The addition of Zn2+ to human carbonic anhydrase B holoenzyme was shown to enhance the protein fluorescence, and this enhancement was correlated with the inhibition of the p-nitrophenyl acetate esterase activity. The affinity for the inhibitory Zn2+ was increased when the ionic inhibitors, acetate or chloride, were added, suggesting that the inhibitory Zn2+-binding site is within the region of the protein that undergoes an anion-induced conformational change. A similar fluorescence enhancement was observed when Zn2+ was added to human carbonic anhydrase C and to bovine carbonic anhydrase, demonstrating that the binding site is not a thiol group. Circular-dichroism studies showed that the C isoenzyme but not the B isoenzyme underwent a major conformational change in the presence of Zn2+. A mechanism for the Zn2+-induced fluorescence enhancement was suggested on the basis of studies with simple compounds.
