concentration of protein in a solution.
cocr
I
+ H3N— C— H
Phenylalanine
Tryptophan
A bicyclic nitrogenous aromatic ring system (known
as an indole ring) is attached to the /3-carbon of ala-
nine to form the R-group of tryptophan. Tryptophan is a
precursor of serotonin, melatonin, nicotinamide, and many
