A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor.

TLDR

The frog erythrocyte β‑adrenergic receptor displays two affinity states whose proportion and relative affinity change with agonist intrinsic activity and guanine nucleotide presence. The study introduces a ternary complex model in which the receptor, an additional membrane component, and the agonist form a high‑affinity complex, providing a general framework for agonist‑mediated activation of adenylate cyclase‑coupled receptors. Computer modeling of agonist binding data with this ternary complex model accurately reproduces full and partial agonist behavior, even when the system is altered by guanine nucleotides or reagents such as p‑hydroxymercuribenzoate. Parameter estimates reveal that agonist intrinsic activity correlates with the affinity of component X for the binary complex, guanine nucleotides destabilize the ternary complex—supporting X as the guanine nucleotide binding site—and the model also successfully describes the turkey erythrocyte receptor.

Abstract

The unique properties of agonist binding to the frog erythrocyte beta-adrenergic receptor include the existence of two affinity forms of the receptor. The proportion and relative affinity of these two states of the receptor for ligands varies with the intrinsic activity of the agonist and the presence of guanine nucleotides. The simplest model for hormone-receptor interactions which can explain and reproduce the experimental data involves the interaction of the receptor R with an additional membrane component X, leading to the agonist-promoted formation of a high affinity ternary complex HRX. Computer modeling of agonist binding data with a ternary complex model indicates that the model can fit the data with high accuracy under conditions where the ligand used is either a full or a partial agonist and where the system is altered by the addition of guanine nucleotide or after treatment with group-specific reagents, e.g. p-hydroxymercuribenzoate. The parameter estimates obtained indicate that the intrinsic activity of the agonist is correlated significantly with the affinity constant L of the component X for the binary complex HR. The major effect of adding guanine nucleotides is to destabilize the ternary complex HRX from which both the hormone H and the component X can dissociate. The modulatory role of nucleotides on the affinity of agonists for the receptor is consistent with the assumption that the component X is the guanine nucleotide binding site. The ternary complex model was also applied successfully to the turkey erythrocyte receptor system. The model provides a general scheme for the activation by agonists of adenylate cyclase-coupled receptor systems and also of other systems where the effector might be different.