Protein interaction studies using Isothermal titration calorimetry (ITC) Yilmaz Alguel.
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Transcript of Protein interaction studies using Isothermal titration calorimetry (ITC) Yilmaz Alguel.
Protein interaction studies using Isothermal titration
calorimetry (ITC)
Yilmaz Alguel
Why Microcalorimetry?
• Heat is generated or absorbed in every chemical process
• In-solution• Real-Time & Direct measurements • No molecular weight limitations• Label-free• No optical limitations
Calorimetry in the Life Sciences
Binding Studies • Quick and accurate affinities
• Mechanism of action and conformational changes
• Structure-function relationships
• Specific vs. non-specific binding
Kinetics • KM, Vmax, kcat Enzyme Inhibition
How Do They Work?
• Measuring Temperature Changes in Calorimetry
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Reference cell contains H2O(can be filled with Buffer as well)
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ITC: A Method for Characterizing Binding Interactions
• Mixture of two components at a set temperature
• Heat of interaction is measured
Parameters measured from a single ITC experiment:
• Affinities• Binding mechanism • Number of binding sites• Kinetics
Range of Binding: KA = 102 – 1010 M-1
Isothermal Titration Calorimetry
• Typical ITC Data
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Range of macromolecule concentration in the cell
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C-value (unitless constant) Ka binding (association) constantM tot the total macromolecule concentration in the celln stoichiometry parameter
c-value = Ka Mtot n
Working Range: c-value 5 to 500
Ligand conc. = 10 n Mtot
Enthalpic and Entropic Contributions to Binding Affinity
• Enthalpy and Entropy make up the affinity (G=-RTlnKa)
G = H - TSQuickTime™ and a
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Binding Mechanism
• Same affinity but different binding mechanisms and specificity
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Enthalpy and Entropy
• Entropy• Hydrophobic interactions
• Water release
• Ion release
• Conformational changes
• Enthalpy• Hydrogen bonding
• Protonation events
• More specific
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Energetic Signatures
A is enthalpy driven. • Strong H-bonding• V.d. Waals interactions • coupled to a conformational change
B is entropically driven • Hydrophobic • Interactions and possibly ‘rigid body’
C is mildly enthalpic and entropic• Small negative or positive enthalpy • (expulsion of structured H2O molecules from the binding site)• (Releasing H2O would increase entropy)
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Drug Discovery –Binding of Inhibitors to HIV-1 Protease
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Ohtaka, et al. Protein Sci. 11, 1908-1916 (2002)
ITC –Protein-Protein Interaction
• A: Wild-type cytochrome c titrated into wild-type cytochrome c peroxidase
• B: Mutant cytochrome c titrated into mutant cytochrome c peroxidaseQuickTime™ and a
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Pielak and Wang, Biochemistry 40, 422-428 (2001)
Schematic representation of the regulatory and the induction mechanism of TtgR
TtgR blocks the access of the RNA-POL by binding to the overlapping ttgR-ttgABC operator region
Binding of a ligand to TtgR induces a
conformational change and releases it from the DNA
RNA-POL is able to bind the ttgR-ttgABC operator and transcribe the efflux pump genes ttgABC and the
ttgR repressor gene encoded divergently
TtgR-binding antibiotics and plant antimicrobials
Chloramphenicol Tetracycline
NaringeninQuercetin Phloretin
At least one aromatic ring is the common feature of
the ligands
TtgR in complex with Naringenin and Chloramphenicol
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3.2 l aliquots of 1mM naringenin into native 50M monomer TtgR (I) and into 50M monomer mutant TtgRV66A/L96A
Effector Protein KA
(M -1)
KD
(µM)
∆G
(kcal/mol)
∆H
(kcal/mol)
T∆S
(kcal/mol)
Naringenin TtgR (5.5 ± 0.1) x 104 18 ± 0.3 -6.6 ± 0.1 -10.8 ± 0.1 -4.2 ± 0.1
TtgRL66A/V96A (2.0 ±0.1) x 104 49 ± 2 -6.0 ±0.2 -5.3 ± 0.2 0.7 ± 0.2
Chloramphenicol TtgR (3.1 ±0.3) x 104 32 ± 3 -6.2 ± 0.9 -7.0 ± 1.0 -0.8 ± 0.1TtgRL66A/V96A (9.1±1.1) x 103 109 ± 13 -5.5 ±1.5 -3.5 ± 0.9 2.0 ± 0.9
Titration of TtgR with Phloretin & the role of residue R176
Buffer:25mM PIPES, 250mM NaCl, 5% Glycerol, 10mM MgAc, 10mM KCl
The role of R176 in ligand recognition of TtgR
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Titration of TtgRR176G with phloretin
P. putida DOT-T1E …AAVAMFAYVDGLIRRWLL… 180P. putida KT2440 …AAVAMFAYVDGLIGRWLL… 180
Crystal structure of TtgR and mutant TtgRR176G with phloretin
Two binding sites exhibit positive cooperativity
Effector Protein KA
(M -1 )
KD
(µM)
∆G
(kcal/mol)
∆H
(kcal/mol)
T∆S
(kcal/mol)
Phloretin TtgRa (2.1 ±0.4) x 107
(4.6 ±1.1) x 105
0.05 ± 0.01
2.2 ± 0.5
-10.2 ± 0.1
-7.8 ± 0.2
-15.1 ± 0.1
-1.8 ± 0.2
-4.9 ± 0.2
6.0 ± 0.2
TtgRR176G (1.09 ± 0.06) x 105 9.2 ± 0.5 -7.0 ± 0.1 -21.6 ± 0.5 -14.6 ± 0.2
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Titration of three DNA double-strand oligomers of the wild-type operator with TtgR
(A) Injection of 6 μl aliquots of 40-wt (18.8 μM) into 8.1 μM TtgR (dimer) (B) Injection of 8 μl aliquots of 30-wt (26.5 μM) into 10.2 μM TtgR(C) Injection of 16-μl aliquots of 14 μM 28-wt into 6.1 μM TtgR
40bp-wt: kD = 1.57( ± 0.04) μMΔH = 6.33( ± 0.03) kcal/mol
30bp-wt: kD = 1.23( ± 0.05) μM ΔH = 5.95( ± 0.04) kcal/mol
28bp-wt: no binding
Practical considerations• Typical (macromolecule) concentrations down to 10 micromolar(e.g. 0.25 mg/ml for
a 25kDa protein) in the reaction cell (1.5ml volume), with 15-20x higher concentrations of titrant(ligand) in the injection syringe (min. 300 microlitrerequired).
• At the end of the titration, typically 250ul of ligand will have been added to 1.5ml of macromolecule.
• Both macromolecule and ligand must be in identical buffer/solvent otherwise large heats of dilution will mask the desired observation.
• Dialysis of the macromolecule against appropriate buffer, using the final dialysis buffer to make up the ligand solution.
• Truly quantitative data can only be obtained if molarcon centrations of proteins/macromolecules and ligands are known accurately. This can usually be done UV/visabsorbance measurements, provided molar extinction coefficients are available.
Microcalorimetry Summary
• Affinities and Binding
• Energetic profile of reaction
• Mechanisms of Binding Stoichiometry
• Enzyme Kinetics