Graduate Thesis Or Dissertation
 

Physical and chemical properties of ribonuclease T₁

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  • The spectrophotometric titration of the tyrosine residues, their reaction with N-acetylimidazole, and the effect of pH on the thermally induced conformational transition of ribonuclease T₁ have been investigated. The results of spectrophotometric titration in 0.20 M KCl/0.001 M sodium phosphate suggest that titration of the tyrosine residues occurs in two steps, the first with an apparent pK of 10.7, and the second with an apparent pK greater than 12. About 8 of the 9 tyrosine residues titrate in the first step, which is characterized by a sharp dependence of the degree of ionization on pH. This behavior is suggestive of a cooperative titration. In 8 M urea-0.20 M KCl/0.001 M sodium phosphate the tyrosine residues titrate in one step with an apparent pK of 10.9. Acetylation of ribonuclease T₁ in 0.02 M Veronal, pH 7.5, indicates that about 2 tyrosine residues are readily acetylated with a 60-180 fold molar excess of acetylimidazole, while other tyrosine residues react only at much higher concentrations of acetylating reagent. In 8 M urea-0.02 M Veronal, pH 7.5, about 8 of the 9 tyrosine residues are readily acetylated. Studies on the thermal transition of ribonuclease T₁ showed that, above about pH 4, the transition temperature decreases with increasing pH. Thermal transition occurred with the exposure of tryptophan and tyrosine residues to solvent. The above properties of ribonuclease T₁ have been contrasted with those of bovine pancreatic ribonuclease A. The homogeneity of ribonuclease T₁ was investigated by high-speed sedimentation equilibrium measurements, and the protein was found to be homogeneous under the conditions used in the experiments. Molecular weights of ribonuclease T₁ calculated from the sedimentation equilibrium experiments agreed, within experimental error, with the molecular weight based on the known amino acid sequence.
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