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A Thermodynamic Molecular Switch in Biological Systems: Ribonuclease S؅ Fragment Complementation Reactions

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A Thermodynamic Molecular Switch in Biological Systems: Ribonuclease S؅ Fragment Complementation Reactions View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector 416 Biophysical Journal Volume 78 January 2000 416–429 A Thermodynamic Molecular Switch in Biological Systems: Ribonuclease S؅ Fragment Complementation Reactions Paul W. Chun Department of Biochemistry and Molecular Biology, University of Florida College of Medicine, Gainesville, Florida 32610-0245 USA ABSTRACT It is well known that essentially all biological systems function over a very narrow temperature range. Most typical macromolecular interactions show ⌬H°(T) positive (unfavorable) and a positive ⌬S°(T) (favorable) at low temperature, because of a positive (⌬Cp°/T). Because ⌬G°(T) for biological systems shows a complicated behavior, wherein ⌬G°(T) changes from positive to negative, then reaches a negative value of maximum magnitude (favorable), and finally becomes positive as temperature increases, it is clear that a deeper-lying thermodynamic explanation is required. This communication demonstrates that the critical factor is a temperature-dependent ⌬Cp°(T) (heat capacity change) of reaction that is positive at low temperature but switches to a negative value at a temperature well below the ambient range. Thus the thermodynamic molecular switch determines the behavior patterns of the Gibbs free energy change and hence a change in the equilibrium ⌬ ⌬ ⌬ ⌬ constant, Keq, and/or spontaneity. The subsequent, mathematically predictable changes in H°(T), S°(T), W°(T), and G°(T) give rise to the classically observed behavior patterns in biological reactivity, as may be seen in ribonuclease SЈ fragment complementation reactions. INTRODUCTION Shortly after T. H. Benzinger published a paper in 1971 It has been well established in pure and applied chemistry titled “Thermodynamics, Chemical Reactions and Molecu- of simple molecules that reaction energies at room temper- lar Biology,” the question arose whether thermochemical ature can be understood in terms of two contributions, one descriptions of biochemical systems in the literature were related to energy differences at 0 K (the innate inherent inadequate or wrong or both. bond energy) and the other associated with integrals of heat After years of detailed study on interacting protein sys- capacity data over a range of temperatures (the thermal tems and their thermodynamic parameters, it is possible to agitation energy). The necessity of a comparable separation formulate a viewpoint of thermodynamic reactions that is of energy terms for biological reactions, however, has not both adequate and consistent when applied to biological been obvious to most workers in structural and molecular systems, and which considers the innate thermodynamic biology. The innate thermodynamic quantities (Gibbs, quantities in structural biology. 1878; Planck, 1927; Moelwyn-Hughes, 1957; Lewis and As made apparent in Benzinger’s pioneering studies, in Randall, 1961), particularly the innate temperature-invariant any chemical reaction involving small molecules, differ- enthalpy, represent differences in chemical bonding energy ences in heat capacities between products and reactants of products minus reactants and thus control the heat of could be considered negligible, and consequently, the heat reaction. of reaction consists primarily of the contribution from the In biological systems, which function over a very narrow chemical bonding term and, therefore, is a close approxi- temperature range, the standard Gibbs free energy change, mation of the chemical bond energy. Thus many scientists ⌬GЊ are comfortable thinking about reaction enthalpy in terms of , shows a complicated behavior, changing from positive “bonds formed minus bonds broken,” that is, based upon the to negative, then reaching a negative (and thus favorable) difference in bond energies between products and reactants. minimum, before finally becoming positive as temperature In the case of biological interactions, Benzinger observed increases. that the difference between the heat capacities of products It is reasonable to search for an underlying thermody- and reactants may be substantial enough to totally obscure namic explanation for the greater complexity known to any difference between the chemical bonding term and the occur in typical biological systems. This communication heat capacity integral. Thus the heat of reaction as tradi- proposes that the critical factor driving this process is a tionally defined cannot be used to accurately represent the temperature-dependent heat capacity change of reaction, chemical forces associated with these systems. which is positive at low temperature but switches to a negative value at a temperature well below the ambient range. This thermodynamic molecular switch determines Received for publication 15 July 1999 and in final form 27 August 1999. the behavior patterns of the Gibbs free energy change, and Address reprint requests to Dr. Paul W. Chun, Department of