GPCR stands for G protein-coupled receptor. So far, 900 members have been identified and been classified into 6 to 8 classes depending of the nomenclature. These seven-(pass)-transmembrane domain receptors, or serpentine receptors (serpent is a snake in French), change their conformation in the presence of their specific ligand allowing a transduction of signal in cell.
Today, GPCRs account for more than one-third of all therapeutic drug targets.
But their discovery begins with receptor work.
A story that began nearly 100 years ago.
Where is G protein-coupled receptor located?
Alfred Joseph Clark was fond of calculatus and pharmacology. Applying the first to the second, he rapidly become a greedy investigator of the mode of action of drugs. His works led him to the elaboration of its famous « receptor theory », described in The mode of Action of Drugs (published in 1933) and General Pharmacology (published in 1937).
In fact, the mathematical relationship between concentration and action of a ligand revealed that a reversible molecular reaction occurs between the drug and some receptor in the cell. A decade later, when Alfred observed no direct relationship between the amount of drug entering the cell and the effect of the drug, he concluded that the receptor was finally ON the cell and not IN the cell.
Something we all take for granted today, but which was not so obvious in the thirties. This first discovery rapidly triggers more researches on ligand affinity for these cell surface receptors.
What is an agonist and antagonist drug for GPCR?
In 1948, Reuse expanded Clark’s work and was the first to mention the concept of “agonist” and “antagonist” drugs. Agonist ligands are binders that present both affinity (they bind to the target receptor) and efficacy (produce a intracellular response cascade), whereas antagonist ligands have affinity without any efficacy.
In 1956, Stephenson et al. (1) suggested that the amount of occupancy of receptors was not directly proportional to drug effect. The notion of being able to produce a maximum response without occupying 100% of the receptor population was first referred to as "reserve receptors" or "spare receptors" phenomenon. But Stephenson has then figured that all receptors participate to the response, even if they are not all needed for the generation of a maximum cell signaling effect. He has so qualified the ligands with the terms “full agonists” (triggering a maximal response) or “partial agonists” (triggering suboptimal response), and so broadened Clark’s theory with the notion of “intrinsic efficacy”.
Many years later, in 1980, De Lean postulated the involvement of an additional membrane component promoting the transition from the low to the high affinity form of the receptor. What he called the X component was later renamed the G protein (2) and the studied family of receptors became the G protein-coupled receptors. G protein and the transmembrane receptor were now forming a complex that arouses the curiosity of the scientists.
And Paul Leff was one of them.
Affinity and efficacy of ligands for GPCR
In 1995, Paul demonstrated that the state of the complex formed by GPCR and its G protein could be predicted. The proposed model for two state mechanism of receptor activation is based on the model introduced by Monod, Wyman and Changeux for oxygen and haemoglobin interactions (3). Contrary to the receptor theory, Leff suggested that measured affinity of an agonist for the receptor would be a complex quantity dependent on the two dissociation constants, KA and KA*, and also on the value of the equilibrium constant L.
Because affinity and efficacy depend on the same underlying constants, they cannot be thought of as being independent from one another (4). This simple, but crucial, conclusion allowed scientific community starting a major turning point in the understanding of the conformational mechanisms of GPCRs.
Inactive and active states of G protein-coupled receptors
Conformation is the term used to qualify a certain three dimensional arrangement of a protein at a single point of time. But like many proteins, GPCR is in constant motion at a molecular level, even in the absence of stimulus.
The two states mechanism theorizes that GPCRs molecules switch from an active to an inactive state and vice-versa. The active site of GPCR for the recognition of its ligand, or orthosteric site, makes subtle but important rearrangements of its structure in order to accommodate the ligand in its core. The inactive state is the more stable conformation of GPCRs, but the active state is not highly stable even in the presence of a high-affinity or covalently bound agonist.
In fact, the transition from one state to another should not be seen as a simple on/off switch system. Rather, it is a subtle balance between inactive and active conformations, the GPCR taking an infinite number of conformations over the course of time. These conformations tend to cluster into groups, and a group of close conformations is known as “population”.
And whether it is the binding of a ligand in the orthosteric site, or the binding of a G protein or an arrestin in the allosteric domain, or dimerization with other GPCRs, or a post-translational modification on certain amino acids, all these phenomena ultimately have only one goal: to modify the fraction of time that the GPCR spends in each of its conformational states.
(1) STEPHENSON RP. A modification of receptor theory. Br J Pharmacol Chemother. 1956;11(4):379-393. doi:10.1111/j.1476-5381.1956.tb00006.x
(2) De Lean A, Stadel JM, Lefkowitz RJ. A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor. J Biol Chem. 1980 Aug 10;255(15):7108-17. PMID: 6248546.
(3) Jacque Monod, Jeffries Wyman, Jean-Pierre Changeux, On the nature of allosteric transitions: A plausible model, Journal of Molecular Biology, Volume 12, Issue 1, 1965, Pages 88-118, ISSN 0022-2836, https://doi.org/10.1016/S0022-2836(65)80285-6.
(4) Leff P. The two-state model of receptor activation. Trends Pharmacol Sci. 1995 Mar;16(3):89-97. doi: 10.1016/s0165-6147(00)88989-0. PMID: 7540781.