STRUCTURE OF LYSOZYME .14. ACID-BASE TITRATION OF LYSOZYME

Hydrogen-ion titration of hen egg-white lysozyme [EC 3.2.1.17]was performed in 0.2 M KC1 and in a mixed solvent of 5 M guanidinehydrochloride and 1.2 M urea, at 25C. The number of carboxyl groupstitratable was found to be eight in 0.2 M KC1 and eleven in the denaturingsolvent. A theoretical titration curve which satisfactorily fits theexperimental data of the native lysozyme, was obtained by assumingthat, of the eight carboxyl groups, six have the pKint values of 43 andthe remaining two have the corresponding values of 3.5 and 6.0, respectively,and that the intrinsic pK's of α-amino and histidyl groups arenormal. The assumption of the presence of the carboxyl groups withthe pKint values of 3.5 and 6.0 is based on other available evidence.It was found that the ionization of the lysyl groups was normal, whereasthe ionization behavior of the tyrosyl groups was complicated. © 1968 by the Journal of Biochemistry.