|MadSci Network: Biochemistry|
The degree to which copper may or may not displace iron from the porphyrin ring will depend on the binding constant, not the reactivity. This will also be affected by the relative amounts of the two ions present in solution. As you point out, copper (II) is also known to have a deleterious effect on the three-dimensional structures of many proteins, a major reason for the toxicity of compounds like copper sulfate (see http://pmep.cce.cornell.edu/profiles/extoxnet/ carbaryl-dicrotophos/copper-sulfate-ext.html). Many proteins contain significant numbers of the amino acid Cysteine, the side chain of which contains a sulfhydryl (-SH) group. The sulfhydryl groups of two Cysteine residues can form a covalent disulfide bond (-S-S-) that holds two sections of the protein together.
Copper (II) can cause protein denaturation because of its ability to coordinate to individual sulfhydryl groups. When this happens, the -S-S- linkage breaks, and the sections of the protein that were held together are now free to move apart, which usually allows further structural alterations, and loss of activity.
This type of activity loss will be irreversible, and would give rise to what is called irreversible inhibition. By contrast, if all that his happening is displacement of the iron by copper in the porphyrin, this type if inhibition will be reversible. It will be reversed if the excess copper is removed (for example, by dialysis) and iron is supplied in the buffer.
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