The binding of Cd(II) to the hemocyanin of the Mediterranean Crab Carcinus maenas

The interaction of Carcinus hemocyanin with Cd(II) was studied. The incubation of the apoprotein with the metal yields a derivative containing 1 g-at. of EDTA-stable Cd(II) per 75 kDa. Spectroscopic data ruled out Cd(II) coordination to tryptophan or cysteine residues. The optical activity and fluorescence properties of the protein are affected by Cd(II) binding and indicate a rearrangement of tryptophan residues. The poor Cd(II) binding to the oxy-form and the resistance of Cd(II)-hemocyanin to EDTA treatment and to the regeneration by Cu(I) strongly indicate that Cd(II) binding to apohemocyanin occurs at the copper-free active site. During the metal-binding process, a marked increase of light scattering is observed. This effect, however, is reversible provided that the incubation medium contains SCN- and glycine as exogenous ligands of the metal in the bulk solution.