The crystal structure of human muscle creatine kinase has been determined by the molecular-replacement method and refined at 3.5 Å resolution. The structures of both the monomer and the dimer closely resemble those of the other known structures in the creatine kinase family. Two types of dimers, one with a non-crystallographic twofold symmetry axis and the other with a crystallographic twofold symmetry axis, were found to occur simultaneously in the crystal. These dimers form an infinite `double-helix'-like structure along an unusual long crystallographic 31 axis.
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Shen, Y., Tang, L., Zhou, H., & Lin, Z. (2001). Crystal structure of human muscle creatine kinase. Acta Crystallographica D, 57(8), 1196-1200. http://www.dx.doi.org/10.1107/S0907444901007703