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The first step of the transcription of the genetic information stored in the DNA into a
protein structure occurs in such a way that a complementary messenger-RNA (mRNA) is
synthesised at a specific DNA segment (carrying this information). An RNA-synthase is
needed, and a “pilot“ (the transcription factor) to “direct“
the RNA-synthase to the DNA section in question. This X
transcription takes place in the nucleus of the cell, while F
the locus for the protein synthesis, the so-called
translation, is the ribosome. In many cases, the
2+
transcription factors are Zn based, containing the zinc Y
ion tetrahedrally coordinated to typically 2 Cys and 2
2+
His. Zn structurally stabilises a loop containing C L
specific amino acid moieties necessary for recognising C Zn H
the DNA site, and thus binding to the respective large Z
groove, where the information for the synthesis of the F H
specific protein is contained. An example is shown to the
right: C = Cys(1-), H = His, F = Phe, Y = Tyr, L = Leu, Z = Glx (Glu, Glu(1-) or Gln). Zinc
fingers usually contain more than just one domain, which work in tandem for nucleic acid
recognition; each domain interacts with three base pairs of DNA.

Thioneines

These are small proteins (ca. 6000 Da; 61 amino acids) with a high percentage of
cysteine (ca. 1/3) and serine, and no aromatic amino acids. Thioneines can accommodate up to
2+
seven Zn or other metal ions and probably serve as zinc and/or cysteines storage proteins.
2+
They are further used in the (transient) detoxification of heavy metal ions such as Cd und
2+
Hg Fig. 27 shows the two domains of a common thioneine.

S S OOC
S S S
S NH3 S S
S S S
S S
S S
S S S S
S
S S

2+
Figure 27. Structure of a (M ) 7 thioneines. The metal centres are represented by hatched
circles. The solid line represents the protein back-bone, the arrow the point where the protein
can be enzymatically split into its two domains. One of the domains contains a cyclic M 3S 3
cluster in the chair conformation, the other domain an adamantane-like M 4S 5 cluster..

10. Cadmium and mercury

The two haevier homologues of zinc, cadmium and mercury, are toxic. The toxicity of
2+
2+
Cd can be traced back, in part, to its higher thiophilicity, allowing Cd to replace zinc in its
2+
2+
enzymes. Since Cd has a larger ionic radius than Zn (see Table), it is sufficiently less
2+
2+
Lewis-acid, i.e. a replacement of Zn for Cd commonly results in a deactivation of the
2+
enzyme. An exception is the marine diatom Thalassiosira weissflogii, which contains Cd