Differential modulation of the active site environment of human carbonic anhydrase XII by cationic quantum dots and polylysine.

Due to prevalence of negative charges on the protein surface, opposite to the active site pocket of human carbonic anhydrase XII (hCA XII), both positively charged CdTe quantum dots (Qds(+)) and polylysine electrostatically interact with the enzyme, and such interaction does not influence the catalytic activity of the enzyme. However, both these cationic macromolecules differently modulate the active site environment of the enzyme. The steady-state kinetic data revealed that whereas polylysine exhibited no influence on dansylamide (DNSA) dependent inhibition of the enzyme, Qds(+) overcame such an inhibitory effect, leading to almost 70% restoration of the catalytic activity of the enzyme. We provide evidence that DNSA remains bound to the enzyme upon interaction with both polylysine and Qds(+). Arguments are presented that the above differential feature of polylysine and Qds(+) on hCA XII is encoded in the "rigidity" versus "flexibility" of these cationic macromolecules. Copyright 2010 Elsevier B.V. All rights reserved.