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Ionization Constants of Glu 35 and Asp 52 in Hen, Turkey, and Human Lysozymes<xref ref-type="fn" rid="fn1">^*1</xref>

1974; Oxford University Press; Linguagem: Inglês

10.1093/oxfordjournals.jbchem.a130613

1756-2651

Seiki Kuramitsu, Kiyoshi Ikeda, Kozo HAMAGUCHI, Hajime Fujio, T Amano, Shiro Miwa, Toshihiro Nishina,

Metabolism and Genetic Disorders

The microscopic dissociation constants of Glu 35 and Asp 52 in hen egg-white, turkey egg-white, and human urine lysozymes [EC 3.2.1.17] were determined by measuring the pH dependence of the circular dichroic band at 305 nm, which is due to Trp 108 near the catalytic carboxyls, Glu 35 and Asp 52. The pk value of Glu 35 when Asp 52 is unionized (pk1, Glu) and that of Asp 52 when Glu 35 is unionized (pk1, Asp) for each of the three lysozymes were quite normal. However, the pk value of Glu 35 when Asp 52 is in a deprotonated form (pk2, Glu) and that of Asp 52 when Glu 35 is in a deprotonated form (pk2, Asp) were higher than the normal pk values of glutamyl and aspartyl residues. The macroscopic pK values of Asp 52 and Glu 35 were 3.4 and 5.9 for both hen and turkey lysozymes and 3.4 and 6.8 for human lysozyme at 25°C and 0.1 ionic strength. The high macroscopic pK value of about 6 which has been assigned to Glu 35 of hen lysozyme is not caused by its hydrophobic environment but by electrostatic interaction of ionized Glu 35 with ionized Asp 52. The pK shift of Glu 35 observed when N-acetyl-chitotriose binds to hen lysozyme may be interpreted in terms of a change in the electrostatic interaction between Glu 35 and Asp 52 accompanied by movement of the active-site cleft.