How peptide receptor signalling is studied: binding, activation and what an assay result means

A peptide on its own does nothing. It works, when it works, by being recognised — by fitting a binding site on a receptor protein well enough and specifically enough to change that protein's shape, which changes what the inside of the cell does. Almost every mechanistic claim about a research peptide is ultimately a claim about that event, so it is worth knowing how the event is measured and what the measurement leaves out.

The receptor, and why most peptide receptors are GPCRs

A receptor is a protein that binds a signalling molecule — the ligand — and converts that binding into a cellular response. For peptides the receptor is very often a G protein-coupled receptor: a single chain threading through the cell membrane seven times, with the ligand-binding region facing outward and the machinery it recruits on the inside.

In outline: the ligand stabilises a particular conformation of the receptor, which lets the receptor act on a heterotrimeric G protein sitting against the inner face of the membrane. The G protein's alpha subunit exchanges GDP for GTP, separates from its beta-gamma partners, and goes on to switch an effector enzyme or ion channel. Different G protein families push the cell in different directions — Gs raises cyclic AMP, Gi lowers it, Gq mobilises intracellular calcium — which is why one structural class of receptor can produce opposite readouts (Rosenbaum and colleagues, PMID 19458711).

This family is not a niche: GPCRs are the target of a large share of all approved drugs, a proportion counted repeatedly (Sriram and Insel, PMID 29298813; Hauser and colleagues, PMID 29075003). When a paper on a peptide identifies a receptor, it is usually one of these.

Not every compound has a known receptor. This is a real gap rather than a formality. BPC-157 has been studied for three decades without its receptor being identified, which is why the mechanistic literature on it describes downstream pathways — VEGF receptor signalling, nitric oxide — rather than a target. A compound with no identified receptor cannot be put through the assays below, screened for selectivity, or have its effects predicted from structure. See BPC-157 and connective tissue for what that absence does to a literature.

Affinity, potency and efficacy are three different measurements

These three get used interchangeably in marketing copy and mean quite distinct things.

Affinity is how tightly a ligand binds, irrespective of what the binding does. It is reported as a dissociation constant, Kd — the concentration at which half the receptors are occupied at equilibrium. A lower number means tighter binding. For a compound measured by displacing something else, the equivalent figure is Ki, and converting the raw IC50 from a displacement curve into a Ki requires the Cheng-Prusoff relationship, which corrects for how much labelled ligand was present and how tightly it binds (PMID 4202581). An IC50 quoted without the assay conditions is close to uninterpretable for this reason.

Potency is how much ligand is needed to produce half of the maximum response in a functional assay, reported as EC50. Potency depends on affinity but also on how much receptor the cell expresses and how much amplification sits between the receptor and the readout. The same peptide can show a tenfold different EC50 in two cell lines without any difference in its chemistry.

Efficacy is how large a response the ligand can produce at all. A full agonist reaches the maximum the system can give. A partial agonist produces a submaximal response however much is added — and because it occupies without fully activating, it reduces the effect of a full agonist present at the same time. An antagonist binds and produces no response. An inverse agonist drives activity below the receptor's resting baseline, which only means anything for receptors with measurable constitutive activity.

A compound can therefore have high affinity and no efficacy. That combination is an antagonist, and a useful reminder that "binds the receptor" is not a claim about activity.

What the common assays actually measure

Radioligand binding. A labelled ligand is incubated with membranes or whole cells and the test compound added to compete it off. The output is an occupancy curve, so this measures affinity and selectivity and nothing else. Binding is a necessary condition for an effect and nowhere near a sufficient one.

Second-messenger assays. cAMP accumulation for Gs- and Gi-coupled receptors, intracellular calcium for Gq. These are functional, so they give potency and efficacy. They also sit one step from the receptor and are heavily amplified, which flatters weak agonists.

Reporter-gene assays. A response element drives a luminescent reporter, integrating signal over hours. Sensitive, cheap, and the furthest downstream — many things that are not the receptor can move a reporter.

Recruitment assays. BRET and similar proximity methods watch a labelled partner, typically beta-arrestin or a G protein, arrive at the receptor. These report the receptor's own behaviour rather than a downstream consequence, which is what makes them the tool of choice for the next point.

