Molten-Globule State of Carbonic Anhydrase Binds to the Chaperone-like α-Crystallin

α-Crystallin, a multimeric protein, exhibits chaperone-like activity in preventing aggregation of several proteins. We have studied the chaperone-like activity of α-crystallin toward heat-induced aggregation of bovine and human carbonic anhydrase. Human carbonic anhydrase aggregates at 60°C, while bovine carbonic anhydrase does not aggregate significantly at this temperature. Removal of the enzyme-bound metal ion, Zn2+, by EDTA modulates the aggregation behavior of bovine carbonic anhydrase. Fluorescence and circular dichroism studies show that removal of the metal ion from the bovine carbonic anhydrase by a chelator such as EDTA enhances the propensity of the enzyme to adopt the molten-globule state. α-Crystallin binds to this state of the enzyme and prevents aggregation. Fluorescence and circular dichroism studies on the α-crystallin-enzyme complexes show that the enzymes in the complex are in the molten-globule state. These results are of relevance to the interaction of chaperones with the partially unfolded states of target proteins.