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. 2006 May;15(5):1214-8.
doi: 10.1110/ps.051840806. Epub 2006 Apr 5.

pK values of the ionizable groups of proteins

Richard L Thurlkill  1 , Gerald R Grimsley, J Martin Scholtz, C Nick Pace
Affiliations

pK values of the ionizable groups of proteins

Richard L Thurlkill et al. Protein Sci. 2006 May.

Abstract

We have used potentiometric titrations to measure the pK values of the ionizable groups of proteins in alanine pentapeptides with appropriately blocked termini. These pentapeptides provide an improved model for the pK values of the ionizable groups in proteins. Our pK values determined in 0.1 M KCl at 25 degrees C are: 3.67+/-0.03 (alpha-carboxyl), 3.67+/-0.04 (Asp), 4.25+/-0.05 (Glu), 6.54+/-0.04 (His), 8.00+/-0.03 (alpha-amino), 8.55+/-0.03 (Cys), 9.84+/-0.11 (Tyr), and 10.40+/-0.08 (Lys). The pK values of some groups differ from the Nozaki and Tanford (N & T) pK values often used in the literature: Asp (3.67 this work vs. 4.0 N & T); His (6.54 this work vs. 6.3 N & T); alpha-amino (8.00 this work vs. 7.5 N & T); Cys (8.55 this work vs. 9.5 N & T); and Tyr (9.84 this work vs. 9.6 N & T). Our pK values will be useful to those who study pK perturbations in folded and unfolded proteins, and to those who use theory to gain a better understanding of the factors that determine the pK values of the ionizable groups of proteins.

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Figures

Figure 1.
Figure 1.
Potentiometric titration curves of the side chain Glu carboxyl group in Ac-AAEAA-NH2 (A), and of the side chain His imidazole group in Ac-AAHAA-NH2 (B). The peptides were dissolved in 0.1 M KCl and titrated with HCl at 25°C. The lines are the best fit of the data to Equation 2.
See this image and copyright information in PMC

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