The Role of Phenylalanine 31 in Maintaining the Conformational Stability of Ribonuclease P2 from Sulfolobus solfataricus under Extreme Conditions of Temperature and Pressure

Ribonuclease P2 from the thermophilic archaebacterium Sulfolobus solfataricus is a small protein (7 kDa) with a known three-dimensional structure. Inspection of the structure and molecular dynamics simulation reveal that three aromatic residues (Phe5, Phe31, and Tyr33) from the hydrophobic core have a strong van der Waals interaction energy. We studied the thermodynamics of the heat, cold, and pressure-induced protein conformational changes of the wild type and of the F31A and F31Y mutants by analyzing the protein UV absorbance in the fourth derivative mode. The wild-type protein was extremely stable under all conditions of temperature and pressure. Heat and cold denaturation of both mutants, as well as denaturation by pressure of the F31A mutant, led to significant blue shifts of the derivative spectrum, indicating increased solvent exposure of Tyr33. For the F31Y mutant, high pressure (400 MPa) protected the protein against thermal denaturation. This study, probing the properties of the hydrophobic aromatic core, complements a thermal unfolding study which probes the overall structural changes [Knapp, S., Karshikoff, A., Berndt, K. D., Christova, P., Atanasov, B., & Ladenstein, R. (1996) J. Mol. Biol. 264, 1132−1144]. The differences observed in response to extremes of temperature, pressure, and pH may be rationalized by an unfolding mechanism involving larger parts of the peripheral protein while the integrity of the hydrophobic core is maintained.