198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

36 J.P. Glusker
b
a
Fig. 28 a, b. A zinc finger [72]: a diagram of the protein folding and the tetrahedral binding of
a zinc ion; b stereoview of one of the zinc fingers in the zif268 protein-nucleic acid complex
[72]
with a right hand with an index finger (the E helix), a curled second finger (the
loop), and a thumb (the F helix). This metal-binding motif has a strong affinity
for calcium ions. The motif is generally considered as an octahedral arrangement
around the calcium ion with one site shared by both carboxylate oxygen
atoms of a glutamate side chain to give a coordination number of 7. The affinity
of this arrangement for calcium ions is highest when there is a pair of contiguous
EF hands. For each hand the affinity is related to the net ligand charge of
the loop, and the hydrophobicity of various residues in the EF-hand pair [76].
Nature has utilized this calcium-specific binding motif in several proteins, such
as troponin C, calmodulin, parvalbumin, and lactalbumin.