Biochemistry hence a change in the equilibrium constant, K , and/or and Molecular Biology, University of Florida College of Medicine, P.O. eq Box 100245, Gainesville, FL 32610-0245. Tel.: 352-392-3356; Fax: 352- spontaneity. The subsequent, mathematically predictable 392-2953; E-mail: [email protected]. changes in ⌬HЊ, ⌬SЊ, ⌬WЊ, and ⌬GЊ that arise as a result of © 2000 by the Biophysical Society this thermodynamic molecular switch are demonstrated in 0006-3495/00/01/416/14 $2.00 RNase SЈ fragment complementation reactions. Thermodynamic Molecular Switch in RNase SЈ 417 THE INNATE THERMODYNAMIC QUANTITIES practical reasons, for instance, the relationship between ⌬ H°reaction and the temperature coefficient of the equilibrium The Gibbs free energy, or more correctly, the change in ϭϪ⌬ Њ constant, Keq, dln Keq/d(1/T) H (T)/R. Kirchhoff’s partial molar free energy for the components in a reaction law (Moelwyn-Hughes, 1957; Lewis and Randall, 1961) mixture, undoubtedly does provide the correct thermody- ⌬ Њ ϭ⌬ Њ ϩ͐298 ⌬ states that H298 H (T0) 0 Cp dT, where this last namic criterion of equilibrium. Furthermore, it is often term represents the thermal agitation energy (heat capacity desirable to relate the drive toward equilibrium to a struc- ⌬ Њ integrals), while the constant term H (T0) represents the tural aspect of a system, that is, to understand the underlying enthalpy of reaction at 0 K. For small molecules, reaction ⌬GЊ factors that contribute to the change in as a function of enthalpies are often obtained around room temperature, and temperature for a specific reaction. the heat of reaction is estimated in terms of the innate ⌬GЊ Certain components of the total change in may be temperature-invariant enthalpy (inherent chemical bond en- described as “innate,” that is, the quantity ⌬GЊ has an ⌬ Њ ergy), H (T0). (The innate temperature-invariant enthalpy, extrapolated value even at absolute zero temperature. There ⌬ ⌬ H(T0) (or in some texts, H0), is an equivalent terminol- are also components that change with temperature. We note, ogy for this quantity in the inherent 0 K enthalpy (innate T ϭ T⌬SЊϭ for example, that at 0K, 0. Accordingly, thermodynamic quantity). It represents the chemical bond ⌬GЊϭ⌬HЊϪT⌬SЊ because (Gibbs, 1978; Planck, 1927; energy difference corrected to 0 K.) Moelwyn-Hughes, 1957; Lewis and Randall, 1961), it is T. L. Cottrell (1958) pointed out 40 years ago that ⌬HЊ ⌬HЊ ϭ⌬GЊ ⌬HЊ T ϭ 298 found that at absolute zero K, 0 0 ( ( 0) and ⌬HЊ(T ) differ only by ϳ1% in small molecules, but in ⌬GЊ T 0 ( 0) in some texts). The residual values of these quan- 1971 T. H. Benzinger made the crucial observation that this tities (note that entropy is excluded) are the same at absolute difference is large in biological macromolecules because of zero of the temperature scale and may be said to describe the large magnitude of the heat capacity integrals (thermal the innate thermodynamic stability of the system. agitation energy). In other words, for small molecules, For a chemical reaction, the difference in these quantities ⌬HЊ Ϫ⌬HЊ(T ) is a correction of only a few percent, ⌬HЊ ϭ⌬GЊ 298 0 between reactants and products, that is, 0 0, rep- whereas for biological macromolecules, the heat capacity resents the differences in the innate thermodynamic stability integrals can be large, from 10% to 90% of the total heat of between reactants and products, or the change in stability reaction. At present the scientific literature provides no between reactants and products as the reaction occurs. “silver bullet,” that is, no highly accurate method for eval- ⌬ Њ Ϫ⌬ Њ uating H298 H (T0) in large biological macromole- THE GIAUQUE FUNCTION AND PLANCK- cules; however, Chun’s work (1988, 1994, 1995, 1996b, BENZINGER THERMAL WORK FUNCTION 1997a, 1998) has extensively addressed the problem. One form of the free energy function, the Giauque function (Giauque, 1930a,b; Giauque and Blue, 1930; Giauque and METHODS AND COMPUTATIONAL ЊϪ Њ ϭ Kemp, 1938; Giauque and Meads, 1941), (G0 H0 )/T PROCEDURES Ϫ⌿Њ ⌿Њ ϭ Ϫ Њ /T, where G°T H0, has been extensively used in chemistry and physics. An equivalent formulation has To analyze the thermodynamic processes operating in a recently found application in the biochemical literature as particular biological system, it is necessary to extrapolate the Planck-Benzinger thermal work function (Chun, 1988, the thermodynamic parameters over a much broader tem- 1994, 1996b, 1997a, 1998), where the application to a given perature range. The enthalpy, entropy, and heat capacity situation is quite different. Here the Planck-Benzinger ther- terms are evaluated as partial derivatives of the Gibbs free ⌬ Њ ϭ⌬ Њ Ϫ⌬ Њ energy function defined by Helmholtz-Kelvin’s expression mal work function, W (T) H (T0) G (T), repre- sents the strictly thermal components of any intra- or inter- (Moelywn-Hughes, 1957; Lewis and Randall, 1961): molecular bonding term, that is, energy other than the Ѩ⌬G͑T͒/ѨT ϭ Ϫ⌬S͑T͒, ͕Ѩ⌬G͑T͒/T͖/ѨT ϭ Ϫ⌬H͑T͒/T2 inherent difference in the 0 K portion of the interaction energy. The latter is the only energy term in constant Ѩ⌬H͑T͒/ѨT ϭ ⌬Cp͑T͒, Ѩ⌬S͑T͒/ѨT ϭ ⌬Cp͑T͒/T pressure processes at absolute zero Kelvin.
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