CJC-1295 + Ipamorelin (5mg/5mg) · Research brief

What Is Receptor Binding? (Affinity, Kd and Potency)

Real Peptides Research TeamEditorial

Updated September 30, 20268 min read

55 WORDS

Short answer

A compound that grips its target a thousand times more tightly than a rival compound can still lose in a functional cell assay. That single fact derails more datasheet comparisons than anything else in pharmacology. Real Peptides supplies research-grade peptides for laboratory research, and confusion around binding data is rarely about what receptor binding is.

Key takeaways

Receptor binding is a reversible equilibrium between association and dissociation, and at the Kd concentration exactly half of the available receptors are occupied.
Kd equals koff divided by kon, which means two compounds with identical affinity can differ a hundredfold in how long they stay bound.
Reaching 90% receptor occupancy requires nine times the Kd concentration and 99% requires ninety-nine times, so occupancy scales steeply, not linearly.
A receptor binding domain is the discrete folded region making direct contact with a partner protein, with the SARS-CoV-2 spike and ACE2 pairing as the best-documented case.
Affinity and potency are not interchangeable: EC50 reflects functional output amplified by receptor reserve and can sit well below the measured Kd.
Kd is condition-dependent, so a certificate of analysis confirms a compound's identity and purity, never its affinity.

Real Peptides supplies research-grade peptides for laboratory research, and confusion around binding data is rarely about what receptor binding is. It's about what a Kd value actually predicts, and what it never will.

What is receptor binding?

Receptor binding is the reversible, non-covalent attachment of a ligand to a specific protein receptor, forming a complex that may then trigger a cellular response. At the equilibrium dissociation constant (Kd), exactly 50% of available receptors are occupied. Lower Kd means tighter binding.

The common oversimplification is treating receptor binding as a lock-and-key click that either happens or doesn't. It's a continuous statistical equilibrium instead: molecules associate and dissociate constantly, and affinity is nothing more than the ratio of those two rates. This piece covers the physical binding event, what Kd and residence time each tell you, how receptor binding affinity differs from potency and efficacy, and what a receptor binding domain actually is.

How a ligand actually attaches to its target

Receptor binding is held together by four weak, non-covalent forces working in parallel: hydrogen bonds, ionic attractions between charged side chains, van der Waals contacts, and the hydrophobic effect, which buries greasy surfaces away from water. No single one of these would hold a ligand in place for more than an instant. Spread across a complementary contact surface, they add up.

Two rate constants describe the whole process. The association rate constant, kon, describes how fast ligand and receptor find each other and lock in. The dissociation rate constant, koff, describes how fast the complex falls apart. Their ratio is the equilibrium dissociation constant: Kd = koff / kon, expressed in molar units. A Kd of 1 nanomolar (1 x 10^-9 M) is a thousandfold tighter than a Kd of 1 micromolar.

The arithmetic that follows from this is worth internalising. Fractional occupancy equals [L] / ([L] + Kd). At a free ligand concentration equal to Kd, half the receptors are occupied. Reaching 90% occupancy requires nine times the Kd concentration. Reaching 99% requires ninety-nine times. The last sliver of occupancy costs roughly a hundredfold more ligand than the first half, which is exactly why assay concentration is not a footnote.

Binding is also not activation. An antagonist can occupy a site perfectly and produce no signal at all. A common reporting gap in binding work is a Kd quoted with no free-ligand concentration and no buffer attached to it.

The number most datasheets leave out

Kd is a ratio, and ratios hide their inputs. Two ligands can share an identical Kd of 5 nanomolar while one associates and dissociates rapidly and the other does both a hundred times more slowly. At equilibrium in a tube they look identical. Under conditions where free ligand is diluted or cleared, they behave nothing alike. This is drug-target residence time, the reciprocal of koff, a framework pushed into mainstream pharmacology by Robert Copeland and colleagues. A slow off-rate keeps the complex intact long after free compound has dropped away.

Here's the point most comparison guides get wrong: they treat receptor binding affinity as an intrinsic constant of a molecule. It isn't. Kd is a property of a molecule, a receptor preparation, and the conditions under which the two were mixed. Temperature, pH, ionic strength, whether the target is a whole membrane preparation or a purified extracellular domain, and the choice of radioligand all move the number.

For G protein-coupled receptors the effect is dramatic. Agonists show high-affinity and low-affinity states depending on whether the receptor is coupled to its G protein, and adding a non-hydrolysable GTP analogue collapses the high-affinity state. The same agonist, the same receptor, an order of magnitude difference in reported affinity.

When two sources list different affinity values for the same peptide, the usual explanation isn't that one is wrong. One ran surface plasmon resonance on a purified ectodomain at 25 degrees Celsius and the other ran radioligand displacement on crude membranes at 4 degrees. A receptor binding number without its method line is a number you cannot use.

