A summary of the measured pK values of the ionizable groups in folded proteins

We tabulated 541 measured pK values reported in the literature for the Asp, Glu, His, Cys, Tyr, and Lys side chains, and the C and N termini of 78 folded proteins. The majority of these values are for the Asp, Glu, and His side chains. The average pK values are Asp 3.5 +/- 1.2 (139); Glu 4.2 +/- 0.9 (153); His 6.6 +/- 1.0 (131); Cys 6.8 +/- 2.7 (25); Tyr 10.3 +/- 1.2 (20); Lys 10.5 +/- 1.1 (35); C-terminus 3.3 +/- 0.8 (22) and N-terminus 7.7 +/- 0.5 (16). We compare these results with the measured pK values of these groups in alanine pentapeptides, and comment on our overall findings.