Calculator Inputs
Example Data Table
| Example | Inputs | Formula Route | Calculated Kd | Interpretation |
|---|---|---|---|---|
| Kinetic example | kon = 2.5×106 M-1s-1, koff = 5×10-3 s-1 | Kd = koff / kon | 2 nM | Very strong affinity |
| Free concentration example | [L] = 20 nM, [R] = 10 nM, [LR] = 5 nM | Kd = ([L][R]) / [LR] | 40 nM | Moderate-to-strong affinity |
| Total concentration example | Lt = 120 nM, Rt = 80 nM, LR = 30 nM | Kd = ((Lt − LR)(Rt − LR)) / LR | 150 nM | Moderate affinity |
| Occupancy example | [L] = 75 nM, θ = 0.60 | Kd = [L](1 − θ) / θ | 50 nM | Higher ligand still leaves partial occupancy |
Formula Used
1) Equilibrium definition
Kd = ([L]free × [R]free) / [LR]
Here, lower Kd means a ligand binds more tightly to its target.
2) Kinetic definition
Kd = koff / kon
This connects microscopic binding and unbinding rates with equilibrium affinity.
3) Occupancy relation
θ = [L] / ([L] + Kd)
Kd = [L](1 − θ) / θ
4) Total concentration route
[L]free = Lt − [LR]
[R]free = Rt − [LR]
Kd = ((Lt − [LR]) × (Rt − [LR])) / [LR]
Use consistent concentration units inside one calculation. When using kon and koff, Kd is first produced in molar units, then converted to your selected display unit.
How to Use This Calculator
- Select the calculation mode that matches your available data.
- Choose the concentration unit you want for displayed results.
- Enter either kinetic constants, equilibrium concentrations, totals, or occupancy data.
- Optionally enter an evaluation ligand concentration and Bmax.
- Click Calculate Kd to show results above the form.
- Review Kd, Ka, affinity interpretation, and the binding curve.
- Use the CSV or PDF buttons to export the result summary.
Answers to the Requested Questions
dissociation constant equation example give kd
Example: if free ligand is 20 nM, free receptor is 10 nM, and bound complex is 5 nM, then
Kd = (20 × 10) / 5 = 40 nM.
That value means half-maximal binding would occur when free ligand is roughly 40 nM.
kd dissociation constant enzyme kinetics
In enzyme systems, Kd describes the binding affinity between enzyme and ligand, inhibitor, or substrate during a reversible binding step.
It is not the same as catalytic turnover. When the binding step is reversible, Kd = koff / kon.
Km and Kd are only equal under specific assumptions.
dissociation constant kd value plot
A Kd plot is usually a binding curve with ligand concentration on the x-axis and occupancy or bound fraction on the y-axis. The midpoint of the curve corresponds to Kd. Lower Kd shifts the curve left, showing stronger affinity at lower ligand concentrations.
dissociation constants (kd) of e2
E2 commonly refers to estradiol. There is no single universal Kd value for E2 because it changes with receptor subtype, assay design, temperature, buffer composition, and whether total or free ligand is measured. Use experimentally matched conditions before comparing literature values.
dissociation constant kd equation
The core equilibrium equation is Kd = ([L]free × [R]free) / [LR].
The kinetic form is Kd = koff / kon.
For single-site binding, occupancy is linked by θ = [L] / ([L] + Kd).
FAQs
1. What does a lower Kd mean?
A lower Kd means stronger binding affinity. The ligand reaches meaningful occupancy at lower concentration, so the target-ligand complex is favored more strongly at equilibrium.
2. What units does Kd use?
Kd has concentration units, such as M, mM, µM, nM, or pM. It should be reported in the same concentration scale used for ligand and receptor values.
3. Is Kd the same as Ka?
No. Ka is the association constant and equals 1/Kd. A higher Ka means stronger binding, while a lower Kd means the same thing expressed inversely.
4. Can Kd be calculated from kon and koff?
Yes. When reliable kinetic constants are available, Kd is simply koff divided by kon. This connects time-dependent binding measurements with equilibrium affinity.
5. Can Kd be negative?
No. A physical dissociation constant cannot be negative. Negative results usually signal unit mismatch, wrong inputs, or an incorrect model for the binding system.
6. Is Kd the same as IC50?
No. IC50 depends on assay conditions and competing ligand levels. Kd reflects equilibrium affinity. They can be related in some assays, but they are not interchangeable values.
7. Why is half-maximal binding linked to Kd?
In a simple one-site binding model, when ligand concentration equals Kd, the occupancy becomes 50%. That is why Kd is often read from the midpoint of the binding curve.
8. Does this calculator work for multi-site cooperative binding?
It is designed for basic single-site or simplified equilibrium interpretations. Cooperative or multi-site systems often require Hill analysis or more advanced fitting models.