Analysis of the acid-base titration curve of hen lysozyme

The acid-base titration curves of hen egg-white lysozyme [EC 3.2.1.17] obtained by three groups (Sakakibara & Hamaguchi (1968) J. Biochem. 64, 613--619; Tanford & Roxby (1972) Biochemistry 11, 2192--2198; Pfeil & Privalov (1976) Biophys. Chem. 4, 23--32) were analyzed using the empirical formula of Linderstrøm-Lang. Hen lysozyme has 32 ionizable groups including the alpha-amino and alpha-carboxyl groups. Of the 21 groups other than the 11 arginyl residues, the pK values of 17 ionizable groups have been determined by various methods. Using these pK values, the pK values of the other four ionizable groups were estimated. The apparent pK values obtained were 2.0 (pKmit = 3.4), 2.1 (pKint = 3.5), 2.5 (pKint = 3.8), and 7.9 (pKint = 8.5) at 0.1 ionic strength and 25 degrees C. The titration curves obtained by the above three groups were consistently explained in terms of the same set of pKint values. The results obtained also showed that no buried and untitratable groups are present in the native lysozyme molecule.