Peptide folding, metal-binding mechanisms, and binding site structures in metallothioneins

This minireview specifically focuses on recent studies carried out on structural aspects of metal-free metallothionein (MT), the mechanism of metal binding for copper and arsenic, structural studies using x-ray absorption spectroscopy and molecular mechanics modeling, and speciation studies of a novel cadmium and arsenic binding algal MT. Molecular mechanics-molecular dynamics calculations of apo-MT show that significant secondary structural features are retained by the polypeptide backbone upon sequential removal of the metal ions, which is stabilized by a possible H-bonding network. In addition, the cysteinyl sulfurs were shown to rotate from within the domain core, where they are found in the metallated state, to the exterior surface of the domain, suggesting an explanation for the rapid metallation reactions that were measured. Mixing Cu6beta-MT with Cd4alpha-MT and Cu6alpha-MT with Cd3beta-MT resulted in redistribution of the metal ions to mixed metal species in each domain; however, the Cu+ ions preferentially coordinated to the beta domain in each case. Reaction of As3+ with the individual metal-free beta and alpha domains of MT resulted in three As3+ ions coordinating to each of the domains, respectively, in a proposed distorted trigonal pyramid structure. Kinetic analysis provides parameters that allow simulation of the binding of each of the As3+ ions. X-ray absorption spectroscopy provides detailed information about the coordination environment of the absorbing element. We have combined measurement of x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) data with extensive molecular dynamics calculations to determine accurate metal-thiolate structures. Simulation of the XANES data provides a powerful technique for probing the coordination structures of metals in metalloproteins. The metal binding properties of an algal MT, Fucus vesiculosus, has been investigated by UV absorption and circular dichroism spectroscopy and electrospray ionization-mass spectrometry. The 16 cysteine residues of this algal MT were found to coordinate six Cd2+ ions in two domains with stoichiometries of a novel Cd3S7 cluster and a beta-like Cd3S9 cluster.