The Aromatic Residues of Bovine Pancreatic Ribonuclease Studied by 1H Nuclear Magnetic Resonance

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Abstract
1. The aromatic proton resonances in the 360‐MHz 1H nuclear magnetic resonance (NMR) spectrum of bovine pancreatic ribonuclease were divided into histidine, tyrosine and phenylalanine resonances by means of pH titrations and double resonance experiments.2. Photochemically induced dynamic nuclear polarization spectra showed that one histidine (His‐1 19) and two tyrosines are accessible to photo‐excited flavin. This permitted the identification of the C‐4 proton resonance of His‐119.3. The resonances of the ring protons of Tyr‐25, Tyr‐76 and Tyr‐1 15 and the C‐4 proton of His‐12 were identified by comparison with subtilisin‐modified and nitrated ribonucleases. Other resonances were assigned tentatively to Tyr‐73, Tyr‐92 and Phe‐46.4. On addition of active‐site inhibitors, all phenylalanine resonances broadened or disappeared. The resonance that was most affected was assigned tentatively to Phe‐120.5. Four of the six tyrosines of bovine RNase, identified as Tyr‐76, Tyr‐115 and, tentatively, Tyr‐73 and Tyr‐92, are titratable above pH 9. The rings of Tyr‐73 and Tyr‐115 are rapidly rotating or flipping by 180° about their Cβ‐Cγ bond and are accessible to flavin in photochemically induced dynamic nuclear polarization experiments. Tyr‐25 is involved in a pH‐dependent conformational transition, together with Asp‐14 and His‐48. A scheme for this transition is proposed.6. Binding of active‐site inhibitors to bovine RNase only influences the active site and its immediate surroundings. These conformational changes are probably not connected with the pH‐dependent transition in the region of Asp‐14, Tyr–25 and His‐48.7. In NMR spectra of RNase A at elevated temperatures, no local unfolding below the temperature of the thermal denaturation was observed. NMR spectra of thermally unfolded RNase A indicated that the deviations from a random coil are small and might be caused by interactions between neighbouring residues.