Structural Characteristics That Govern Binding to, and Modulation through, the Cardiac Ryanodine Receptor Nucleotide Binding Site

Comparative molecular field analysis (CoMFA) predicts that the large electrostatic field around the phosphate groups of ATP plays a crucial role in stabilizing the open state of the cardiac ryanodine receptor (RyR) channel. We therefore investigated the effects of adenosine-5-(,-methylenetriphosphate) (AMPPCP), an ATP analog with lower negative charge in this region, on the gating of the cardiac RyR channel. In the presence of 10 M cytosolic Ca 2 , AMP-PCP exhibited approximately 50% of the efficacy of ATP and optimal doses increased open probability (Po) to only 0.441 0.156 (n 4), thus confirming the predictive ability of our preliminary CoMFA model. We also reveal that AMP-PCP has a higher affinity than ATP for the cardiac RyR, demonstrating that the structural properties required for tight binding to RyR differ from those necessary for recruiting long open states and high Po values. CoMFA identified very strong correlations between the structures of adeninebased ligands and their affinity for RyR and different (but also highly significant) correlations between structure and the ability to activate the channel. Analysis indicates that ATP may be more effective than other adenine nucleotides because it can convert the greatest amount of binding energy into conformational changes that stabilize the open channel state.