Binding Affinity vs Potency: Kd, Ki, EC50, IC50 and Emax
Distinguish binding affinity, functional potency and maximum response, with hypothetical examples showing why assay endpoints cannot be ranked interchangeably.
Binding affinity describes a ligand–target interaction; potency describes the concentration associated with a defined effect in a particular assay. A lower Kd and a lower EC50 therefore answer different questions. Neither value, by itself, establishes a larger maximum response, greater selectivity or an outcome outside the tested system.
Five endpoints that should remain in separate columns
| Endpoint | Usual interpretation | Important limitation |
|---|---|---|
| Kd | Equilibrium dissociation constant for binding | Requires the binding model and conditions |
| Ki | Inhibition constant estimated from an appropriate competition model | Not simply any observed IC50 |
| EC50 | Concentration producing a defined halfway response | Depends on the response definition and system |
| IC50 | Concentration producing a defined halfway inhibition | Depends on what is inhibited and how it is measured |
| Emax | Maximum measured or fitted response | Keep the reference, scale and uncertainty visible |
The Assay Guidance Manual separates receptor binding measurements from functional characterisation. Its binding chapter also distinguishes an observed competition IC50 from the Ki calculated under a specified model. Converting one into the other requires the relevant assumptions and assay information. [1]
Affinity is not a promise about the size of a response
A ligand can be described by a binding measurement without that measurement showing whether it activates the chosen functional readout. A functional experiment asks an additional question. The GPCR characterisation chapter discusses affinity, potency and efficacy as related but distinct properties, and describes how receptor expression can influence measured responses. [2]
When reading a peptide paper, first classify the experiment. Does it measure competition for binding, a downstream cellular signal, or a different endpoint? If a summary says only “strong activity,” return to that distinction before recording a number. A concentration with no endpoint definition is incomplete information.
Also record units. Nanomolar and micromolar are different scales; a unit conversion can correct a presentation difference, but cannot turn a functional endpoint into a binding constant. Both the number and what it measures must match the intended comparison.
A lower EC50 does not imply a higher maximum
Hypothetical same-assay example: suppose curve A has an EC50 of 2 nM and a maximum of 40 response units. Curve B has an EC50 of 20 nM and a maximum of 100 units, with a common zero baseline. A reaches half of its own maximum at a lower concentration, but B reaches the larger maximum. Ranking only the EC50 would conceal that difference.
Under this stated convention, the halfway values are 20 units for A and 50 for B. These are not the same absolute response. If a report instead defines a midpoint relative to a common reference response, read that definition before reusing its EC50. The Assay Guidance Manual’s data-standardisation chapter explains why result types, normalisation and reference compounds must be specified. [3]
The example is deliberately not a ranking of real compounds. It shows why a comparison table needs both midpoint and maximum fields. Where a curve has no supported plateau, describe the maximum and fitted midpoint as uncertain rather than reporting precise values as though the missing region had been measured.
The experimental system can change apparent potency
Receptor abundance and downstream signal amplification can alter the relationship between occupancy and measured response. The GPCR chapter discusses receptor reserve: a system can reach its maximal response without every receptor being occupied. Consequently, a functional midpoint need not equal a binding dissociation constant. [2]
This creates a practical reading rule: compare like with like before interpreting differences. Two numbers from different cell systems are not a controlled head-to-head experiment merely because both use nanomolar units. Preserve receptor species, cell background, readout and exposure timing where supplied. If an author leaves one of those fields unstated, keep the gap visible.
For the same reason, a curve shift after changing a construct or assay cannot automatically be attributed to stronger molecular binding. Look for the experiment that addresses the proposed explanation. Our experimental-controls guide discusses how a comparison supports a specific inference.
A comparison record that preserves the question
Use a row containing the full entity, target, assay type, endpoint definition, units, reference, midpoint, maximum and uncertainty. This is an editorial extraction template, not a substitute for the paper’s methods. It prevents a clean-looking spreadsheet from erasing the conditions that made each value meaningful.
For example, “A: 2 nM” is insufficient. “A: functional EC50, half its own response span, system X, maximum 40 units, hypothetical example” states what the number means. If the paper instead reports binding competition, label that row accordingly. Do not silently move a Ki into an EC50 column to fill an empty cell.
Common questions
Is IC50 always a measure of binding affinity?
No. It can describe inhibition in different assay types. A competition-binding IC50 also requires an appropriate model and conditions before interpretation as a Ki.
Does a lower EC50 mean a compound works better?
It indicates a lower midpoint concentration under the stated definition and conditions. It does not alone establish a larger maximum, selectivity or an outcome in another system.
Can binding and potency values be placed side by side?
Yes, when clearly labelled and interpreted as different endpoints. They should not be combined into a single ranking without a defensible model.
Related reading and scope
Read how research systems differ, why material content is another measurement, or the peptide science library. Scope: interpreting laboratory endpoints, not original pharmacology data or a systematic comparison of commercial products. HealthyWelness operates a commercial research-material catalog.
Cover: AI-generated conceptual editorial artwork, not an exact molecular structure, measured result or a photograph of a HealthyWelness facility.