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 transit ion 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 Angs trom (enzyme + Mg-ATP) and to 48.2 Angstrom (enzyme + TSAC). The exper iments 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 beh aviour: a change of the radius of gyration from 21.5 Angstrom (free en zyme) 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 substrat e may be of local order only. In creatine kinase, however, further mov ements must be involved in the large conformational change.