What a receptor binding domain actually is

A receptor binding domain is the discrete, independently folded region of a protein that makes direct physical contact with its partner. The most heavily documented example is the receptor binding domain of the SARS-CoV-2 spike protein, the compact segment that engages the ACE2 receptor on host cells. Most neutralising antibodies target that surface precisely because it is the contact patch. On the receptor's side, the matching structure is the binding pocket, and it comes in two flavours: orthosteric, the site the endogenous ligand uses, and allosteric, a structurally separate site whose occupancy reshapes how the orthosteric site behaves.

Peptide ligands at class B GPCRs, the family that includes the growth hormone-releasing hormone receptor (GHRHR) and the GLP-1 receptor, follow a two-domain model. The peptide's C-terminal region docks into the receptor's large N-terminal extracellular domain and supplies most of the binding energy, while the N-terminal residues insert into the transmembrane bundle and drive activation. That split explains why a truncated GHRH analogue such as modified GRF(1-29), catalogued as CJC-1295 No DAC, still engages the receptor, and why Tesamorelin, a GRF(1-44) analogue carrying a trans-3-hexenoyl modification, is altered for stability rather than contact. Ipamorelin doesn't touch GHRHR at all; the literature describes it as an agonist at the growth hormone secretagogue receptor GHS-R1a, the ghrelin receptor.

Not everything works by receptor binding. 5-Amino-1MQ is characterised in published work as an inhibitor of nicotinamide N-methyltransferase (NNMT), an intracellular enzyme with a catalytic pocket, not a cell-surface receptor. Calling that receptor binding would be wrong, and the distinction matters when you choose an assay.

Affinity, potency and efficacy: what each number measures

These three terms get used interchangeably and they describe completely different things. Receptor binding affinity answers how tightly, potency answers how much, and efficacy answers how much response.

Parameter	What it measures	How it is usually obtained	Bottom line
Kd (equilibrium dissociation constant)	Free ligand concentration at which half the receptors are occupied at equilibrium	Saturation radioligand binding, surface plasmon resonance, isothermal titration calorimetry	The cleanest single measure of binding strength, valid only under the conditions it was measured in
Ki (inhibition constant)	Affinity of an unlabelled competitor for the same site, corrected for the tracer used	Competition binding, converted from IC50 using the Cheng-Prusoff equation	Use Ki rather than raw IC50 whenever comparing competitors across papers
IC50	Concentration producing 50% inhibition of a signal under one specific condition set	Functional or displacement assay	Shifts with tracer concentration and assay design; rarely comparable between labs as reported
EC50 (potency)	Concentration producing half of the compound's own maximum functional effect	Cell-based functional assay such as cAMP accumulation, calcium flux or reporter gene	Potency, not affinity; amplification and spare receptors can push it far below the Kd
Emax / intrinsic efficacy	How large a response the ligand produces once bound	The same functional assays, read at the plateau	Separates full agonists, partial agonists, antagonists and inverse agonists; affinity says nothing about this
Residence time (1/koff)	How long the ligand-receptor complex survives before dissociating	Kinetic SPR, washout binding experiments	Often the better predictor of sustained occupancy when free ligand is being cleared

What If: Binding Data Scenarios

What if two compounds show the same Kd but different results in a cell assay?

Check the binding kinetics and the functional readout before assuming an assay error. Identical Kd values can arise from very different kon and koff pairs, and a slow-dissociating ligand holds receptors occupied long after a fast one has washed away. The second explanation is efficacy: receptor binding tells you the ligand is present, not what it does once bound, so a partial agonist and a full agonist with matched affinity will plateau at different Emax values.

What if a supplier lists potency but no affinity data?

Treat the potency figure as assay-specific and ask which system produced it. An EC50 from a cAMP reporter line with high receptor expression can sit an order of magnitude below the Kd from a binding assay, because spare receptors let a small fraction of occupancy generate a maximal signal. Neither figure is dishonest. They answer different questions, and only one of them is a binding measurement.

What if the published Kd came from a different species receptor?

Assume it may not transfer and check the sequence identity of the binding region first. Single residue substitutions inside a binding pocket can shift affinity by orders of magnitude, which is why rodent and human data for the same peptide frequently diverge. Cross-species affinity differences are a mechanism, not noise, and they are one of the recurring reasons a promising in vitro result fails to reproduce in a different model system.

What if the binding curve has a Hill slope well below 1?

Re-examine the model before reporting a single Kd. A Hill slope meaningfully below 1 usually points to two or more sites with different affinities, negative cooperativity, or a heterogeneous receptor preparation. Fitting a one-site model to that data produces a number that describes nothing real. Refit with a two-site model and report both constants rather than averaging them into a single misleading value.

