Electrostatic Energetics of Bacillus subtilis Ribonuclease P Protein Determined by Nuclear Magnetic Resonance-Based Histidine pKa Measurements.

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The pKa values of ionizable groups in proteins report the free energy of site-specific proton binding and provide a direct means of studying pH-dependent stability. We measured histidine pKa values (H3, H22, and H105) in the unfolded (U), intermediate (I), and sulfate-bound folded (F) states of RNase P protein, using an efficient and accurate nuclear magnetic resonance-monitored titration approach that utilizes internal reference compounds and a parametric fitting method. The three histidines in the sulfate-bound folded protein have pKa values depressed by 0.21 ± 0.01, 0.49 ± 0.01, and 1.00 ± 0.01 units, respectively, relative to that of the model compound N-acetyl-l-histidine methylamide. In the unliganded and unfolded protein, the pKa values are depressed relative to that of the model compound by 0.73 ± 0.02, 0.45 ± 0.02, and 0.68 ± 0.02 units, respectively. Above pH 5.5, H22 displays a separate resonance, which we have assigned to I, whose apparent pKa value is depressed by 1.03 ± 0.25 units, which is ∼0.5 units more than in either U or F. The depressed pKa values we observe are consistent with repulsive interactions between protonated histidine side chains and the net positive charge of the protein. However, the pKa differences between F and U are small for all three histidines, and they have little ionic strength dependence in F. Taken together, these observations suggest that unfavorable electrostatics alone do not account for the fact that RNase P protein is intrinsically unfolded in the absence of ligand. Multiple factors encoded in the P protein sequence account for its IUP property, which may play an important role in its function.





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Mosley, Pamela L, Kyle G Daniels and Terrence G Oas (2015). Electrostatic Energetics of Bacillus subtilis Ribonuclease P Protein Determined by Nuclear Magnetic Resonance-Based Histidine pKa Measurements. Biochemistry, 54(35). pp. 5379–5388. 10.1021/acs.biochem.5b00138 Retrieved from https://hdl.handle.net/10161/10596.

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Terrence Gilbert Oas

Professor of Biochemistry

Our laboratory is primarily interested in the mechanisms of protein folding. We use nuclear magnetic resonance (NMR) and other types of spectroscopy to study the solution structure, stability and folding reactions of small protein models. These include monomeric λ repressor, the B domain of protein A (BdpA) and various regulator of G-protein signalling (RGS) domains. Our biophysical studies are used to inform our investigations of the role of folding mechanism in the function of proteins in the cell. For example, a naturally occuring cancer-causing mutation in the RGS domain of axin appears to lower the thermodynamic stability of the domain. We are developing methods to compensate for such destabilizing mutations, thereby restoring normal function to the protein.We are also developing computational models of protein folding as a way to better understand the mechanisms and as a tool in the design of new experiments.

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