Calculator Inputs
Enter thermodynamic, binding, and rate values. Leave manual standard energy blank to derive it from Keq.
Example Data Table
Use these example values to compare weak binding, tight binding, fast turnover, and pH shift cases.
| Case | Temperature | Keq | Q | Kd | kcat | Meaning |
|---|---|---|---|---|---|---|
| Balanced reaction | 25 °C | 1 | 1 | 100 uM | 10 s⁻¹ | Near equilibrium with moderate turnover. |
| Forward favored | 37 °C | 10000 | 0.2 | 20 nM | 250 s⁻¹ | Negative reaction energy and tight binding. |
| Product loaded | 30 °C | 50 | 200 | 5 uM | 80 s⁻¹ | Product accumulation can reduce forward drive. |
| pH linked | 25 °C | 500 | 1.5 | 1 uM | 150 s⁻¹ | Proton coefficient changes apparent energy. |
Formula Used
Reaction Free Energy
ΔG°′ = −RT ln(Keq)
ΔG = ΔG°′ + RT ln(Q) + 2.303RT nH(pH − pHref)
R is the gas constant. T is absolute temperature in kelvin.
Binding and Activation Energy
ΔGbinding = RT ln(Kd / 1M)
ΔG‡ = RT ln(kBT / hkcat)
ΔΔG‡ = −RT ln(kcat / kuncat)
How to Use This Calculator
- Enter temperature in Celsius. The calculator converts it to kelvin.
- Enter Keq to calculate standard free energy, or enter manual ΔG°′.
- Enter the reaction quotient Q for the current reaction mixture.
- Add Kd, Ki, or Km to estimate binding free energy.
- Add kcat and uncatalyzed rate to estimate activation effects.
- Use pH values and proton coefficient only when the reaction is proton linked.
- Press calculate. Results appear above the form and below the header.
- Download CSV or PDF for reports, records, and comparison tables.
Enzyme Free Energy in Physics
Enzymes change the speed of reactions. They do not change equilibrium. Free energy explains that useful difference. A reaction with negative Gibbs free energy is thermodynamically favorable. A reaction with positive free energy needs coupling, concentration change, or external work.
Why Free Energy Matters
In physical chemistry, energy links motion, heat, and molecular probability. The Gibbs equation uses temperature, the gas constant, and a logarithm. This makes small concentration changes important. It also explains why equilibrium constants can be huge or tiny.
Enzyme systems often need more than one view. Reaction energy describes direction. Binding energy estimates affinity. Activation free energy describes the barrier that controls rate. This calculator combines these views in one place.
Interpreting Reaction Results
Standard free energy comes from the equilibrium constant. A large equilibrium constant gives a negative standard value. Actual free energy uses the reaction quotient. When products build up, the reaction quotient rises. The reaction may become less favorable. When substrates dominate, the value can drop.
Temperature also matters. Higher temperature changes the RT term. It can magnify logarithmic effects. For proton linked reactions, pH can shift the apparent value. The pH correction is a practical estimate. It is most useful when the proton coefficient is known.
Binding and Activation Views
Binding free energy uses the dissociation constant. Strong binding usually has a small Kd. Because Kd is compared with one molar standard state, tight binding gives negative values. This helps compare inhibitors, substrates, or protein partners.
Activation free energy uses the Eyring relationship. Faster catalytic turnover gives a lower barrier. If an uncatalyzed rate is available, rate acceleration can show transition state stabilization. This is often the clearest enzyme physics insight.
Practical Use
Use consistent units. Enter concentrations as dimensionless reaction quotients. Keep temperature realistic for the enzyme. Treat Km as an approximation when true Kd is missing. Export the table for lab notes. Use the chart to inspect how reaction quotient changes energy. Always confirm biological conclusions with experiments.
The calculator gives estimates, not proof. Real enzymes can have conformational steps, solvent effects, and ionic strength effects. Use results as a guide for planning future tests.
FAQs
What does enzyme free energy mean?
It describes the thermodynamic drive, binding strength, or activation barrier linked to an enzyme reaction. The meaning depends on which input set you use.
Does an enzyme change equilibrium free energy?
No. Enzymes lower activation barriers and speed reactions. They do not change the final equilibrium position or the standard reaction free energy.
What is the difference between Keq and Q?
Keq describes equilibrium. Q describes the current mixture. Actual reaction energy uses Q, so product buildup can change the reaction direction estimate.
Can I use Km instead of Kd?
Km can be used as an approximation when Kd is unavailable. It is not always the same because Km also depends on catalytic rate constants.
Why is tight binding usually negative?
The binding formula compares Kd with a one molar standard state. Very small Kd values give negative logarithms and negative binding free energy.
What does activation ΔG‡ show?
It estimates the energetic barrier related to turnover rate. Lower activation free energy usually means faster catalytic conversion under the same conditions.
How should I enter the proton coefficient?
Use the sign based on your reaction convention. Positive and negative values move the pH correction in opposite directions, so define the reaction carefully.
Are these results experimental proof?
No. They are model based estimates. Use them to compare cases, plan assays, and understand physics before confirming results with measurements.