Mg-nucleotides induced dissociation of liposome-bound creatine kinase: reversible changes in its secondary structure and in the fluidity of the bilayer.

MgADP binding to mitochondrial creatine kinase (mtCK) adsorbed on liposomes was induced by the photorelease of caged ADP. The nucleotide binding produced two types of structural changes. One was related to the well-established release of mtCK from the liposomes. The other corresponded to reversible structural changes induced by nucleotide binding to mtCK as demonstrated here. Infrared spectroscopy data show that the MgADP-induced desorption of mtCK from vesicles led to a slight increase in alpha-helix structures in mtCK at the expense of a small decrease in beta-sheet structures and a concomitant increase in the fluidity of the membranes. The desorption of mtCK induced by MgADP and MgATP was almost complete, as shown by centrifugation and enzymatic activity measurements. The photorelease of MgADP in a reactive medium containing phosphocreatine and mtCK associated with liposomes led to nucleotide binding and to the formation of MgATP and creatine. Addition of phosphocreatine also desorbed mtCK from liposomes, while addition of creatine did not. Interpretation of these results would suggest that ADP, ATP or phosphocreatine induce the release of mtCK from membranes, increase the phospholipid bilayer fluidity, and may also decrease the number of contact sites between inner and outer mitochondrial membranes, thus affecting the activity of other mitochondrial enzymes. It is tempting to propose that membrane mtCK binding regulation by nucleotide and PCr concentrations may serve as a physiological adaptation for energy supply.