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Salting the Charged Surface: pH and Salt Dependence of Protein G B1 Stability

Department of Biophysical Chemistry, Lund University, Lund, Sweden

Correspondence: Address reprint requests to Stina Lindman or Sara Linse, Biophysical Chemistry, Lund University, Chemical Center, SE-22100 Lund, Sweden. Tel.: 46-46-222-7092; E-mail: stina.lindman{at}bpc.lu.se or sara.linse{at}bpc.lu.se.

This study shows significant effects of protein surface charges on stability and these effects are not eliminated by salt screening. The stability for a variant of protein G B1 domain was studied in the pH-range of 1.5–11 at low, 0.15 M, and 2 M salt. The variant has three mutations, T2Q, N8D, and N37D, to guarantee an intact covalent chain at all pH values. The stability of the protein shows distinct pH dependence with the highest stability close to the isoelectric point. The stability is pH-dependent at all three NaCl concentrations, indicating that interactions involving charged residues are important at all three conditions. We find that 2 M salt stabilizes the protein at low pH (protein net charge is +6 and total number of charges is 6) but not at high pH (net charge is –6 and total number of charges is 18). Furthermore, 0.15 M salt slightly decreases the stability of the protein over the pH range. The results show that a net charge of the protein is destabilizing and indicate that proteins contain charges for reasons other than improved stability. Salt seems to reduce the electrostatic contributions to stability under conditions with few total charges, but cannot eliminate electrostatic effects in highly charged systems.

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				S. Lindman, S. Linse, F. A. A. Mulder, and I. Andre pKa Values for Side-Chain Carboxyl Groups of a PGB1 Variant Explain Salt and pH-Dependent Stability Biophys. J., January 1, 2007; 92(1): 257 - 266. [Abstract] [Full Text] [PDF]