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

Abstract

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 pK_a 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 pK_a values. We find a correlation between the changes in the chemical shift of His 12 and the corresponding inhibition constants (K_i).