Chemistry Reference
In-Depth Information
Hydrolysis may proceed in several stages until the
last coordinated water molecule is removed. The proc-
ess of hydrolysis may be interrupted if at one stage an
insoluble compound is produced. Hydrolysis occurs
most readily with metal ions that strongly polarize the
coordinated water molecules.
6.2 Metal-Containing Biological Molecules
Many metals play important roles for the biologi-
cal activity of enzymes and vitamins when consti-
tuting either part of the structure or as a cofactor of
these entities. Zinc, for example, plays an essential
role in the zinc-dependent enzyme alcohol dehydro-
genase, and cobalt is essential to vitamin B
12
. Aspects
on the biochemistry in this topic are given in Hughes'
monograph (1972).
Metalloproteins (also called conjugated proteins)
consist of a protein and a prosthetic group or cofactor
that consists of a metal. Metalloenzymes are defi ned as
holoenzymes and a prosthetic group or cofactor that
consists of a metal.
Hemoglobin, hemerythrin, and myoglobin carry
oxygen bound to iron. Examples of redox proteins are
iron sulfur proteins like cytochromes
c
and
b5
. Redox
enzymes are cyt-p450, catalase, and peroxidase.
A number of proteins are metal carriers, as in the
case of the blood proteins shown in Table 3.
6 OTHER ASPECTS OF METAL
CHEMISTRY OF BIOLOGICAL AND
TOXICOLOGICAL INTEREST
6.1 Main Group and Transition Metals
According to the
IUPAC Nomenclature of Inorganic
Chemistry
(Freiser and Nancollas, 1987), the main group
metals are those belonging to the periodic system group
1 (alkali metals Na, K, Rb, Cs, Fr), group 2 (alkaline
earths Be, Mg, Ca, Sr, Ba, Ra), group 13 (Al, Ga, In, Tl),
group 14 (Ge, Sn, Pb), group 15 (Sb, Bi), and group 16
(Po). The transition metals are those belonging to group
3 (Sc, Y, La, Ac), group 4 (Ti, Zr, Hf), group 5 (V, Nb,
Ta), group 6 (Cr, Mo, W), group 7 (Mn, Tc, Re), group 8
(Fe, Ru, Os), group 9 (Co, Rh, Ir), group 10 (Ni, Pd, Pt),
group 11 (Cu, Ag, Au), and group 12 (Zn, Cd, Hg).
Elements with partly fi lled d- or f-orbitals are usu-
ally defi ned as transition elements. A broader defi ni-
tion would be elements in any oxidation state in which
they form compounds with partly fi lled d- or f-orbit-
als. By this defi nition also Cu, Ag, and Au would be
included. There are 56 transition elements of the d- and
f-block. All have the same common properties:
6.2.1 Metalloporphyrins
The metalloporphyrins include two important cat-
egories: the chlorophyll molecule and the molecules
carrying the heme group. The ability of chlorophyll to
absorb light is related to the conjugated polyene struc-
ture of the porphyrin ring. Magnesium ions that are
coordinated to the nitrogen atoms of the four pyrrole
rings have at least two functions. They provide the
necessary structural rigidity, and they increase the rate
of conversion of the singlet-excited state resulting from
photon absorption into the triplet state that enables the
transfer of the excitation energy into the redox chain.
The two main functions of iron-containing bio-
logical complexes are the transport of oxygen and the
mediation in electron transfer chains. The heme group
is in all cases associated with a protein molecule as in
hemoglobin, myoglobin, cytochromes, and enzymes
such as catalase and peroxidase. Cytochromes serve
as electron carriers, and the heme-containing enzymes
catalase and peroxidase catalyze the decomposition
of hydrogen peroxide. Catalase is involved in the
oxidation of mercury vapor.
a. They are all metals.
b. They exhibit variable oxidation states with a few
exceptions.
c. Because of partly fi lled d- and f-orbitals, they
form at least some paramagnetic compounds.
d. Their ions and compounds are colored in one or
all oxidation states.
Properties b and c are of great biological importance
because of their role in biological catalysis and by their
electron transport function.
The transition elements are further subdivided into
three main groups:
A. The main transition elements or d-block
elements
B. The lanthanide elements
C. The actinide elements
TABLE 3
Examples of Metal-Carrying Proteins
Protein
Metal
Transferrin
Fe
Ceruloplasmin
Cu
The lanthanides and actinides are classifi ed as
elements of the f-series.
http://www.iupac.org/reports/periodic_table/IUPAC_
Periodic_Table-14Jan05-CI.pdf
Metallothionein
Zn, Cd, Hg, Cu
Albumin
Zn, Cd
Phosphoproteins
Ca
