The
antibiotic valinomycin and several other antibiotics work in the
same way as EDTA. Valinomycin is known to make it easier for potassium
ions to get through biological membranes, and x-ray crystallographers
found out why recently when they solved the structure of the potassium
salt of the antibiotic. The valinomycin molecule is a closed ring
of 36 atoms, with 12 carbonyl (C=O) groups. It wraps around the
potassium ion with six of the carbonyl oxygens as octahedral ligands,
and effectively gives the ion an organic molecule "overcoat." The
coated ion can slip through a membrane more easily because its charge
is disguised. Part of the toxicity of this antibiotic to microorganisms
is believed to lie in its upsetting of the natural balance of potassum
ions within the cell membranes.
The iron atom in the oxidation-reduction protein cytochrome c is
octahedrally coordinated, with four of the six ligands coming from
one large, planar porphyrin molecule, as shown below. This ironporphyrin
complex is called a heme group. In cytochrome c the fifth and sixth
octahedral ligands, above and below the plane of the heme group,
are provided by a nitrogen and a sulfur atom from the protein wrapped
around the heme. The delocalized electrons in the porphyrin ring
are shared with the iron, thereby changing its oxidation--reduction
behavior. Hemoglobin is another combination of heme groups and protein,
in which the fifth octahedral ligand is a nitrogen atom from the
protein, and the sixth position is left open for binding the
molecule that each hemoglobin molecule carries in the bloodstream.
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Four of the six octahedral ligand positions
of are occupied
by four nitrogen lone pairs from the flat porphyrin ring, and the
fifth and sixth positions are filled by other chemical groups. The
double-bond electrons in the porphyrin ring actually are delocalized
over the entire ring system. |