肌酸激酶
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肌酸激酶 (Creatine Kinase, CK) (ATP: Creatine N-phosphotransferase EC 2.7.3.2)通常存在于动物的心脏、肌肉以及脑等组织的细胞浆和线粒体中，是一个与细胞内能量运转、肌肉收缩、ATP再生有直接关系的重要激酶[1,2]，它可逆地催化肌酸与ATP之间的转磷酰基反应。
目录 |
形式
肌酸激酶有四种同功酶形式：肌肉型(MM)、脑型(BB)、杂化型(MB)和线粒体型(MiMi)。MM型主要存在于各种肌肉细胞中，BB型主要存在于脑细胞中，MB型主要存在于心肌细胞中，MiMi型主要存在于心肌和骨骼肌线粒体中。肌肉型肌酸激酶分子是由两个相同的亚基组成的二聚体。根据目前已经测定的兔、人、鸡、鼠肌酸激酶的一级结构[3-6]，M型亚基由387个氨基酸残基组成，分子量为43 KDa左右，分子内有8个巯基，但无二硫键。大熊猫肌肉型肌酸激酶也是二聚体酶，每个亚基由376个氨基酸残基组成，分子量为42 KDa[7]。
临床价值
肌酸激酶的同功酶在临床诊断中有十分重要的意义[2，8-10]，在各种病变包括肌肉萎缩和心肌梗塞发生时，人的血清中肌酸激酶水平迅速提高，目前认为在心肌梗塞的诊断中测定肌酸激酶的活性比做心电图更为可靠。心肌梗死时，肌酸激酶在起病6小时内升高，24小时达高峰，3-4日内恢复正常。其中肌酸激酶的同工酶CK-MB诊断的特异性最高。肌酸激酶因其具有重要的生理功能和临床应用价值已引起人们广泛的重视和深入的研究。
实验作用及理由
肌酸激酶作为研究蛋白质折叠的理想模型基于以下理由：i) 肌肉型肌酸激酶分子是由两个相同的亚基组成的二聚体，目前兔肌CK的2.35 Å高分辨率晶体结构已经解出[11]，每个亚基具有一个小的N-末端结构域和一个大的C-末端结构域。人肌CK的3.5Å分辨率晶体结构也已经得到[12]。ii)多种条件下变性或修饰后的CK在体外仍可再折叠为天然构象[13-16]。iii). CK是一个大的二聚体蛋白质，比小的二聚体或单体蛋白质分子更复杂，再折叠过程中可以得到更多的中间体[16-18]，聚沉与正确折叠之间的竞争也被观察到[19,20]。
天然的肌酸激酶分子是一个紧密的球状结构。近来关于肌酸激酶构象变化和活力变化关系的研究显示了酶分子活性部位构象的柔性[17，21，22]，即酶分子活性部位的微区构象在变性剂作用下易发生改变而导致酶分子快速失活，此时酶分子整体构象尚未发生明显变化。周海梦等人[23]用荧光探针标记兔肌肌酸激酶的活性部位，监测了荧光衍生物微区构象变化与相应酶活力丧失速度，发现二者几乎一致，为酶活性部位柔性的假说提供了有力的证据。
参考文献
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[2] Seraydrarian M W and Abbot B C. The role of the creatine phosphokinase system in muscle. J. Mol. Cell. Cardiol. 1976, 8: 741~746
[3] Shain S A. Creatine kinase and lactate dehydrogenase: stability of isoenzymes and their activity in stored human plasma and prostatic tissue extracts and effect of sample dilution. Clin. Chem., 1983, 29: 832~835
[4] Kwiatkowski R W, Schweinfest C W and Dottin R P. Molecular cloning and the complete nucleotide sequence of the creatine kinase-M cDNA from chicken. Nucleic. Acids Res. 1984, 12: 6952~6934
[5] Pickering L, Pang H, Biemann K, et al. Two tissue-specific isozymes of creatine kinase have closely matched amino acid sequences. Proc. Natl. Acad. Sci. USA, 1985, 82: 2310~2314
[6] Muhlebach S M, Gross M, Wirz T, et al. Sequence Homology and Structure Predictions of the Creatine-Kinase Isoenzymes. Mol.Cell. Biochem., 1994, 133: 245~262
[7] Benfield P A. Isolation and sequence analysis of cDNA clones coding for rat skeletal muscle creatine kinase. J. Biol. Chem., 1984, 259: 14979~14984
[8] Sobel B E, Markham J and Roberts R. Factors influencing enzymatic estimates of infarct size. Am. J. Cardiol., 1977, 39: 130~132
[9] Sobel B E. Applications and limitations of estimation of infarct size from serial changes in plasma creatine phosphokinase activity. Acta Med. Scand. Suppl., 1976, 587: 151~167
[10] Kouttinen A. Purification of human and canine creatine kinase isozymes. Acta Med.Scand.Suppl. 1978, 623: 115~117.
[11] Rao J K, Bujacz G, and Alexander W. 1998. Crystal structure of rabbit muscle creatine kinase. FEBS Lett. 439: 133–137.
[12] Shen Y Q, Tang L, Zhou H M et al. and Lin Z J. Structure of human muscle creatine kinase. ACTA CRYSTALLOGR D-BIOL CRYST, 2001, 57:1196-1200.
[13]Bickerstaff, G.F., Paterson, C., and Price, N.C. 1980. The refolding of denatured rabbit muscle creatine kinase. Biochim. Biophys. Acta 621: 305–314.
[14]Hou, L.X., Zhou, H.M., Yao, Q.Z., and Tsou, C.L. 1983. A comparative study of renaturation and reactivation kinetics of the guanidine denatured creatine kinase. Acta Biochim. Biophys. Sin. 15: 393–397.
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[16]Zhou, H.M. and Tsou, C.L. 1986. Comparison of activity and conformation changes during refolding of urea-denatured creatine kinase. Biochim. Biophys. Acta 869: 69–74.
[17]Wang, Z.F, Yang, Y., and Zhou, H.M. 1995. Conformational changes of active sites during refolding of urea-denatured creatine kinase. Biochimie 77: 953–956.
[18]Yang, Y., Park, Y.D., Yu, T.W., and Zhou, H.M. 1999. Reactivation and refolding of a partially folded creatine kinase modified by 5,5_-Dithio-bis(2-nitrobenzoic acid). Biochem. Biophys. Res. Commun. 259: 450–454.
[19]Webb, T., Jackson, P.J., and Morris, G.E. 1997. Protease digestion studies of an equilibrium intermediate in the unfolding of creatine kinase. Biochem. J. 321: 83–88.
[20]Zhou, J.M., Fan, Y.X., Kihara, H., Kimura, K., and Amemiya, Y. 1997. Unfolding of dimeric creatine kinase in urea and guanidine hydrochloride as measured using small angle X-ray scattering with synchrotron radiation. FEBS Lett. 415: 183–185.
[21] Yao Q Z, Zhou H M, Hou L X, et al. A comparison of denaturation rates of creatine kinase in guanidine solution. Sci. Sin. Ser. B. (Engl. Ed.), 1982, 25: 1296~1302
[22 Wang Z F, Huang M Q, Zou X M, et al. Unfolding , conformational change of active site and inactivation of creatine kinase in SDS solutions. Biochim. Biophys. Acta, 1995, 1251: 109~114
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