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catalytic or a structural function; see the classification below. The daily requirement for zinc is
3-25 mg, a demand which, since zinc is ubiquitous, is commonly satisfied by our nutrients.
Diseases due to zinc deficiency encompass disturbance of growth, arthritis-related health
problems, break-down of the immune system and loss of taste. They are usually a consequence
of impaired zinc absorption rather than of undersupply. In the blood stream, zinc is transported
by albumin and transferrin. Zinc storage is achieved by thioneins (vide infra).

Interlude: Inorganic chemistry of zinc

Important minerals: ZnS (zinc blende, wurtzite, sphalerite), ZnCO 3 (zinc spar, calamine).
Earth’s outer sphere (17 km crust + hydro- + atmosphere) contains 0.007 % (by weight) of
zinc.

Metallic zinc is obtained by firing of ZnS (→ ZnO + SO 2) followed by reduction of ZnO with
coal, or by electrolytic reduction of aqueous zinc sulphate. Applications include alloys (such
as brass, a Cu-Zn alloy), galvanisation (of iron), torch batteries (Lechlanché element).

0
+
The redox potential is E = -0.763 V, i.e. Zn is oxidised by H . In air, Zn is, however, stable
due to passivation [formation of compact layers consisting of ZnO, Zn(OH) 2 and
2+
Zn(OH)(HCO 3)]. In aqueous media, Zn exists in the form of the Brønstedt acid
2+
2+
[Zn(H 2O) 6] ; Zn itself is a Lewis acid (and this is determinant for its enzymatic actions).
-
+
2+
Zinc hydroxide is amphoteric: Zn(OH) 2 + 2H → Zn + 2H 2O; Zn(OH) 2 + 2OH →
2-
[Zn(OH) 4] . With halogenated alkanes RX, zinc forms reagents of composition RZnX, which
transform to ZnX 2 and ZnR 2 on heating. RZnX and ZnR 2 are alkylating agents.

2+
2-
Zn forms complexes mainly of coordination numbers 4 ([Zn(SR) 4] , tetrahedral;
[Zn(Gly) 2], square planar), 5 ([Zn(acac) 2H 2O] [acac = acetylacetonate(1-), square-pyramidal]
and 6 (octahedral).

Ointments containing zinc (in the form of ZnO, Zn(OH) 2, zinclactate) have already
been employed in the Middle Age and are still employed today in wound healing. The
essentiality of zinc for life had been discovered in 1869 by Raulin (inhibition of the growth of
the mould Aspergillus niger caused by undersupply of zinc). In 1940, the first zinc-dependent
enzyme, carboanhydrase, was isolated by Keilin and Mann, followed by the discovery, in
1954, of the second enzyme, pancreatic bovine carboxypeptidase-A. In 1985, the role of zinc in
genetic transcription (“zinc fingers”) became established, and in 1995, the Zn-based Ada repair
2+
protein (demethylation of DNA) was characterised. A role of Zn in synaptic transmission was
found in 2006.

Classification of zinc proteins according to function:

1. Enzymatic:
- Ligases and synthases (C-C-bond formation, e.g. aldolases)
2+
- Hydrolases: Here, Zn is coordinated by three amino acid side-chains of the protein matrix
(His, Cys and/or Asp) plus water (inactive, resting form) or a hydroxy group (active form); see
the scheme below. Examples: carboxypeptidase-A (an exopeptidase acting at the C-terminus of
the peptide), thermolysin (a thermophilic exopeptidase acting at the N-terminus).