Subunit Interaction Slows the Unfolding of the N-Terminal Domain of Creatine Kinase in Urea

Z. Guo, Z. Wang, and X. C. Wang^*

Department of Biological Science and Biotechnology, School of Life Science and Engineering, Tsinghua University, Beijing 100084, China; fax: 8610-62772248; E-mail: wangxic@mail.tsinghua.edu.cn

^* To whom correspondence should be addressed.

Received April 25, 2002; Revision received July 5, 2002

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Fluorescence emission intensity changes with two different excitation wavelengths were used to measure the unfolding rate constants of different domains of muscle type creatine kinase (CK-MM) according to the heterogeneity of aromatic amino acid distributions in the crystal structure of CK-MM. The results were compared with those of brain type creatine kinase (CK-BB) and dithio-bis(succinimidyl propionate) cross-linked CK-MM. CK-BB differed greatly in its distribution of aromatic amino acids in each domain and the unfolding process of cross-linked CK-MM was not accompanied by the dissociation of the dimer. The N-terminal domain of CK-MM was shown to be well protected by subunit interaction during the unfolding of CK-MM in 4 M urea. Dissociating the CK dimer in high urea concentration (>=6 M) eliminated the subunit protection. Subunit interactions are also important in preserving secondary structure and forming contracted conformation at low urea concentration.

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KEY WORDS: creatine kinase, N-terminal domain, dithio-bis(succinimidyl propionate) cross-link, subunit interaction

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