title: pK(a) values for side-chain carboxyl groups of a PGB1 variant explain salt and pH-dependent stability.
abstract: Determination of pK(a) values of titrating residues in proteins provides a direct means of studying electrostatic coupling as well as pH-dependent stability. The B1 domain of protein G provides an excellent model system for such investigations. In this work, we analyze the observed pK(a) values of all carboxyl groups in a variant of PGB1 (T2Q, N8D, N37D) at low and high ionic strength as determined using (1)H-(13)C heteronuclear NMR in a structural context. The pK(a) values are used to calculate the pH-dependent stability in low and high salt and to investigate electrostatic coupling in the system. The observed pK(a) values can explain the pH dependence of protein stability but require pK(a) shifts relative to model values in the unfolded state, consistent with persistent residual structure in the denatured state. In particular, we find that most of the deviations from the expected random coil values can be explained by a significantly upshifted pK(a) value. We show also that (13)C backbone carbonyl data can be used to study electrostatic coupling in proteins and provide specific information on hydrogen bonding and electrostatic potential at nontitrating sites.
authors.affiliation: Department of Biophysical Chemistry, Lund University, Chemical Center, SE-22100 Lund, Sweden. Stina.Lindman@bpc.lu.se
authors.forename: Stina
authors.initials: S
authors.lastname: Lindman
authors.forename: Sara
authors.initials: S
authors.lastname: Linse
authors.forename: Frans A A
authors.initials: FA
authors.lastname: Mulder
authors.forename: Ingemar
authors.initials: I
authors.lastname: André
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D001703:Biophysics / Q000379:methods*; D002244:Carbon / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007477:Ions; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009154:Mutation; D016011:Normal Distribution; D011487:Protein Conformation; D011489:Protein Denaturation; D017434:Protein Structure, Tertiary; D012492:Salts / Q000494:pharmacology; D055672:Static Electricity; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D007477:Ions; D012492:Salts; D002244:Carbon