Based on a large body of circumstantial evidence, the non-stoichiometric floating receptor model for hormone sensitive adenylyl cyclase is proposed. It is thought to be the most economical model able to rationalize the available data. According to this model, the adenylyl cyclase system consists of two protein components: receptors and enzymatic units, each capable of assuming two conformations. Hormones activate adenylyl cyclase by association with receptors, which then have an increased affinity for the enzymatic units in the catalytically active conformation. There is no fixed stoichiometric relationship between the number of enzymatic units and the number of hormone receptors in a cell, but they only form 1 : 1 complexes with each other. Hormone receptors are assumed to be capable of lateral diffusion in the membrane.
The model is also formulated in physico-chemical terms in order to elucidate the consequences. The non-stoichiometric floating receptor model (1) underscores the absence of homotropic allosterism with respect to hormonal activation. (2).It explains, at least qualitatively, the differing maximal effects of various hormones. (3) It gives a clue to the effect of hormone combinations, describing them as being competitive in origin, rather than caused by positive or negative heterotropic allosterism. (4) It explains how one system can be responsive to many agents.
(5) It explains both the easy ontogenetic as well as phylogenetic adaptation of the adenylyl cyclase system. (6) It suggests a basic structural similarity of all receptors for hormones affecting adenylyl cyclase.
Finally, the model constitutes a suitable working hypothesis for further experimentation and suggests new approaches.