The blunt truth about affinity numbers on a datasheet

Here's the honest answer: an affinity value printed without its assay conditions is marketing, not data. Kd, Ki and IC50 are interpretable only alongside temperature, buffer, receptor preparation, tracer identity and ligand concentration. Strip that context away and the number can't be compared to anything. The second uncomfortable part is bigger. Strong receptor binding predicts almost nothing about behaviour in a living system, where solubility, proteolytic stability, clearance and off-target engagement decide the outcome. Affinity is necessary. It was never sufficient.

What a supplier can legitimately document is narrower and more useful: identity, purity and mass confirmation for the exact material in the vial. Third-party HPLC and mass spectrometry results on the Real Peptides certificates of analysis cover that, and it is the precondition for any binding work meaning anything, since a misidentified or contaminated peptide invalidates an assay before the first data point. Compounds across the Real Peptides peptide catalog are supplied strictly for laboratory research and are not for human or veterinary use; anyone with a question about an animal's health should talk to their veterinarian rather than reading a research catalog. Facility and shipping details sit on the Real Peptides location page.

Receptor binding is the most measurable step in a long chain and the least predictive of everything that follows it. That tension is worth sitting with. A picomolar Kd feels like an answer, and it is, to exactly one question: how tightly does this molecule hold this particular target, in this buffer, at this temperature. Almost everything a researcher actually cares about happens downstream of that figure. Treat affinity as the opening coordinate of an investigation rather than its verdict, and the data stops surprising you.

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Questions

Receptor binding is the reversible, non-covalent attachment of a ligand to a specific protein receptor, forming a complex that may or may not trigger a cellular response. It is governed by an association rate and a dissociation rate, and at the equilibrium dissociation constant exactly half of the available receptors are occupied.

Receptor binding affinity describes how tightly a ligand holds its target, expressed as the equilibrium dissociation constant Kd in molar units. Lower values mean tighter binding, so a 1 nanomolar Kd is a thousandfold tighter than 1 micromolar. Kd equals the dissociation rate constant divided by the association rate constant.

A receptor binding domain is the discrete, independently folded region of a protein that makes direct physical contact with its receptor partner. The SARS-CoV-2 spike receptor binding domain, which engages the ACE2 receptor, is the most widely documented example. On the receptor side, the matching structure is called the binding pocket.

Affinity sets the concentration needed to occupy a target, which drives candidate selection and selectivity screening. A compound with high affinity for its intended receptor and much weaker affinity elsewhere has a wider selectivity window. Affinity alone predicts nothing about efficacy, stability or clearance, so it works as a filter rather than a verdict.

Affinity measures how tightly a ligand binds and is reported as Kd or Ki. Potency measures how much compound is required to produce half of its maximum functional effect and is reported as EC50. Signal amplification and spare receptors can push EC50 far below Kd, so a potent compound is not automatically a high-affinity one.

Three approaches dominate. Saturation and competition radioligand binding on membrane preparations returns Kd and Ki values. Surface plasmon resonance reports kon and koff directly against an immobilised target, giving kinetics as well as affinity. Isothermal titration calorimetry measures the heat released during binding and yields affinity plus thermodynamic parameters in a single run.

Usually not. Kd shifts with temperature, pH, ionic strength, receptor preparation, tracer choice and, for GPCRs, the presence of GTP analogues. Values are only comparable when the methods match closely. Comparing an SPR value from a purified ectodomain with a radioligand value from crude membranes is a common and misleading error.

The orthosteric site is where the endogenous ligand normally binds. An allosteric site is a structurally separate pocket, and occupying it changes the receptor's conformation, altering affinity or efficacy at the orthosteric site without direct competition. Allosteric modulators can be positive or negative and often show ceiling effects that orthosteric agonists do not.

Affinity ignores efficacy, kinetics, solubility, proteolytic stability, clearance and off-target engagement. A high-affinity antagonist and a high-affinity full agonist can share a Kd and produce opposite biological outcomes. For laboratory work, the practical risk is designing an experiment around a number measured under conditions your assay does not reproduce.

No. Some research compounds act on intracellular enzymes rather than cell-surface receptors. 5-Amino-1MQ is characterised in the literature as an inhibitor of nicotinamide N-methyltransferase, an enzyme with a catalytic pocket. Others, such as copper-binding peptides, act through metal coordination chemistry, which is a different mechanism entirely from receptor engagement.

Reputable suppliers publish batch-specific third-party results, typically HPLC for purity and mass spectrometry for identity confirmation. Real Peptides lists these on its certificates of analysis page, organised by compound. A certificate confirms what is in the vial and at what purity; it does not and cannot certify binding affinity, which is measured separately in an assay.

RESEARCH USE ONLY · NOT EVALUATED BY THE FDA