Kd Binding Affinity Calculator

Calculate fraction bound, binding affinity Kd, and binding occupancy from Kd and ligand concentration. Uses saturation binding equation: fraction bound = [L] / (Kd + [L]). Essential for protein-ligand interactions.

🧬 Biophysics📐 FB = [L] / (Kd + [L])🧬 Biology
Dissociation constant Kd (nM)
Ligand concentration [L] (nM)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Kd Binding Affinity Calculator—FB = [L] / (Kd + [L])nM or uM

Step-by-Step Examples

Example 1
Antibody-Antigen Binding

Monoclonal antibody Kd=1nM. Antigen at 10 nM in serum.

  • FB = 10/(1+10) = 10/11 = 90.9%
  • Almost fully saturated at 10x Kd
  • For 50% binding, need [antigen]=Kd=1nM
✓ 90.9% occupancy at 10x Kd
Example 2
Drug-Receptor Binding

Drug with Kd=100nM. Effective concentration in blood = 50nM.

  • FB = 50/(100+50) = 50/150 = 33.3%
  • Drug at 0.5x Kd gives only 33% receptor occupancy
  • Need higher dose or more potent drug for therapeutic effect
✓ 33% receptor occupancy
Example 3
Protein-DNA Binding

Transcription factor Kd=0.5nM for promoter. Nuclear concentration = 5nM.

  • FB = 5/(0.5+5) = 5/5.5 = 90.9%
  • High occupancy: gene will be strongly activated
  • Kd << [protein] ensures robust regulation
✓ 90.9% promoter occupancy

Real-World Applications

💊
Drug Development
Kd determines drug potency. Drugs need Kd < free drug concentration at target for efficacy.
🧪
Biochemistry Research
Quantifying protein-protein, protein-DNA, protein-RNA, and enzyme-substrate interactions.
🏥
Antibody Characterization
Antibody Kd (dissociation constant) determines sensitivity of immunoassays and clinical antibodies.
📊
Biophysical Methods
ITC, SPR, FP, and BLI all directly measure Kd for molecular interactions.

Common Mistakes to Avoid

⚠️
Confusing Kd with Ki

Kd is the dissociation constant of ligand from receptor/protein. Ki is inhibition constant. They are related but not identical.

⚠️
Using nM vs uM inconsistently

Check units! A Kd of 10 nM = 0.01 uM. Mix-ups of 1000-fold are common when switching between nM and uM.

⚠️
Ignoring cooperativity

Simple Kd equation assumes 1:1 binding. For cooperative binding (like hemoglobin-O2), use Hill equation with Hill coefficient n.

Frequently Asked Questions

What does Kd mean? ▾
Kd is the concentration of ligand at which 50% of binding sites are occupied. Lower Kd = tighter binding = higher affinity.
What is the difference between Kd and Ka? ▾
Ka = 1/Kd (association constant). Ka measures binding affinity in M-1. Kd is more commonly used because it has intuitive units (concentration at half-max binding).
How is Kd measured experimentally? ▾
SPR (surface plasmon resonance), ITC (isothermal titration calorimetry), fluorescence polarization, ELISA, and pull-down assays with quantification.
What is a good Kd for a drug? ▾
Drugs targeting receptors: 1 nM - 1 uM typical. Antibodies: 0.01-10 nM. Weak interactions: > 1 uM. Tight-binders: < 1 pM (avidin-biotin: 10-15 M).
What is the difference between Kd and IC50? ▾
IC50 is functional (inhibition of activity) and depends on assay conditions. Kd is thermodynamic (equilibrium binding). Convert IC50 to Ki using Cheng-Prusoff equation.

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