Engineering Chemistry S Datta

ROLE OF METALS IN BIOLOGY 563
The processes and reaction in which iron participates are crucial for the survival of
organisms and they include DNA synthesis, respiration, photosynthesis, nitrogen reduction,
oxygen transport and oxygenations.
Deficiency of iron leads to iron-deficiency anemia, in which the number of red blood
cells are normal but the amount of hemoglobin in the cells is low.
Copper
Copper is essential in the diet because it is involved in the proper utilization of iron
especially for the synthesis of cytochrome oxidase, which contains both iron and copper. It is
required to the extent of 2.5 to 5 mg/day and is found in meat, seafood, nuts, and vegetables.
Copper and iron proteins participate in many of the biological reactions:
• for cytochrome oxidase synthesis which catalyzes the transfer of electrons from water
to dioxygen during respiration.
• reversible binding of dioxygen, e.g. hemoglobin, hemocyanin etc.
• activation of dioxygen.
• dismutation of superoxide by Cu or Fe as the active metal.
Use of stored iron is reduced by copper deficiency, which suggests that iron metabolism
may depend on copper proteins.
Copper is also present in the active group of lysyl oxidase, an enzyme responsible for
cross-linking in collagen and elastin. Hence animals that are copper deficient develop defective
collagen molecules (due to absence of cross-linking) leading to rupture of arteries.
In the photosynthetic electron transport, the actual carrier of electrons is the blue copper
protein—plastocyanin. Here the copper atom carries the electron undergoing Cu(I)–Cu(II) cycles.
Zinc
Zinc is an essential component of nearly about 80 to 100 different enzymes as part of
their prosthetic groups. The enzymes include alcohol dehydrogenase, carbonic anhydrase, DNA
and RNA polymerases, carboxy peptidase. It is also present in many NAD and NADP linked
dehydrogenases which promote the transfer of hydride ions from substrates to NAD + /NADP + .
As a part of the carbonic anhydrase, it catalyzes the hydration of CO 2
to H 2
CO 3
, whereas
in carboxypeptidases, it participates in proteolytic activity. Zinc is found in high concentration
in sperm cells, prostate gland and eyes. The hormone insulin is stored complexed with zinc.
Zinc also has a role in the functioning of taste and smell receptors respectively of tongue
and nose.
Mechanism
Zinc has such an important role as the prosthetic group of large number of enzymes
since it can co-ordinate with four, five or six ligands without any preference. The ligands can
be easily exchanged with one water molecule. Zinc acts as a Lewis acid, it does not change its
protein ligand nor does it change its oxidation state but only changes the co-ordination sphere
on the side exposed to water/solvent. Zinc has a high affinity for nitrogen, oxygen and sulfur. It
is therefore found to be bound to histidines, glutamates or aspartates. It does not always
participate in the catalytic steps but its presence increases catalytic rate. Thus from above it
can be said that zinc plays both structural and regulating roles for the enzymes.
Daily requirement of zinc for an adult is 15 mg. It is present in abundance in meat, egg,
sea foods, milk and liver but in low amount in fruit and green vegetables.