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Isothermal Titration Calorimetry and Differential Scanning Calorimetry As Complementary Tools to Investigate the Energetics of Biomolecular Recognition

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Isothermal Titration Calorimetry and Differential Scanning Calorimetry As Complementary Tools to Investigate the Energetics of Biomolecular Recognition JOURNAL OF MOLECULAR RECOGNITION J. Mol. Recognit. 1999;12:3–18 Review Isothermal titration calorimetry and differential scanning calorimetry as complementary tools to investigate the energetics of biomolecular recognition Ilian Jelesarov* and Hans Rudolf Bosshard Department of Biochemistry, University of Zurich, CH-8057 Zurich, Switzerland The principles of isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) are reviewed together with the basic thermodynamic formalism on which the two techniques are based. Although ITC is particularly suitable to follow the energetics of an association reaction between biomolecules, the combination of ITC and DSC provides a more comprehensive description of the thermodynamics of an associating system. The reason is that the parameters DG, DH, DS, and DCp obtained from ITC are global properties of the system under study. They may be composed to varying degrees of contributions from the binding reaction proper, from conformational changes of the component molecules during association, and from changes in molecule/solvent interactions and in the state of protonation. Copyright # 1999 John Wiley & Sons, Ltd. Keywords: isothermal titration calorimetry; differential scanning calorimetry Received 1 June 1998; accepted 15 June 1998 Introduction ways to rationalize structure in terms of energetics, a task that still is enormously difficult inspite of some very Specific binding is fundamental to the molecular organiza- promising theoretical developments and of the steady tion of living matter. Virtually all biological phenomena accumulation of experimental results. Theoretical concepts depend in one way or another on molecular recognition, have developed in the tradition of physical-organic which either is intermolecular as in ligand binding to a chemistry. It is difficult to adapt these concepts in a macromolecule and in the formation of macromolecular straightforward way to complex biomacromolecules. Pro- complexes, or intramolecular as in protein folding. Here we teins and nucleic acids are so large that in solution they may deal with intermolecular recognition, that is, with binding be regarded as individual macroscopic systems surrounded reactions. The specificity and precision of binding reactions by solvent (Privalov and Potekhin, 1986). They behave has fascinated biologists and chemists from the very cooperatively and often undergo structural rearrangements beginning of modern biochemistry, and one of the most during binding reactions. The changes range from the subtle rapidly advancing fields today is the study of molecular adjustments of dihedral angles to the prominent rearrange- recognition between macromolecules. The quantitative ment and even refolding of entire molecular domains description of the forces that govern the formation of (Padlan, 1996). For example, in a DNA binding protein the biomolecular complexes is part of this endeavor. It has great domain that binds to the DNA target site can be unstructured practical significance to the knowledge-based development in the free protein and becomes folded only when it is bound of new drugs, vaccines and other medicinal compounds. to DNA (Patikoglou and Burley, 1997). The energetic The number of high resolution crystal structures of consequence of such binding-induced refolding is far from biomolecular complexes is growing fast and there is a large being understood, but the energetic cost is thought to be data base describing the complementarity of interacting large. From this perspective, binding specificity has to be surfaces and the precise orientation of interacting groups. redefined. It is no longer a simple function of spatial From the many detailed, yet static, pictures of biomolecular complementarity and of accumulation of favorable interac- complexes we have learned how molecules interact, but we tions between molecules that can be treated as rigid bodies. less well know why they do so. This means we have to find Binding between large and flexible biomolecules has a complicated energy profile involving different energetic expenditures in going from the free components to the final complex. The free energy of complex formation turns out to * Correspondence to: I. Jelesarov, Department of Biochemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. be a very small number resulting from a delicate balance Abbreviations used: DSC, differential scanning calorimetry; ITC, isothermal between large favorable and large unfavorable contribu- titration calorimetry. tions. Moreover, binding takes place only if the total free Copyright # 1999 John Wiley & Sons, Ltd. CCC 0952–3499/99/010003–16 $17.50 4 I JELESAROV