Comparison of the solution conformations of human [Zn7]-metallothionein-2 and [Cd7]-metallothionein-2 using nuclear magnetic resonance spectroscopy

The solution structure of native human [Zn₇]-metallothionein-2 has been compared with the previously determined structure of human [Cd₇]-metallothionein-2. The comparison was based on complete sequence-specific ¹H nuclear magnetic resonance assignments for human [Zn⁷]-metallothionein-2 obtained using the sequential assignment method. The secondary structure was found to be very similar in the [Zn₇]- and |Cd₇]- forms of the protein. Only seven amide protons in [Zn₇]- metallothionein-2 were found to have exchange rates lower than ~0.2 min^-1 at pH 7.0 and 10 °C, which corresponds closely to the results of amide proton exchange studies with the [Cd₇]- form of the protein. Finally, the ¹H-¹H distance constraints determined from nuclear Overhauser enhancement spectroscopy for human [Zn₇]-metallothionein-2 were checked for compatibility with the [Cd₇]-metallothionein-2 structure. Overall, although no direct method is available for identifying the metal polypeptide co-ordinative bonds in the Zn²⁺-containing protein, these measurements provided several independent lines of evidence showing that the [Zn₇]- and [Cd₇]- forms of human metallothionein-2 have the same molecular architecture.