Page 69 - Color Atlas of Biochemistry

P. 69

60 Biomolecules

Proteinogenic amino acids The aromatic amino acids (class III) contain
resonancestabilized rings. In this group, only
phenylalanine has strongly apolar properties.
A. The proteinogenic amino acids
Tyrosine and tryptophan are moderately polar,
The amino acids that are included in the ge- and histidine is even strongly polar. The imi-
netic code (see p. 248) are described as “pro- dazole ring of histidine is already protonated
teinogenic.” With a few exceptions (see p. 58), at weakly acidic pH values. Histidine, which is
only these amino acids can be incorporated only aromatic in protonated form (see p. 58),
into proteins through translation.Onlythe can therefore also be classified as a basic
side chains of the 20 proteinogenic amino amino acid. Tyrosine and tryptophan show
acids are shown here. Their classification is strong light absorption at wavelengths of
based on the chemical structure of the side 250–300 nm.
chains, on the one hand, and on their polarity The neutral amino acids (class IV) have
on the other (see p. 6). The literature includes hydroxyl groups (serine, threonine)or amide
several slightly different systems for classify- groups (asparagine, glutamine). Despite their
ing amino acids, and details may differ from nonionic nature, the amide groups of aspara-
those in the system used here. gine and glutamine are markedly polar.
The carboxyl groups in the side chains of
For each amino acid, the illustration names: the acidic amino acids aspartic acid and glu-
• Membership of structural classes I–VII (see tamic acid (class V) are almost completely
below; e. g., III and VI for histidine) ionized at physiological pH values. The side
• Name and abbreviation, formed from the chains of the basic amino acids lysine and
first three letters of the name (e. g., histi-
dine, His) arginine are also fully ionized—i. e., positively
• The one-letter symbol introduced to save charged—at neutral pH. Arginine, with its
positively charge guanidinium group, is par-
space in the electronic processing of se- ticularly strongly basic, and therefore ex-
quence data (H for histidine)
• A quantitative valuefor thepolarity of the tremely polar.
Proline (VII) is a special case. Together with
side chain (bottom left; 10.3 for histidine). the α-C atom and the α-NH 2 group, its side
The more positive this value is, the more chain forms a fivemembered ring. Its nitrogen
polar the amino acid is.
atom is only weakly basic and is not proto-
In addition, the polarity of the side chains is nated at physiological pH. Due to its ring
indicated by color. It increases from yellow, structure, proline causes bending of the pep-
through light and dark green, to bluish green. tide chain in proteins (this is important in
For ionizing side chains, the corresponding collagen, for example; see p. 70).
pK a values are also given (red numbers). Several proteinogenic amino acids cannot
The aliphatic amino acids (class I) include be synthesized by the human organism, and
glycine, alanine, valine, leucine, and isoleucine. therefore have to be supplied from the diet.
These amino acids do not contain heteroa- These essential amino acids (see p. 360) are
toms (N, O, or S) in their side chains and do marked with a star in the illustration. Histi-
not contain a ring system. Their side chains dine and possibly also arginine are essential
are markedly apolar. Together with threonine for infants and small children.
(see below), valine, leucine, and isoleucine
form the group of branched-chain amino
acids.The sulfurcontaining amino acids cys-
teine and methionine (class II), are also apolar.
However, in the case of cysteine, this only
applies to the undissociated state. Due to its
ability to form disulfide bonds, cysteine plays
an important role in the stabilization of pro-
teins (see p. 72). Two cysteine residues linked
by a disulfide bridge are referred to as cystine
(not shown).

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