Changes of creatine kinase structure upon ligand binding as seen by small-angle scattering

Small-angle X-ray and neutron scattering have been used to investigate structural changes upon binding of individual substrates or a transition state analogue complex (TSAC), consisting of Mg-ADP, creatine and KNO3 to creatine kinase isoenzymes (dimeric M-CK and octameric Mi-CK) and monomeric arginine kinase (AK). Considerable changes in the shape and the size of the molecules occurred upon binding of Mg-ATP and TSAC, whereas creatine alone had only a small effect. In Mi-CK, the radius of gyration was reduced from 55.6 Angstrom (free enzyme) to 48.9 Angstrom (enzyme + Mg-ATP) and to 48.2 Angstrom (enzyme + TSAC). The experiments performed with M-CK showed similar changes from 28.0 Angstrom (free enzyme) to 25.6 Angstrom (enzyme + Mg-ATP) and to 25.5 Angstrom (enzyme + TSAC). Creatine alone did not lead to significant changes in the radii of gyration, nor did free ATP or ADP. AK showed the same behaviour: a change of the radius of gyration from 21.5 Angstrom (free enzyme) to 19.7 Angstrom (enzyme + MG-ATP), whereas with arginine alone only a minor change could be observed. The primary change in structure as seen with monomeric AK seems to be a magnesium-nucleotide induced domain movement relative to each other, whereas the effect of substrate may be of local order only. In creatine kinase, however, further movements must be involved in the large conformational change.