Binding Affinity and Reactivity

3 Binding Affinity and Reactivity 3.1 Metal Binding Affinities 

A variety of metal ions has been reported to bind to MT in vivo and in vitro. The cadmium metalloform has not only been found naturally but its generation in vitro has been proved indispensable in structural studies. In all Cd-MTs studied so far Cd(n) is tetrahedrally coordinated by solely Cys or both Cys and His ligands. 

At neutral pH closely similar average apparent stability constants in the order of 10 have been determined for various Cd-MTs with sole cysteine thiolate coordination by different methods. 

Most widely used methods are based on (i) the competition of protons with the bound metal ions for thiolate ligands, i.e., the pH stability of the metal-thiolate clusters or (ii) the competition with a metal chelator. In the former case the pH stability of the Cd-thiolate clusters is followed by absorption spectroscopy of the CysS-Cd(II) LMCT band at 250 nm. From the apparent p/Ca values the apparent stability constants of the clusters can be derived [103]. The apparent p/sTa values are determined either by taking the pH values of half-maximum absorbance or using a non-linear curve fit of the pH plot [104]. The other method is based on the competition for a single metal ion between the chelator 5F-BAPTA (1,2-bis-(2-amino-5-fluorophenoxy)ethane-A,W,A ,A -tetraacetic acid) and the protein followed by NMR spectroscopy [71]. Although this method was established for the zinc metalloforms of MTs, its applicability to cadmium metalloforms has also been demonstrated [105]. In this case, by analogy with zinc finger proteins a lower affinity for mixed Cys/His coordination of Cd(II) in MTs compared to sole Cys coordination has been shown. 

Apart from the conformational flexibihty of the polypeptide chain, dynamic processes within the metal-thiolate clusters have also been recognized. The best evidence for metal fluxionality in CdyMT was provided by Cd NMR saturatimi transfer experiments, which established the presence of inter- and/or intramolecular metal exchange within the 3-metal cluster of the p-domain with a half-Ufe of the order of 0.5 s ([109], and refs cited therein). The confirmation of similar processes taking place within the 4-metal cluster, but with a half-life of about 16 minutes. 

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