Modulation of ligand binding affinity of tumorigenic carbonic anhydrase XII upon interaction with cationic CdTe quantum dots

The structural data of tumorigenic carbonic anhydrase (CA) XII revealed that the enzyme surface opposite to the active site pocket was negatively charged, and thus it had potential to interact with the positively charged surfaces. We investigated the influence of cationic CdTe quantum dots on the catalytic and ligand binding properties of the enzyme. Although cationic quantum dots interacted with CAXII (with a K_d value of 2.1 μM), they did not impair the enzyme's catalytic activity, suggesting that the accessibility of the enzyme's active site remained unaffected by the above interaction. When CAXII bound dansylamide (serving as a fluorescence probe as well as a potent inhibitor of the enzyme) was titrated with cationic quantum dots, the fluorescence spectral profiles revealed a marked transfer of the excited state energy between the above species. However, the binding of quantum dots to CAXII weakened the affinity of dansylamide for the enzyme, and thus obviated the inhibitory feature of the ligand. Since the quantum dots and dansylamide are bound at the opposite sides of CAXII, their influence must be mediated via changes in the protein conformation at a distal region as observed with allosteric enzymes. The mechanistic insights gained from these studies may lead toward developing diagnostic protocols for tumorigenic CAXII.