An equilibrium binding constant describes a balance, while kinetic measurements describe how binding changes over time. Two peptide ligands can have similar affinity yet differ in how quickly their complexes form and dissociate.
A ratio does not specify both of its components
In a simple reversible binding model, the equilibrium dissociation constant is the dissociation rate constant divided by the association rate constant: Kd = koff/kon. GraphPad’s kinetic-model documentation explicitly separates their units and roles.GraphPad — Association and dissociation kinetic model (opens in a new tab)
The ratio alone cannot recover both rates. Multiplying the numerator and denominator by the same factor preserves the ratio while changing the timescale.
This is why a table containing only Kd may miss a meaningful difference between candidate ligands. The table can correctly describe their equilibrium affinity while remaining silent about the route to equilibrium.
The simple relationship should remain attached to its model. A complex trace involving multiple states or other unresolved processes should not be forced into this interpretation merely because software reports two fitted rate constants.
Work through two equal-affinity scenarios
Consider two hypothetical one-step interactions. For A, kon is 1,000,000 M⁻¹s⁻¹ and koff is 0.01 s⁻¹. For B, kon is 100,000 M⁻¹s⁻¹ and koff is 0.001 s⁻¹. Both ratios give Kd = 10⁻⁸ M, or 10 nM.
| Parameter | Interaction A | Interaction B |
|---|---|---|
| Kd | 10 nM | 10 nM |
| koff | 0.01 s⁻¹ | 0.001 s⁻¹ |
| 1/koff | 100 s | 1,000 s |
For the assumed single-exponential dissociation, the mean bound lifetime is 1/koff. B therefore has a tenfold longer model lifetime despite the equal equilibrium constants.
These are constructed examples, not measurements of a product. They isolate the mathematical point: equal affinity does not uniquely determine a dissociation timescale.
A peptide panel demonstrated a wider kinetic spread
A study of 12 GnRH peptide agonists compared radioligand and time-resolved FRET approaches to receptor-binding kinetics. The two approaches gave highly correlated results.Characterisation of 12 GnRH peptide agonists: a kinetic perspective (opens in a new tab)
The tested peptides differed more in their binding kinetics than in affinity. Reported residence times ranged from 5.6 minutes for goserelin to 125 minutes for deslorelin in that study.Characterisation of 12 GnRH peptide agonists: a kinetic perspective (opens in a new tab)
Those values demonstrate the usefulness of measuring a time-dependent property in addition to equilibrium binding. They do not establish those times as clinical durations or predict the behaviour of a different receptor system.
When summarising such a panel, retain the assay conditions and target. A kinetic ranking observed in one experimental configuration should not become a condition-free ranking of the molecules.
Keep the biological inference separate
A longer bound lifetime can motivate a hypothesis about persistence, but a biological response still needs its own measurement. Continued signalling, receptor trafficking and clearance are not all measured by the dissociation curve.
Similarly, time spent in the circulation is a different quantity from time spent in a particular receptor complex. Shared units of minutes or hours do not make the two interchangeable.
A useful comparison reports affinity, association and dissociation information together, then identifies any separate functional experiment. This preserves the additional insight from kinetics without asking one fitted timescale to explain every aspect of activity.
Sources and further detail
- GraphPad — Association and dissociation kinetic model (opens in a new tab)
Official equations read for Kd = koff/kon and exponential dissociation. Worked rates and derived lifetimes are original illustrative calculations.
- Characterisation of 12 GnRH peptide agonists: a kinetic perspective (opens in a new tab)
Complete original abstract read. Published online 2015, 2016 issue; 5.6–125 minute range retained only for the tested receptor assays.
Sources checked 20 September 2026. Numerical examples are illustrative unless identified as published observations. This article has not undergone independent scientific peer review.