A general protocol for the use of isothermal titration calorimetry to monitor the binding thermodynamics for biological systems with moderate binding affinities is presented.
Isothermal titration calorimetry (ITC) is a useful tool for understanding the complete thermodynamic picture of a binding reaction. In biological sciences, macromolecular interactions are essential in understanding the machinery of the cell. Experimental conditions, such as buffer and temperature, can be tailored to the particular binding system being studied. However, careful planning is needed since certain ligand and macromolecule concentration ranges are necessary to obtain useful data. Concentrations of the macromolecule and ligand need to be accurately determined for reliable results. Care also needs to be taken when preparing the samples as impurities can significantly affect the experiment. When ITC experiments, along with controls, are performed properly, useful binding information, such as the stoichiometry, affinity and enthalpy, are obtained. By running additional experiments under different buffer or temperature conditions, more detailed information can be obtained about the system. A protocol for the basic setup of an ITC experiment is given.
Isothermal titration calorimetry (ITC) is a well established technique that can determine all the thermodynamic parameters (affinity, enthalpy and stoichiometry) of a binding interaction in one experiment.1 ITC works by titrating one reactant into a second reactant under isothermal conditions. The signal measured is the heat released or absorbed upon interaction (binding) of the two reactants. A series of injections are performed and the heat signal will approach zero as the limiting reactant becomes saturated. Fitting of the isotherm gives the thermodynamic parameters. Several reviews are available that describe the instrumentation as well as the math of data collection and analysis. 2,3 While other calorimeters are available (most notably the ITC200 with small volumes), here we describe a general protocol for the VP-ITC manufactured by MicroCal (now part of GE Healthcare).
1. Preparing Samples
2. Setting up the Experiment
3. Analyzing the Data
4. Representative Results:
Figure 1. A representative example of a well-behaved titration for the binding of the cofactor NADPH to E. coli chromosomal dihydrofolate reductase (ecDHFR). Panel (A) shows the raw thermogram; (B), the binding isotherm from the integrated thermogram fit using the one-site model in the Origin software; and (C) the fit of the isotherm using the one binding site model from SEDPHAT along with fit residuals. From the Origin software, using the one site binding model, n = 1.09 ± 0.02, Kd = 0.194 ± 0.001 μM, ΔH = -22.7 ± 0.4 kcal/mol, TΔS = -13.1 ± 0.4 kcal/mol, and ΔG = -9.16 ± 0.01 kcal/mol. Fits of the data using Sedphat afford an n of 0.94 ± 0.01, a Kd of 0.195 ± 0.013 ΔM, a ΔH of -22.5 ± 0.2 kcal/mol, TΔS of -13.39 kcal / mol and ΔG of -9.15 kcal/mol.
ITC has been used extensively in studying ligand macromolecule interactions,11 with studies looking at protein-ligand,12 DNA-ligand13 and RNA-macromolecule14 studies. ITC can even be run with solid materials, such as nanoparticles, that form uniform suspensions.15 Further, ternary systems, where one ligand is already bound to the macromolecule and a second ligand is titrated, can be used to determine the thermodynamics of, for example, binding of substrate to an enzyme-cofactor system.7 Studies can also be performed for molecules with very high binding affinities that would normally exceed the detection limit of the ITC by performing competition binding assays with weaker binding ligands.16 Information on weakly binding ligands can also be obtained by competition assays.17 The role of water in binding can be explored by ITC,18 along with the dependence of the enthalpy on solvent reorganization.19 Recently, the thermodynamics of conformational change of DNAK variants were measured upon binding of ADP and ATP.20 Protein-protein association can be studied by ITC, yielding information on hetero complexes21 as well as on homo-association.22 Temperature effects on the binding enthalpy will give the heat capacity of the binding event.2 The number of protons absorbed or released upon binding can also be determined from experiments performed in buffers with different enthalpies of ionization.5 If additional ITC studies are done at different pH values, then the pKa of the group involved can potentially be determined. 23
To summarize, accurate measurements of the concentration of the macromolecule and ligand are imperative for a good ITC experiment. Sample concentrations also need to be within a proper range to get reliable data. Care should be taken with the buffer as small molecule impurities and pH mismatches will cause artifacts in the thermogram.
The authors have nothing to disclose.
This work was supported by NSF grant MCB-0817827.
Name of the reagent | Company | Catalogue number |
---|---|---|
1.7 mL microcentrifuge tubes | Fisher Scientific | 02-681-282 |
15 mL falcon tubes | Fisher Scientific | 14-959-49B |
2.5 mL Hamilton syringe | MicroCal | SYN161714 |