It encloses a brief information about ITC its experimental instrumentation, working, results, and applications to other fields like pharmaceuticals, drug discovery etc.
2. Introduction
Isothermal Titration Calorimetry (ITC) is a technique used in
quantitatively in measurement of a wide variety of biomolecular
interactions.
A direct measurement of heat generated or absorbed when molecules
interact.
Directly measure the heat that is either released or absorbed during a
biomolecular binding reaction.
simultaneously determine all binding parameters in a single experiment
such as binding constant (KD), reaction stoichiometry (n), enthalpy
(∆H) and entropy (ΔS).
No need of modification of binding patterns, either with fluorescent
tags or labeling or through immobilization.
It goes beyond binding affinities and can elucidate the mechanisms
underlying molecular interactions.
3. Principle
Isothermal Titration Calorimetry is used to measure interactions
between biomolecules.
It determine binding affinity, stoichiometry, and entropy and enthalpy
of the binding reaction in solution, without any labeling.
When binding occurs, heat is either absorbed or released and this is
measured by the sensitive calorimeter during gradual titration of the
ligand into the sample cell containing the biomolecule of interest.
5. Instrumentation
1. The thermal core
In the microcalorimeter there are two cells, one of which contains water and acts as a
reference cell, the other contains the sample.
The microcalorimeter needs to keep these two cells at exactly the same temperature.
The heat sensing devices detect temperature difference between the cells when
binding occurs and give feedback to the heaters, which compensate for this
difference and return the cells to equal temperature.
2. Making a measurement
The reference cell and the sample cell are set to the desired experimental
temperature.
The ligand is loaded into a syringe which sits in a very accurate injection device.
The injection device is inserted into the sample cell containing the protein of interest.
A series of small amounts of ligand are injected into the protein solution.
If there is a binding of the ligand to the protein, heat changes of a few millionths of a
degree Celsius are detected and measured.
8. Experiment
Ligand in syringe
Biomolecule in sample cell
Interaction heat is measured
Following parameters are
measured from a single
experiment:-
1. Binding constant(K)- Affinity
2.Enthalpy (change in H)-
Energy
3.No. of binding sites.-n
Reference:-
https://www.sciencedirect.com/science/articl
e/pii/S0076687915004267
9. First injection
As the first injection made all ligand bound to target molecule.
In 2 signal returns to baseline before next injection.
Reference:
https://www.huck.psu.edu/assets/uploads/content-
images/_1024xAUTO_fit_center-center_80/Titration3.png
10. As the injections continue,
the target becomes
saturated with compound,
so less binding occurs and
the heat change starts to
decrease.
Reference:-
https://www.huck.psu.edu/assets/uploads/content-
images/_1024xAUTO_fit_center-
center_80/Titration3.png
11. RESULTS
•A binding curve is obtained
from a plot of the heats from
each injection against the ratio
of ligand and binding partner in
the cell.
•The binding curve is analyzed
with the appropriate binding
model to determine K (binding
affinity), n (number of binding
sites), and ΔH(enthalpy).
Reference:-
https://www.huck.psu.edu/assets/uploads/content-
images/_1024xAUTO_fit_center-
center_80/BindingCurve.png
12. Determination of thermodynamic
characterization
A full thermodynamic profile is then obtained
using the relationships
∆G ◦ = ∆H ◦ − T∆S ◦ = −RT ln Kb
Above mentioned ∆G ◦ , ∆H ◦ and ∆S ◦ are the
Gibbs free energy, enthalpy and entropy of
binding, respectively. T is the absolute
temperature and R = 1.98 cal mol−1 K −1 is
the ideal gas law constant Reference:
https://bic.nd.edu/instrumentation/itc/
13. Advantages
Complete basic thermodynamic characterization i.e.
1. stoichiometry constant,
2. association constant,
3. binding enthalpy in a single experiment.
No need of labeling e.g. chromophores, fluorophores.
Direct determination of the binding enthalpy.
Interaction in solution
Possibility of performing experiment with optically dense solutions or
unusual systems e.g. dispersions, intact organelles or cells
Considered to be faster
14. Disadvantages
Heat is a universal signal and each process contributes to the global
measured heat, thus complicating the evaluation of the contribution
because of binding
Large amount of sample is needed.
Low throughput cannot suitable for HTS
Kinetically slow processes may be overlooked.
A limited range for consistently measured binding affinities.
15. Applications
• Drug discovery:
• Hit validation and characterization
• Optimization of candidates
• Mechanism of action
• Quantitative measurement of interaction between any two biomolecules
(protein, RNA, DNA, lipids, drugs and inhibitors):
• Confirm binding and activity
• Determine thermodynamic parameters and stoichiometry
• Assessing effect of molecular structure changes on binding mechanisms
• Analysis of enzyme kinetics