The accuracy of the Microenvironment Modulated_Screened Coulomb Potential (MM_SCP)1 to predict pKa values for titratable residues in biological macromolecules is analyzed. This self-consistent approach calculates electrostatic energetics using a modified form of the SCP-continuum solvent model2, which is modulated by the hydrophobicity of the local environment around the titratable group. The analysis has been applied to 345 ionizable groups (Asp, Glu, His, Cys, Tyr, Lys, N- and C-termini) in 59 proteins belonging to multiple structural classes. Among these residues, 82% were predicted with errors < 1 unit), although half of residues with large shifts were correctly predicted (error <1 unit). Further analysis showed that the MM_SCP accounts for the pKa shifts of ionizable groups in hydrophobic microenvironment3 and treats interaction energies very well. Most errors originate from steric clashes or improper handling of certain interaction pairs (e.g., ionic H-bonds). A feature in the MM_SCP was used to identify steric clashes between the titratable residue and other parts of the sequence; after local minimization the calculated pKa improved by half a unit in more than half the cases. Other methods to improve the predictions will be discussed, such as optimizing the H-bond network, or using refined reference pKa values4 which improves the root mean square deviations by at least 0.1 unit for 25 proteins.1. Mehler, E.L. and Guarnieri, F., Biophysics J.,1999. 77: p. 3.2. Hassan, S.A., et al. J. Phys. Chem. B,2000. 104: p. 6478.3. Mehler, E.L., et al. PROTEINS: Stru. Func. Genet.,2002. 48: p. 283.4. Thurlkill, R.L., Grimsley, G.R., Scholtz, J.M., and Pace, C.N., Protein Science,2006. 15 p. 1214.