Desensitisation, internalisation and bias

Receptors do not respond indefinitely. Sustained stimulation leads to phosphorylation by GPCR kinases, recruitment of arrestins, physical removal of the receptor from the surface into endosomes, and eventually degradation or recycling — a sequence worked out over the 1990s and reviewed thoroughly by Ferguson (PMID 11171937). This is why an acute assay result and a chronic one can disagree completely, and why continuous exposure in a model is a different experiment from intermittent exposure.

Arrestin recruitment is not only a shutdown mechanism. It initiates signalling of its own, and ligands differ in how strongly they drive G protein signalling versus arrestin signalling at the same receptor. That phenomenon — biased agonism — means two agonists at one receptor can produce qualitatively different cellular outcomes, and it is now a design objective in drug discovery rather than an oddity (Kenakin and Christopoulos, PMID 23411724). It also means "agonist at receptor X" is an incomplete description of what a peptide does.

What a receptor result does not establish

A clean binding curve and a clean EC50 tell you a molecule engages a target in a system built to make that engagement visible. They do not tell you the target is engaged at concentrations reachable in a living animal, that the peptide survives in plasma long enough to get there, that the cell type carrying the receptor in vivo responds as the engineered line did, or that any of it produces a change worth measuring in an organism.

Overexpressed receptors in immortalised cells are the standard tool precisely because they exaggerate signal — a feature for detecting an interaction and a hazard for estimating its importance. The step from a receptor assay to an animal model, and from an animal model to anything about a person, is where most of the attrition in this field happens: the subject of how to read a peptide study.

For the chemistry and vocabulary underneath all of this, see peptides explained and the glossary. Nothing here is medical, dosing, or therapeutic guidance; compounds named on this site are supplied for laboratory research use only and are not authorised for human use.

Common questions

If a peptide binds its receptor, does it work? No. Binding establishes engagement, not consequence. An antagonist binds tightly and produces no response at all, and a weak partial agonist can bind tightly while doing very little. Function has to be measured separately.

Why do two papers report different EC50 values for the same peptide? Because potency is a property of the assay as much as the molecule. Receptor expression level, cell line, incubation time, temperature, buffer and readout all shift it. Comparing potencies across papers is only meaningful when the systems match, which is why good studies run a reference agonist in parallel.

What does it mean that a compound has no identified receptor? That its mechanism is described downstream of wherever it acts. Observed effects may be real and still unexplained, but without a target there is no selectivity screen, no structure-activity work and no way to anticipate off-target activity.

Is a cell-culture result worth anything? Yes, for the question it answers. In vitro work establishes that an interaction is possible, maps a pathway and orders compounds by potency, with controls that are impossible in an animal. It cannot establish that the interaction matters in a whole organism, and it is not evidence about people.

Selected Research References

1. Rosenbaum DM, Rasmussen SGF, Kobilka BK. The Structure and Function of G-Protein-Coupled Receptors. Nature. 2009;459(7245):356-363. pubmed.ncbi.nlm.nih.gov/19458711
2. Cheng Y, Prusoff WH. Relationship Between the Inhibition Constant (Ki) and the Concentration of Inhibitor Which Causes 50 Per Cent Inhibition of an Enzymatic Reaction. Biochemical Pharmacology. 1973;22(23):3099-3108. pubmed.ncbi.nlm.nih.gov/4202581
3. Ferguson SSG. Evolving Concepts in G Protein-Coupled Receptor Endocytosis: The Role in Receptor Desensitization and Signaling. Pharmacological Reviews. 2001;53(1):1-24. pubmed.ncbi.nlm.nih.gov/11171937
4. Kenakin T, Christopoulos A. Signalling Bias in New Drug Discovery: Detection, Quantification and Therapeutic Impact. Nature Reviews Drug Discovery. 2013;12(3):205-216. pubmed.ncbi.nlm.nih.gov/23411724
5. Sriram K, Insel PA. G Protein-Coupled Receptors as Targets for Approved Drugs: How Many Targets and How Many Drugs? Molecular Pharmacology. 2018;93(4):251-258. pubmed.ncbi.nlm.nih.gov/29298813
6. Hauser AS, Attwood MM, Rask-Andersen M, Schiöth HB, Gloriam DE. Trends in GPCR Drug Discovery: New Agents, Targets and Indications. Nature Reviews Drug Discovery. 2017;16(12):829-842. pubmed.ncbi.nlm.nih.gov/29075003

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