32
chapter
2
Amino Acids
O
O
Ninhydrin
Amino acid
H
^ R --C ^
+
C02
O
Purple-colored ion
+
H20
Ninhydrin
2
HjO
FIGURE 2-11
Reaction of an œ-amino acid with ninhydrin. Two molecules of ninhydrin
and the nitrogen atom of the amino acid are involved in the production of
the purple product.
Ammonia, some amines, and some proteins and pep-
tides will also yield a colored product but will not gen-
erate CO
2
. Thus, the colorimetric analysis is not spe-
cific for amino acids unless CO
2
release is measured or
the amino acids are purified and freed from interfering
materials (the usual procedure). The color reaction with
ninhydrin is used extensively in manual and automated
procedures.
CO
2
adds reversibly to the un-ionized amino group of an
amino acid. The product is a carbamate (or carboxyamino)
derivative.
0
0
II
l / 0 '
c
— 0
H
\
c
1
1
1—C—H
-
>
-C—H
1
/
1
R
n ^C- n -
R
+
H+
This type of reaction accompanies transport of CO
2
in the blood (Chapter 1). In tissue capillaries, CO
2
combines with free a-amino groups of hemoglobin to
form carbaminohemoglobin; in pulmonary capillaries, this
reaction is reversed to release CO
2
into the alveoli. This
mode of transport is limited to only about
1 0
% of the car-
bon dioxide transported in the blood.
Metal ions
can form complexes with amino acids. Metal
ions that function in enzymatic or structural biochemi-
cal systems include those of iron, calcium, copper, zinc,
magnesium, cobalt, manganese, molybdenum, nickel, and
chromium. The functional group that binds a metal ion is
called a
ligand.
Ligands are electron donors that form non-
covalent bonds with the metal ions, usually two, four, or
six ligand groups per ion. When four ligand groups bind a
metal ion, the complex is either a plane or a tetrahedron;
when six ligand groups participate, octahedral geometry
results. The term
chelation
is applied to a metal-ligand
interaction when a single molecule provides two or more
ligands (e.g., chelation of iron with four nitrogens in one
porphyrin molecule; see Chapter 14).
Metal ions can also react with amino acid functional
groups to abolish the biological activity of proteins. Heavy
metal ions that form highly insoluble sulfides (e.g., HgS,
PbS, CuS, Ag
2
S) characteristically react with sulfhydryl
groups of cysteinyl residues. If the reactive -SH group is
involved in biological activity of the protein, the displace-
ment of the hydrogen and the addition of a large metal
atom to the S atom usually cause a major change in pro-
tein structure and loss of function. Hence, heavy metals
are often poisons.
In contrast, amino acid residues in proteins may un-
dergo nonenzymatic chemical reactions that may or may
not alter biological activity. An example of this type
of reaction is the formation of glycated proteins. The
amino groups of proteins combine with carbonyl groups of
sugars (glucose) to form labile aldimines (Schiff bases),
which are isomerized (Amadori rearrangement) to yield
stable ketoamine (fructosamine) products (Figure 2-12).
The degree of glycation achieved in a protein is deter-
mined by the concentration of sugar in the environment of
the protein. In glycated hemoglobin, a Schiff-base adduct
forms between the sugar and the N-terminal group of the
-chains of hemoglobin.
The Amadori sugar-amino acid residue adducts in
proteins are produced with prolonged hyperglycemia and
undergo progressive nonenzymatic reactions involving
dehydration, condensation, and cyclization. These com-
pounds are collectively known as
advanced glycosylation
end products
and are involved in the chronic complica-
tions of
diabetes mellitus
(cataracts and nephropathy)
(Chapter 22).
