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Electrostatic forces in two lysozymes: Calculations and measurements of histidine pKa values
Author(s) -
Takahashi Takuya,
Nakamura Haruki,
Wada Akiyoshi
Publication year - 1992
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360320802
Subject(s) - chemistry , ionic strength , dipole , lysozyme , histidine , globular protein , side chain , ionic bonding , solvent , helix (gastropod) , residue (chemistry) , computational chemistry , thermodynamics , crystallography , analytical chemistry (journal) , ion , aqueous solution , chromatography , organic chemistry , enzyme , ecology , biochemistry , physics , snail , biology , polymer
In order to examine the electrostatic forces in globular proteins, pK a values and their ionic strength dependence of His residues of hen egg white lysozyme (HEWL) and human ly−sozyme (HUML) were measured, and they were compared with those calculated numerically. pK a values of His 30 residues in HEWL, HUML, and short oligopeptides were determined from chemical shift changes of His side chains by 1 H−nmr measurements. The associated changes in pK a values in HEWL and HUML were calculated by solving the Poisson‐Boltzmann equations numerically for macroscopic dielectric models. The calculated pK a changes and their ionic strength dependence agreed fairly well with the observed ones. The contribution from each residue and each α‐helix dipole to the pK a values and their ionic strength dependence was analyzed using Green's reciprocity theorem. The results indicate that (1) the pK a of His residues are largely affected by surrounding ionized and polar groups; (2) the ionic strength dependence of the pK a values is determined by the overall charge distributions and their accessibilities to solvent; and (3) α‐helix dipoles make a significant contribution to the pK a when the His residue is close to the helix terminus and not fully exposed to the solvent.