Determination of the complex stability of zinc with carbonic anhydrase in sea-water

Carbonic anhydrase (CA) is inactive unless associated with zinc, with possi ble substitution by cobalt. In this work, the complexation of zinc by CA wa s determined in sea-water using cathodic stripping voltammetry (CSV) with l igand competition. The zinc was found to be released from the CA over a per iod of 3 h when equilibrated with a competing complexing ligand and the com plex was re-formed with the CA when zinc was added. A value of 8.90 +/- 0.2 7 was found for logK' (ZnCA) where K' (ZnCA) is the conditional stability c onstant for the complex of Zn2+ with CA in pH 8 sea-water. A value for the molecular weight of CA was calculated from its equivalent ligand concentrat ion (in nM) obtained by titrations with zinc at various CA concentrations ( 1-4 mg l(-1)). The value found (34740 g mol(-1)) for the molecular weight i s consistent with values found previously by other methods (29000-31000 g m ol(-1)) confirming that the stoichiometry of the complex between zinc and C A is 1:1. This work confirms that the zinc-CA complex is reversible and tha t the interaction between zinc and CA can be determined using CSV with liga nd competition.