Influence of Active Site Ligands and Nanoparticle Surfaces on Human Carbonic Anhydrase Isozymes

Abstract

Carbonic anhydrase (CA) is an ubiquitously distributed zinc containing metallo enzyme that catalyzes the reversible hydration of carbon dioxide to form bicarbonate and a proton. Existence of 16 isoenzymes of CA in the animal kingdom has been known so far with varying subcellular and tissue distributions. Due to their involvement in many physiological and pathological processes, these isozymes have been the target for drug designing for the past 6 decades. The present study was designed with the aim of understanding the effect of active site ligands and nanoparticle surfaces on human carbonic anhydrase isozymes. In an effort to identify a fluorescent probe for carbonic anhydrases, the quantum yields and binding affinities of a variety of naphthalenesulfonamide derivatives with human carbonic anhydrase isozymes (hCAs) were determined. In this pursuit, a highly sensitive fluorescent probe, JB2-48 was identified. Experiments involving the above flurophore with hCA I unraveled the contributions of the sulfonamide moiety and the hydrophobic regions of the ligand structure on the spectral, kinetic, and thermodynamic properties of the enzyme-ligand complex. The fluorescence data revealed that the deprotonation of the sulfonamide moiety of the enzyme-bound ligand increases the fluorescence emission intensity as well as the lifetime of the ligand. This is manifested via the electrostatic interaction between the active site resident Zn2 + cofactor and the negatively charged sulfonamide group of the ligand. Evidence was provided that the anionic and neutral forms of JB2-48 are stabilized by the complementary microscopic/conformational states of the enzyme. Investigations on the binding of the sulfonamide inhibitor, benzene sulfonamide (BS), with hCA isozymes II and VII, revealed that the binding is stabilized by polar interactions in the former case and hydrophobic interactions in the latter case. In addition, it was found that the binding of BS with hCA II is enthalpically driven at low temperatures, whereas it is entropically driven for its binding with hCA VII. Due to the prevalence of bipolar distribution of charges on hCA XII, the effects of the interaction of differently charged quantum dots, liposomes and polylysine on hCA XII were investigated. These charged particles were found to differently modulate the active site of the enzyme. The data revealed that whereas poly lysine and liposomes exhibited no influence on the binding and catalytic features of the enzyme, quantum dots had significant influence on the above features. Arguments were presented that the above differential feature exhibited by quantum dots, liposomes and poly lysine is encoded in the rigidity versus flexibility of the charged molecules. Studies on the denaturation of hCA isozymes II and XII unraveled their unfolding mechanism. It was found that the unfolding ofhCA XII followed a simple two state model from native to unfolded state; however hCA II unfolded with the formation of a stable intermediate.