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J. Biochem, 1974, Vol. 76, No. 4 671-683
© 1974 Japanese Biochemical Society

research-article

Ionization Constants of Glu 35 and Asp 52 in Hen, Turkey, and Human Lysozymes*1

Seiki KURAMITSU, Kiyoshi IKEDA, Kozo HAMAGUCHI, Hajime FUJIO*2, Tsunehisa AMANO*2, Shiro MIWA*3 and Toshihiro NISHINA*4

Department of Biology, Faculty of Science, Osaka University Toyonaka
*2Department of Immunochemistry, Research Institute for Microbial Diseases, Osaka University Suita, Toranomon Hospital, Tokyo
*3The Third Department of Internal Medicine, Yamaguchi University School of Medicine Ube, Toranomon Hospital, Tokyo
*4Division of Hematology Research, Toranomon Hospital Tokyo

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 [EC] ] 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.

*1This work was suppported in part by a grant for scientific research and a cancer research subsidy from the Ministry of Education.


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