Crystal structure of Cd,Zn metallothionein

Area of Science:

• Biochemistry
• Structural Biology
• Metalloprotein Chemistry

Background:

• Metallothioneins are crucial for metal homeostasis and detoxification.
• Understanding the structure of cadmium, zinc (Cd,Zn) metallothionein isoform II is vital for its biological function.
• Rat liver isoform II provides a model for studying metalloprotein-metal interactions.

Purpose of the Study:

• To elucidate the three-dimensional crystal structure of rat liver cadmium, zinc (Cd,Zn) metallothionein isoform II.
• To characterize the metal-binding sites and coordination geometry within the protein.
• To investigate the structural relationship between the alpha and beta domains.

Main Methods:

• Anomalous scattering data from cadmium (Cd) in the native protein.
• Direct methods for solving the crystal structure.
• Iterative single-wavelength anomalous scattering (SAS) to calculate a 2.3 Å resolution electron density map.

Main Results:

• The crystal structure revealed two distinct domains: a beta domain (residues 1-29) and an alpha domain (residues 30-61).
• The beta domain contains a three-metal cluster (one Cd, two Zn) coordinated by cysteine thiolates.
• The alpha domain contains a four-metal cluster (Cd,Zn) coordinated by cysteine thiolates, with all seven metal sites exhibiting tetrahedral geometry.

Conclusions:

• Rat liver cadmium, zinc (Cd,Zn) metallothionein isoform II folds into two topologically similar domains with opposite chirality.
• Cysteine-rich sequences play a conserved role in metal cluster formation across both domains.
• The structure provides insights into the coordination of multiple metal ions by metallothioneins.