Conformational flexibility and structure of creatine kinase.

The structural flexibility of creatine kinase has been investigated with the covalent hydrophobic probe 2-[4'-(2"-iodoacetamido) phenyl] aminonaphthalene-6-sulfonic acid (IAANS) which reacts at vastly different rates with the two subunits to give a protein conjugate with fluorescence characteristic of reaction with a site in a hydrophobic cleft. Binding of purine nucleotides greatly enhances the probe fluorescence while pyrimidine nucleotides quench the fluorescence. Small anions bind to nucleotide-free creatine kinase near the location of the transferable phosphoryl group and quench both the IAANS fluorescence of modified creatine kinase and the tryptophan fluorescence of native creatine kinase. Chloride and nitrate non-competitively inhibit MgADP binding both with and without creatine. Fluorescence energy transfer demonstrates that the active sites of creatine kinase are well separated and become further apart after the nucleotide-induced conformational change.