Different behavior of sulfonamides with respect to copper-substituted bovine and human carbonic anhydrases

Long-range paramagnetic NMR data can provide a closer look on metal coordination in metalloproteins

Paramagnetic NMR data can be profitably incorporated in structural refinement protocols of metalloproteins or metal-substituted proteins, mostly as distance or angle restraints. However, they could in principle provide much more information, because the magnetic susceptibility of a paramagnetic metal ion is largely determined by its coordination sphere. This information can in turn be used to evaluate changes occurring in the coordination sphere of the metal when ligands (e.g.: inhibitors) are bound to the protein. This gives an experimental handle on the molecular structure in the vicinity of the metal which falls in the so-called blind sphere. The magnetic susceptibility anisotropy tensors of cobalt(II) and nickel(II) ions bound to human carbonic anhydrase II in free and inhibited forms have been determined. The change of the magnetic susceptibility anisotropy is directly linked to the binding mode of different ligands in the active site of the enzyme. Indication about the metal coordination sphere in the presence of an inhibitor in pharmaceutically relevant proteins could be important in the design of selective drugs with a structure-based approach.

Simultaneous and independent versus antagonistic inhibition of muscle carbonic anhydrase (CA III) by acetazolamide and cyanate

The inhibition by cyanate and acetazolamide of pig muscle carbonic anhydrase III (CA III) CO2 hydratase activity was studied in order to explore mechanistic features possibly unique to the muscle isoenzyme. The turnover number for CO2 hydration was found to be 6000 sec-1 with a Km of 83 mM for CO2. Cyanate inhibition (Ki, 3 microM) and acetazolamide inhibition (Ki, 44 microM) were both found to be noncompetitive with respect to CO2. Significantly, acetazolamide and cyanate displayed non-exclusive binding to pig muscle carbonic anhydrase. The similarity of mode and degree of inhibition of muscle carbonic anhydrase by cyanate as compared with the inhibition of the erythrocyte isoenzymes suggests the existence of a similar metal environment. However, the observation that cyanate and acetazolamide bind simultaneously to CA III and the comparatively large Ki for acetazolamide per se appear to be more compatible with a different mode of coordination of the zinc with the sulfonamide, thus supporting a five-coordinate zinc in the catalytic mechanism of CO2 hydration for CA III.

Investigation of the system copper(II) carbonic anhydrase and HCO3-/CO2

Copper(II) substituted human and bovine carbonic anhydrases B in the presence of bicarbonate have been investigated in solution through water-solvent proton nuclear magnetic resonance (nmr) at variable magnetic fields. HCO3-, contrary to all the other monoanionic inhibitors, partially reduces the water proton relaxation rates. This has been accounted for on the basis of the availability within the active cavity of two coordination positions partially overlapping. 13C-nmr measurements on both CO2 and HCO3- confirm that HCO3- binds the metal, whereas CO2 interacts with the paramagnetic center at nonbonding distance. The upper limit for the CO2 in equilibrium HCO3- interconversion has been estimated to be 10 sec-1.