Page 67 - Color Atlas of Biochemistry

P. 67

58 Biomolecules

Amino acids: chemistry and mensional structure as follows: firstly, the
properties tetrahedron is rotated in such a way that the
most oxidized group (the carboxylate group)
is at the top. Rotation is then continued until
–
the line connecting line COO and R (red) is
A. Amino acids: functions
level with the page. In L-amino acids, the
+
The amino acids (2-aminocarboxylic acids) NH 3 group is thenonthe left,while in D-
fulfill various functions in the organism. amino acids it is on the right.
Aboveall, they serveas the components of
peptides and proteins. Only the 20 proteino-
genic amino acids (see p. 60) are included in C. Dissociation curve of histidine
the genetic code and therefore regularly All amino acids have at least two ionizable
found in proteins. Some of these amino acids groups, and their net charge therefore de-
undergo further (post-translational) change pends on the pH value. The COOH groups at
following their incorporation into proteins the α-C atom have pK a values of between 1.8
(see p. 62). Amino acids or their derivatives and2.8 andare thereforemore acidicthan
are also form components of lipids—e. g., ser- simple monocarboxylic acids. The basicity of
ine in phospholipids and glycine in bile salts. the α-amino function also varies, with pK a
Several amino acids function as values of between 8.8 and 10.6, depending
neurotransmitters themselves (see p. 352), on the amino acid. Acidic and basic amino
while others are precursors of neurotransmit- acids have additional ionizable groups in their
ters, mediators, or hormones (see p. 380). side chain. The pK a values of these side chains
Amino acids are important (and sometimes arelistedon p. 60. Theelectrical charges of
essential) components of food (see p. 360). peptides and proteins are mainly determined
Specific amino acids form precursors for other by groups in thesidechains, as most α-car-
metabolites—e. g., for glucose in gluconeogen- boxyl and α-amino functions are linked to
esis, for purine and pyrimidine bases, for peptide bonds (see p. 66).
heme, and for other molecules. Several non- Histidine can beusedhereas an example of
proteinogenic amino acids function as inter- thepH-dependence of thenet charge of an
mediates in the synthesis and breakdown of amino acid. In addition to the carboxyl group
proteinogenic amino acids (see p. 412) and in and the amino group at the α-C atom with pK a
the urea cycle (see p.182). values of 1.8 and 9.2, respectively, histidine
also has an imidazole residue in its side chain
with a pK a value of 6.0. As the pH increases,
B. Optical activity
the net charge (the sum of the positive and
The natural amino acids are mainly α-amino negative charges) therefore changes from +2
acids, in contrast to β-amino acids such as β- to –1. At pH 7.6, the net charge is zero, even
alanine and taurine. Most α-amino acids have though the molecule contains two almost
four different substituents at C-2 (Cα). The α completely ionized groups in these condi-
atom therefore represents a chiral center—i. e., tions. This pH value is called the isoelectric
there are two different enantiomers (L- and point.
D-amino acids; see p. 8). Among the proteino- At its isoelectric point, histidine is said to
genic amino acids, only glycine is not chiral (R be zwitterionic,as ithas both anionic and
=H). In nature,itisalmostexclusively cationic properties. Most other amino acids
L-amino acids that are found. D-Amino acids are also zwitterionic at neutral pH. Peptides
occur inbacteria—e. g.,inmurein(see and proteins also have isoelectric points,
p. 40)—and in peptide antibiotics. In animal which can vary widely depending on the
metabolism, D-Amino acids would disturb composition of the amino acids.
the enzymatic reactions of L-amino acids
and they are therefore broken down in the
liver by the enzyme D-amino acid oxidase.
The Fischer projection (center) is used to
present the formulas for chiral centers in bio-
molecules. It is derived from their three-di-

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