The reversal of the Gpp(NH)p-activated state of adenylate cyclase by GTP and hormone is by the "collision coupling" mechanism

Abstract

The guanylyl imidodiphosphate-activated state of turkey erythrocyte cyclase can be reversed to the basal state by the simultaneous action of beta-agonists and GTP. The rate of reversal diminishes progressively with decreasing concentration of beta-adrenergic receptors on the membrane surface, whereas the extent of reversal is always maximal. The rate of reversal is found to be linearly dependent on the concentration of beta-adrenergic receptors within the membranes. This result supports the notion that the interaction of the enzyme unit and the beta-adrenergic receptor is catalytic and therefore of the "collision coupling" type. The dependence of the rate of reversal reaction on epinephrine concentration is noncooperative with an apparent dissociation constant of KD = 3.0 X 10(-6) M. The fraction of guanylyl imidodiphosphate-activated cyclase system which can be reversed by GTP and a beta-agonist strongly depends on temperature and reveals a sharp transition at 24 degrees C which is the freezing temperature of the inner monolayer. It is suggested that the GTP regulatory unit is inactive when the inner monolayer is frozen.