Probing the energetics of dissociation of carbonic anhydrase-ligand complexes in the gas phase

This paper describes the use of electrospray ionization-fourier transform i on cyclotron mass spectrometry (ESI-FTICR-MS) to study the relative stabili ties of noncovalent complexes of carbonic anhydrase II (CAII, EC 4.2.1.1) a nd benzenesulfonamide inhibitors in the gas phase. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) was used to determine the energetics of dissociation of these CAII-sulfonamide complexes in the gas phase. When two molecules of a benzenesulfonamide (1) were bound simult aneously to one molecule of CAII, one of them was found to exhibit signific antly weaker binding (Delta E-50 = 0.4 V, where E-50 is defined as the ampl itude of sustained off-resonance irradiation when 50% of the protein-ligand complexes are dissociated). In solution, the benzenesulfonamide group coor dinates as an anion to a Zn(II) ion bound at the active site of the enzyme. The gas phase stability of the complex with the weakly bound inhibitor was the same as that of the inhibitor complexed with apoCAII (i.e., CAII with the Zn(II) ion removed from the binding site). These results indicate that specific interactions between the sulfonamide group on the inhibitor and th e Zn(II) ion on CAII were preserved in the gas phase. Experiments also show ed a higher gas phase stability for the complex of para-NO2-benzenesulfonam ide-CAII than that for ortho-NO2-benzenesulfonamide-CAII complex. This resu lt further suggests that steric interactions of the inhibitors with the bin ding pocket of CAII parallel those in solution. Overall, these results are consistent with the hypothesis that CAII retains, at least partially, the s tructure of its binding pocket in the gas phase on the time scale (seconds to minutes) of the ESI-FTICR measurements.