THE PYRROLE LOCUS IS THE MAJOR ORIENTING FACTOR IN RYANODINE BINDING

Ryanodine, a natural product, is a complex modulator of a class of int racellular Ca2+ release channels commonly called the ryanodine recepto rs. Ryanodine analogs can cause the channel to persist in long-lived, subconductance states or, at high ligand concentrations, in closed, no nconducting states. In this paper, we further explore the relationship between structure and ryanodine binding to striated muscle. Ryanodine , 3-epiryanodine, and 10-ryanodine are three structural isomers of rya nodine. The dissociation constants of these compounds were measured us ing rabbit skeletal muscle ryanodine receptors. Placing the pyrrole ca rbonyl group at the 3-epi- and 10-positions of ryanodol largely restor es the large loss of binding energy observed when ryanodine is hydroly zed to ryanodol. Comparative molecular field analysis successfully pre dicts the enhanced binding and indicates that the pyrrole group contro ls the orientation of ligand binding. We propose that the ryanoids are reorientated in the binding site of the ryanodine receptors such that the pyrrole always occupies the same subsite. By applying this model, the binding constants of other ryanoids are predicted.