A study on the interaction of ribonuclease St with guanosine 3′-monophosphate by circular dichroism

Abstract The interaction of ribonuclease St with guanosine 2′(3′)-monophosphate, a competitive inhibitor for the enzyme, has been studied by means of their circular dichroism spectra. Ribonuclease St exhibits a strong positive band at 234 mμ and a negative band at 280 mμ. Upon addition of the nucleotide to the enzyme, a significant change in the circular dichroism spectrum occurs in the 280 mμ region alone, and this has been used to characterize the interaction of nucleotide and enzyme. Mixing enzyme which had been inactivated by modification with iodoacetate with the nucleotide did not produce any change in the spectrum, though the circular dichroism spectra of the native and the modified enzyms, are virtually identical. The results are discussed together with the results obtained using bovine pancreatic ribonuclease and cytidine 2′(3′)-monophosphate.