CHAPTER
Amino Acids
Proteins are the most abundant class of organic compounds
in the healthy, lean human body, constituting more than
half of its cellular dry weight. Proteins are polymers of
amino acids and have molecular weights ranging from ap-
proximately 10,000 to more than one million. Biochemical
functions of proteins include catalysis, transport, contrac-
tion, protection, structure, and metabolic regulation.
Amino acids are the monomeric units, or building
blocks, of proteins joined by a specific type of covalent
linkage. The properties of proteins depend on the charac-
teristic sequence of component amino acids, each of which
has distinctive side chains.
Amino acid polymerization requires elimination of a
water molecule as the carboxyl group of one amino acid
reacts with the amino group of another amino acid to form
a covalent amide bond. The repetition of this process with
many amino acids yields a polymer, known as a
polypep-
tide.
The amide bonds linking amino acids to each other
are known as
peptide bonds.
Each amino acid unit within
the polypeptide is referred to as a
residue.
The sequence
of amino acids in a protein is dictated by the sequence of
nucleotides in a segment of the DNA in the chromosomes
and the uniqueness of each living organism is due to its
endowment of specific proteins.
2.1
L-a-Amino Acids: Structure
Almost all of the naturally occurring amino acids in pro-
teins are L-a-amino acids. The principal 20 amino acids in
proteins have an amino group, a carboxyl group, a
hydrogen atom, and an R-group attached to the »-carbon
(Figure 2-1).
Proline
is an exception because it has
a cyclic structure and contains a secondary amine group
(called an imino group) instead of a primary amine group
(called an amino group). Amino acids are classified
according to the chemical properties of the R-group.
Except for glycine (R = H), the amino acids have at
least one asymmetrical carbon atom (the »-carbon). The
absolute configuration of the four groups attached to
the a-carbon is conventionally compared to the config-
uration of L-glyceraldehyde (Figure 2-2). The D and L
designations specify absolute configuration and not the
dextro (right) or levo (left) direction of rotation of plane-
polarized light by the asymmetrical carbon center. In or-
ganic chemistry, the assignment of absolute configurations
of an asymmetrical center is made by the R and S classifi-
cation of isomers. This system prioritizes the substituents
linked to the asymmetrical carbon atom (e.g., decreasing
atomic number or valence density) and the assignment
is based upon the clockwise (R) or the counterclockwise
(S) positioning of the three higher priority groups.
2.2 Classification
A useful classification of the amino acids is based on the
solubility (i.e., ionization and polarity) characteristics of
the side chains (R-groups). The R-groups fall into four
classes:
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