AND H BOSSHARD energy change decreases, regardless of the actual amount of can be expressed as (mol fraction)1, where 1 mol l1 favorable free energy change accumulated by direct equals 1.8 Â 102 mol fraction (corresponding to 1/55.56 molecular contact between the associating biomolecules. mol H2O per 1). The choice of the standard state is a matter This is to say, solvent effects are as important to the energy of convention yet is important when comparing values of balance as are direct noncovalent interactions (Collins, DG and DS. 1997; Jelesarov et al., 1998). There are many ways to measure KA. Let us begin with the simple equilibrium between L, M and ML: K!A L M ML Basic Thermodynamic Relationship Describing Binding Phenomena Techniques like equilibrium dialysis, ultracentrifugation, or radio-ligand binding assay directly yield values for [ML], The Gibbs free energy change, DG, of an association [M], or [L] to calculate KA. More indirect and mostly reaction is temperature dependent and is described by spectroscopic methods yield an observable p the change of Z T which is proportional to the degree of saturation, Y, ÁG T ÁH TR ÁCpdT TÁS TR according to: TZR 1 Áp ML K L T i A Y Á 4 T ÁCpd ln T pmax M ML 1 KA L TR Dpi is a signal change (e.g. a change of fluorescence DH and DS are the change in enthalpy and entropy, accompanying the formation of the complex, ML), and D respectively, Cp is the heat capacity change, and TR is an Dp is the maximum signal change at full saturation D max appropriate reference temperature. If Cp is temperature- (Y = 1). From Y measured at different temperatures, independent in the temperature interval of interest, Eq. (1) KA(T) and DG(T) are obtained. DH(T) and DS(T) can be simplifies to derived using the integrated form of the van’t Hoff equation: ÁG T ÁH TR TÁS TR 2 Z Z Á = T2 T2 ÁH Cp T TR T ln T TR d ln K dT 5a A RT 2 Equations (1) and (2) show that the free energy of T1 T1 association has an enthalpy and an entropy component. The and Z Z changes of enthalpy and entropy depend on temperature T2 T2 according to ÁG ÁSdT 5b d ÁH d ÁS T1 T1 ÁCp T 3 D dT dT Further, taking the second derivative of G(T) with respect to temperature, one obtains DCp. The non-calori- To characterize the thermodynamics of a binding reaction metric approach to the thermodynamics of an association means to determine DG, DH and DS at a given reference D reaction, commonly called a van’t Hoff analysis, has severe temperature and to obtain Cp to predict the change of the drawbacks. First, for technical reasons, experiments can be above three parameters with temperature. performed only in a limited temperature range, and experimental errors propagate into large errors of DH, DS, and of DCp in particular. Second, KA very often appears Non-calorimetric determination of binding energetics temperature-independent in the experimentally accessible D T-range because of enthalpy/entropy compensation between G is accessible through many types of binding experi- large and strongly temperature-dependent DH and DS. ments because DG is related to the association constant KA D Therefore, the van’t Hoff analysis of a binding reaction is by G = -RT ln KA where R is the gas constant (8.314 J often flawed. K1mol1) and T is the absolute temperature in degrees Kelvin. In the simplest case a molecule L reacts with another molecule M to the complex ML.1 The association constant is KA = [ML]/[M][L] where the brackets indicate concentration 1 1 expressed in units of mol l and KA has units of l mol . To Isothermal Titration Calorimetry take the logarithm, KA must be a dimensionless ratio. This is achieved by normalizing the molar concentrations to the ITC and DSC are the only methods for the direct 1 D standard state concentration of 1 mol l . Alternatively, KA determination of H. The principle of ITC is now reviewed first. It is the most direct method to measure the heat change on formation of a complex at constant temperature. Since a 1 titration experiment is performed in which L is titrated into a In this article the components of a complex are designated L and M, solution of M (or M into a solution of L), K and the irrespective of their size and chemical nature. Thus, L and M can be proteins, A nucleic acids, low molecular weight metabolites, salt ions, and so on. L often stoichiometry, n, of the complex are also obtained by ITC. is named the ligand and M the receptor. However, this assignment is Moreover, ITC experiments performed at different tem- arbitrary. peratures yield DCp defined by Eq. (3). Copyright # 1999 John Wiley & Sons, Ltd. J. Mol. Recognit. 1999;12:3–18 ITC AND DSC—REVIEW 5 computer-controlled stepper motor. The contents of the sample cell are stirred to effect rapid mixing of the reactants. In commercially available instruments volumes of sample cells are in the range 0.2–1.4 ml. The amount of titrand required per experiment depends on the magnitude of the heat change; 10–100 nmol of protein are typical. Figure 1A shows the raw data of an ITC experiment.
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