Metallothioneins evolution

Kizek etal. [191] have involved catalytic reaction at mercury electrode to electrochemical determination of metallothioneins. This determination was based on catalytic processes occurring at very negative potentials and accompanied by catalytic evolution (SH group) of hydrogen. 

With few exceptions, metallothioneins consist of relatively simple amino acids, aromatic amino acids and histidine only being found in a small number of species [329]. This amino acid composition suggests that metallothioneins evolved early in the evolution of life, probably even before the oxygenation of the atmosphere. A further clue is one of their functions. As metal-transport and storage proteins, thioneins are capable of binding metal ions but release them relatively easily as well. Metallothioneins can therefore be considered a transition from non-metal to metalloproteins. It is improbable, however, that the known copper proteins evolved from copper metallothioneins as there are no homologies between them and other copper proteins or enzymes. 

Kagi JHR, Kojima Y, Berger C, Kissling MM, Lerch K, Vasak M (1979) Metallothionein structure and evolution. In Weser U (ed) Metalloproteins. Georg Thieme, Stuttgart New York, p 194... 

Binz pa and Kagi JHR (1999) Metallothionein Molecular evolution and classification. In Klaassen CD, ed. Metallothionein IV, pp. 7-13. Birk-hauser Verlag, Basel. 

Kagi JHR (1991) Overview of metallothionein. Methods Enzymol 205 613-626 Kagi JHR (1993) Evolution, structure and chemical activity of class I metal-lothioneins an overview. In Suzuki KT, Imura N, Kimura K (eds) Metallothionein III. Birkhauser, Basel, pp 29-55 Klaassen CD (1978) Effect of metallothionein on the hepatic disposition of metals. Am J Physiol 234 E47-E53... 

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]. 

Figure 5. (a) Probable scheme of the catalytic hydrogen evolution in the Brdicka reaction, (b) Real voltammograms of 20 fmol metallothionein (red line) and the Brdicka s solution as a supporting electrolyte containing 1 mM Co(NH3)5Cl3 and 1 M NH3(aq ) + NH4CI, pH 9.6 (black line). Parameters of DPV method were as follows initial potential -0.35 V, end potential -1.8 V, modulation time 0.057 s, time interval 0.2 s, step potential 1.05 mV/s, modulation amplitude 250 mV, time of accumulation of MT 120 s, Eads = 0 V. All measurements were carried at temperature of 5 °C. 

---