Using proton nuclear magnetic resonance to study the mode of ribonuclease A inhibition by competitive and noncompetitive inhibitors

The C(2) proton resonances of the active site histidines (His 12 and His 119) of ribonuclease A have been exploited to study the inhibition pattern of both noncompetitive (four green tea polyphenols and their copper complexes) and competitive (3′-O-carboxy esters of thymidine and 3′-amino derivatives of uridine) inhibitors. Competitive inhibitors devoid of any phosphate group have the ability to change the pKa of the histidine residues at the active site. Their mode of inhibition, albeit competitive, is found to be different compared to known phosphate inhibitors 2′-CMP and 3′-CMP as revealed by changes in the pKa values. We find a correlation between the changes in the chemical shift of His 12 and the corresponding inhibition constants (Ki).