Catalysis metallothionein

LASER-FLASH KINETIC ANALYSIS METAL ION CATALYSIS METALLOTHIONEINS Metal-nucleotide complex,... 

Complete removal of zinc and thus inactivation of the enzyme can be accomplished in these systems at low D-PEN concentrations if a secondary scavenger chelator is added to the system. Such chelators bind metal that has been released from the enzyme but do not participate in the release.In the case of carboxypeptidase A, aM thionein (apo-metallothionen see Metallothioneins) inhibits catalysis by only about 10% over a 15-min period consistent with its action as a secondary chelator. However, in the presence of 250 aM D-PEN and aM thionein total inhibition is achieved in less than 15 min. D-PEN accelerates zinc equilibration between carboxypeptidase A and thionein (Scheme 1). This is accomplished by D-PEN catalyzing the release of Zn from the enzyme. Since D-PEN is in vast excess over both the enzyme and thionein, the enzyme-released zinc would be expected to bind to D-PEN first. However, since thionein binds zinc more tightly than D-penicillamine and can accept 7 moles of zinc per mole of thionein, it should be the ultimate acceptor of the released zinc. 

Binding those metal ions in a metalloprotein usually prevents them from entering into these types of reactions. For example, transferrin, the iron-transport enzyme in serum, is normally only 30 percent saturated with iron. Under conditions of increasing iron overload, the empty iron-binding sites on transferrin are observed to fill, and symptoms of iron poisoning are not observed in vivo until after transferrin has been totally saturated with iron. Ceruloplasmin and metallothionein may play a similar role in preventing copper toxicity. It is very likely that both iron and copper toxicity are largely due to catalysis of oxidation reactions by those metal ions. 

Tomschik, M., Havran, L., Palecek, E., Heyrovsky, M. (2000). The presodium catalysis of electroreduction of hydrogen ions on mercury electrodes by metallothionein an investigation by constant current derivative stripping chronopotentiometry. Electroanalysis 12, 274-279. 

Scheme 23.8 Cu-catalyzed desulfitative catalysis using metallothionein mimics.

Detection of metallothionein is connected with specific structure of MT, i.e. the absence of aromatic amino acids in the native state and disulfide bonds. However, its structure is adapted to incorporate heavy metal ions by sulfhydryl groups, thus, these specific features may be used for the electrochemical determination. The protection by reduction agents is required, because sulfbydryl groups have a tendency to be oxidized. On the other hand, this property allows an application of electrochemical determination. The other opportunity how to detect metallothionein is the catalysis of hydrogen evolution from a supporting electrolyte [38-41]. 

---