Bioinorganic Chemistry
637
¥
Bioinorganic chemistry focuses on the roles played by noncarbon elements in life
processes. Yet, bioinorganic chemistry is inseparable from the general chemistry
of life. Carbon itself cycles among the many bioorganic compounds and inorganic
carbon dioxide and carbonates. More than 80% of all the carbon in the earth’s
crust occurs as CaCO
3
. About 30% of all enzymes contain metal ion cofactors. The
most common metal ion, zinc, appears in over 100 enzymes; iron and copper, in a
substantial number; manganese, cobalt, and molybdenum, in a few cases. Selenium
appears in the enzyme glutathione peroxidase. Metal ions stabilize nucleic acid
polymers, which bear a negative charge on each residue. Though blood is loaded with
an array of organic molecules, its main constituent is NaCl and that of intracellular
fluids is KCl. Even among the vitamins, a word coined from amines essential to life,
the action of vitamin B
12
depends upon a cobalt ion. Metal ions and nonmetals other
than carbon are intimately and inseparably involved in life processes.
1
Essentiality
Twenty-one elements are essential to hu-
mans. An essential nutrient is one for
which a deFciency results in an impair-
ment of function that is relieved only by
administration of that substance. Vitamins
by deFnition and some minerals are essen-
tial. The signiFcance of essentiality may be
illustrated by burlesquing an old adage.
‘‘±or want of a nail the shoe is lost, for want
of a shoe the horse is lost, for want of a horse
the rider is lost.’’
George Herbert,
Jacula
Prudentum
(Out-
landish Proverbs), 1640
‘‘±or want of a nutrient the enzyme is lost,
for want of an enzyme the function is lost, for
want of a function the life is lost.’’
Bruce Martin,
Summa Veritatis
(Lofty Truth),
1989
±our essential elements H, O, C, and
N comprise more than 99 atom % and
abou
t96w
t%ofthehumanbody
.These
4 and 14 other essential elements occur
among the Frst 30 elements (through zinc)
of the periodic table. Three heavier trace
elements Se, Mo, and I are also essential
in humans. ±or 17 tabulated essential
elements, Table 1 shows the predominant
elemental form at pH 7, typical adult
concentrations in the blood plasma or
serum, the approximate amount found
in a 70 kg adult, and a recommended
adult daily allowance. In addition to the
four basic elements, the essential elements
include two alkali metal ions, two alkaline
earth metal ions, seven transition metals
(the
most
common,
iron,
contributes
to less than 0.01% of body weight),
phosphorus, sulfur, selenium, and three
halogens. Table 1 shows that most of the
remaining 4% of body weight consists
of 2 elements found in bone, Ca and
P. Many of the elements do not exist
predominantly in their pH 7 forms in the
serum as they are combined with other
components. ±or example, ±e
3
+
does not
precipitate as the hydroxide but is retained
by tightly chelating ligands. There is little
free iodide; it occurs as part of the thyroid
hormones. ±or sulfur, Table 1 lists the
total serum concentration, most of which
appears in proteins; there is only about
1 mM nonprotein sulfur. Sulfur is not
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