SECTION
2.2
Classification
21
FIGURE 2-4
UV absorption spectra of phenylalanine, tyrosine, and tryptophan.
naturally occurring medicinal compounds derived from
plants. It is an essential amino acid. The indole group
absorbs UV light at 280 nm—a property that is useful for
spectrophotometric measurement of protein concentration
(Figure 2-4).
Tryptophan and tyrosine both show fluorescence; how-
ever, tryptophan absorbs more intensely. When molecules
are raised to a higher energy state by absorption of
radiant energy, they generally are unstable and return
to the ground state. The energy released in this process
manifests as heat (radiation energy) or light. The pro-
cess of light emission is known as
fluorescence.
The
quantum of energy re-emitted as fluorescence is always
less than that of absorbed energy. Thus, the fluorescent
light always appears at a longer wavelength (lower en-
ergy) than the original absorbed light energy (Figure 2-5).
Tryptophan fluorescence studies can provide valuable in-
formation regarding protein and protein-ligand confor-
mations due to the effects of surrounding amino acid
residues.
COO“
I
+ H3N—C—H
CH2
H
Tryptophan
Methionine
This essential amino acid contains an R-group with
a methyl group attached to sulfur. Methionine serves as
donor of a methyl group in many transmethylation reac-
tions, e.g., in the synthesis of epinephrine, creatine, and
FIGURE 2-5
Tryptophan fluorescence spectrum. The emission spectrum appears at
longer wavelengths as compared to the absorption spectrum.
melatonin. Almost all of the sulfur-containing compounds
of the body are derived from methionine.
COO“
I
+ H3N—C— H
I
CH2
I
CH2
I
s
I
CH3
Methionine
Proline
Proline contains a secondary amine group, called an
imine, instead of a primary amine group. For this reason,
proline is called an imino acid. Since the three-carbon
R-group of proline is fused to the a-nitrogen group, this
compound has a rotationally constrained rigid-ring struc-
ture. As a result, prolyl residues in a polypeptide introduce
restrictions on the folding of chains. In collagen, the prin-
cipal protein of human connective tissue, certain prolyl
residues are hydroxylated (Figure 2-6). The hydroxylation
occurs during protein synthesis and requires ascorbic acid
(vitamin C) as a cofactor. Deficiency of vitamin C causes
formation of defective collagen and
scurvy
(Chapters 25
and 38).
COO“
I
+ H2N-----------C—H
Proline
