Anomalous binding of MgADP to myosin of skeletal muscle

A possible solvent effect of adenosine diphosphate influences the binding of 1,N6 ethenoadenosine diphosphate to myosin from skeletal muscle

Skeletal muscle myosin displays two independent and equivalent binding sites for 1,N6 ethenoadenosine diphosphate, with a dissociation constant of 24.7 microM. MgADP, 10 to 40 microM, behaves as a pure competitive type inhibitor (K(SI)=8-9 microM) for the binding of 1,N6 ethenoadenosine diphosphate to skeletal muscle myosin. On the contrary, the inhibition by MgADP, 0.11-1.54 mM, is neither competitive nor non-competitive nor mixed, as is revealed by the analysis with the general kinetic equation (K.J. Laidler, P.S. Bunting, The Chemical Kinetics of Enzyme Action, 2nd ed., Clarendon, Oxford, 1973, p. 94). To explain our finding we propose that MgADP operates a complex type of inhibition, acting both directly as a competitor for myosin active sites, and indirectly by perturbing the regions of the solvent near to the protein.