J. Biochem, 1980, Vol. 87, No. 4 1015-1027
© 1980 Japanese Biochemical Society
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Binding of Substrate Analogs to Hen Lysozyme in Which Trp 62 is Modified to Kynurenine1
*Department of Biology, Faculty of Science, Osaka University Toyonaka, Osaka 560
**Division of Protein Chemistry, Institute for Protein Research, Osaka University Suita, Osaka 565
***Department of Agricultural Chemistry, Faculty of Agriculture, Kyushu University Higashi-ku, Fukuoka, Fukuoka 812
2To whom correspondence should be sent.
The interactions of ß-methyl-GlcNAc, (GlcNAc)2, and (GlcNAc)3 with hen egg-white lysozyme [EC 3.2.1.17 [EC] ] in which Trp 62 is modified to kynurenine (Kyn 62-lysozyme) were studied by measuring the changes in the CD band and fluorescence due to the kynurenine at various pH values. The pH profiles of the binding constants of these saccharides to Kyn 62-lysozyme were very similar to those to intact lysozyme, although the binding constants were lower for the modified lysozyme than for intact lysozyme. The pK values of Asp 52, Glu 35, Asp 48, and Asp 101 in Kyn 62-lysozyme and in its complexes with ß-methyl-GlcNAc and with (GlcNAc)2 were in agreement with those of intact lysozyme and its complexes. The pK values of Asp 52 and Glu 35 in the modified lysozyme-(GlcNAc)3 complex were also in agreement with those of the complex with intact lysozyme, but the pK shift of Asp 101 was smaller for Kyn 62-lysozyme than for intact lysozyme. The significance of the decreased binding constants to Kyn 62-lysozyme is discussed.
The pH dependence of the CD band due to the kynurenine in Kyn 62-lysozyme was interpreted in terms of the participation of the catalytic groups, Asp 52 (apparent pK 3.5) and Glu 35 (apparent pK 6.0), and the amino group of the kynurenine (apparent pK 0.75). This indicates that the ionization of the catalytic groups affects the state around Trp 62 and supports the previous proposal that there is a relation between the state around Trp 62 and the ionization of Glu 35 (Ikeda, K. & Hamaguchi, K. (1973) J. Biochem. 74, 221–230; (1975) ibid. 77, 1–16; Nakae et al. (1975) J. Biochem. 77, 993–1006). The intrinsic pK value of the amino group of the kynurenine at position 62 shifted from 2.1 to 0.8 on complexing with (GlcNAc)3, indicating interaction between the kynurenine and the sugar residue at subsite B.
1This work was supported in part by a grant for scientific research from the Ministry of Education, Science and Culture of Japan.
3Present address: Department of Biochemistry, Miyazaki Medical College, Kiyotake, Miyazaki, Miyazaki 889–16.
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