Receptors and signalling: a short primer for peptide research

Two receptor families dominate

Most peptide targets fall into two structural classes. G-protein-coupled receptors are seven-transmembrane proteins that, on ligand binding, activate heterotrimeric G proteins; the class B subfamily to which GLP-1, GIP, GHRH and VIP receptors belong is characterised by a large extracellular domain that captures the peptide N-terminus. Receptor tyrosine kinases, such as the IGF-1 receptor, instead dimerise and autophosphorylate on ligand binding, initiating intracellular cascades directly.

Which family a target belongs to predicts the assay. GPCR work is usually read out as cAMP accumulation or calcium mobilisation; receptor tyrosine kinase work is read out as phosphorylation state by western blot or phospho-specific immunoassay.

Agonist vocabulary

A full agonist produces the maximal response the receptor system can generate. A partial agonist binds and activates but plateaus below that maximum, and in the presence of a full agonist can act functionally as an antagonist. An inverse agonist suppresses constitutive activity below baseline. An allosteric modulator binds away from the orthosteric site and changes the response to the natural ligand rather than producing one itself.

Potency and efficacy are distinct and frequently conflated. Potency is the concentration required for half-maximal effect, reported as EC50. Efficacy is the size of the maximal effect. A compound can be far more potent than a comparator while producing a smaller maximal response.

Biased signalling

A single receptor can couple to multiple downstream pathways — classically G-protein signalling and beta-arrestin recruitment. Ligands that preferentially engage one over the other are described as biased. Bias matters experimentally because two agonists with matched cAMP potency can produce entirely different receptor internalisation and desensitisation behaviour, and therefore different responses over long incubations.

Half-life engineering

Much of modern peptide design is not about receptor binding at all but about clearance. Lipidation, albumin-binding linkers, pegylation and protease-resistant substitutions all extend residence time without changing the receptor interaction. When comparing analogs, it is worth separating what a modification does to affinity from what it does to exposure — they are independent variables and are frequently confused in the secondary literature.

From binding event to measurable readout

Every receptor assay is a chain of inference, and knowing where the chain is measured explains most apparent contradictions in the literature. Binding assays report affinity — how tightly a ligand occupies the receptor — and say nothing about what happens next. Functional assays report the consequence of occupancy: cAMP accumulation for Gs-coupled receptors, calcium flux for Gq, inhibition of forskolin-stimulated cAMP for Gi, and beta-arrestin recruitment for the desensitisation arm.

Amplification means these numbers rarely agree. Second-messenger cascades amplify signal, so a ligand occupying a small fraction of receptors can produce a near-maximal cAMP response; the functional EC50 then sits well below the binding Kd. Receptor reserve of this kind is a property of the cell line and expression level, not of the compound, which is why potency values are only comparable when the assay system is the same.

Time is the other overlooked variable. Acute readouts taken minutes after addition capture initial coupling; readouts after hours capture the net of coupling, internalisation, recycling and degradation. Two agonists that look identical at ten minutes can diverge completely over a four-hour incubation, and this divergence is usually the point of a biased-signalling comparison.

Comparing analogs without confusing exposure and activity

Half-life engineering — lipidation, albumin binders, pegylation, protease-resistant substitutions — changes how long a molecule persists, not how strongly it engages the receptor. In an in vitro well with no clearance mechanism, those modifications may show little effect or even slightly reduce potency because the added moiety mildly impairs binding. The same modification can dominate the outcome in a preclinical model where exposure is the limiting factor.

The practical rule is to read comparisons in the light of their system. A statement that one analog is more potent than another is meaningful only alongside the assay, the cell background, the incubation time and whether the readout is proximal or distal. Secondary sources frequently strip that context, which is how single potency ratios acquire a life of their own.

Multi-receptor agonists complicate this further. A dual or triple agonist has a separate potency at each target, and the ratio between them — not the absolute values — usually characterises the compound. Reported ratios should always be traced to the primary paper, because they shift with the species of receptor used in the assay.