Binding Affinity

Understanding Binding Affinity

Binding affinity describes the strength of molecular interactions:

Drug + Receptor ⇌ Drug-Receptor Complex
      kon →
      ← koff

Kd = koff / kon = [Drug][Receptor] / [Drug-Receptor]

Key Principles

Concept	Meaning	Implication
Low Kd	Tight binding	Drug stays bound longer
High Kd	Weak binding	Drug dissociates easily
kon	Association rate	How fast binding occurs
koff	Dissociation rate	How fast drug leaves

The Dissociation Constant (Kd)

Interpreting Kd Values

Kd Range	Affinity Level	Example
pM (10^-12 M)	Extremely high	Therapeutic antibodies
nM (10^-9 M)	High	Most peptide drugs
uM (10^-6 M)	Moderate	Some small molecules
mM (10^-3 M)	Low	Metabolites, substrates

What Kd Physically Means

Kd equals the concentration at which 50% of receptors are occupied:

Receptor Occupancy
100%|         _____
    |        /
 50%|-------*---- At Kd, 50% occupied
    |      /
  0%|_____/____________
         Kd    [Ligand]

Binding Kinetics

Beyond Equilibrium Affinity

Two drugs with identical Kd can behave differently:

Property	Drug A	Drug B
Kd	1 nM	1 nM
kon	10^6 M^-1s^-1	10^8 M^-1s^-1
koff	10^-3 s^-1	10^-1 s^-1
Binding	Slow on, slow off	Fast on, fast off
Result	Long-lasting	Rapidly reversible

Drug A provides sustained effect; Drug B allows rapid response to concentration changes.

Residence Time

Drug-receptor residence time (1/koff) often correlates better with in vivo duration than Kd alone:

• Long residence time: Sustained effect, less frequent dosing
• Short residence time: Rapid onset/offset, easier titration

Binding Affinity in Peptide Research

GLP-1 Receptor Binding

Agonist	GLP-1R Affinity	Notes
Native GLP-1	Kd ~0.5-1 nM	High affinity, rapidly degraded
Semaglutide	Kd ~0.1-0.5 nM	Similar affinity, albumin binding extends action
Tirzepatide	Kd ~0.5-2 nM	Biased agonist, different signaling

Growth Hormone Secretagogue Receptor

Ligand	GHS-R Affinity	Notes
Ghrelin	Kd ~1-10 nM	Endogenous ligand
Ipamorelin	Kd ~1-5 nM	Selective synthetic agonist
MK-677	Kd ~1-5 nM	Orally active, non-peptide

Affinity vs Function

Affinity Doesn’t Equal Activity

High affinity is necessary but not sufficient:

Scenario	Affinity	Efficacy	Result
Full agonist	High	High	Strong activation
Partial agonist	High	Moderate	Limited activation
Antagonist	High	Zero	Blocks receptor
Weak agonist	Low	High	Needs high concentration

Receptor Reserve

Systems with receptor reserve can achieve maximum response without full receptor occupancy:

• Maximum effect at 20% occupancy = high receptor reserve
• Maximum effect at 90% occupancy = low receptor reserve

This explains why EC50 (functional) is often lower than Kd (binding).

Methods to Measure Affinity

Common Techniques

Method	Measures	Advantage
Radioligand binding	Kd, Bmax	Classic, quantitative
Surface plasmon resonance (SPR)	kon, koff, Kd	Real-time kinetics
Isothermal calorimetry (ITC)	Kd, thermodynamics	Label-free
Fluorescence polarization	Kd	High-throughput

Frequently Asked Questions

Why is a lower Kd better?

Lower Kd means tighter binding, so the drug is effective at lower concentrations. This typically translates to lower doses, potentially fewer off-target effects, and often longer duration of action. However, very tight binding isn’t always desirable if rapid reversibility is needed.

How does binding affinity relate to potency?

Affinity (Kd) measures binding strength; potency (EC50) measures functional effect. They’re related but not identical. A drug with high affinity usually has high potency, but signaling efficiency and receptor reserve also contribute. EC50 is often lower than Kd due to signal amplification.

Can binding affinity be too high?

Yes. Extremely high affinity (very low Kd) can cause problems: slow receptor turnover, prolonged effects that are hard to reverse, potential for accumulation, or “trapping” at non-target sites. Optimal affinity balances efficacy with manageable pharmacokinetics.