Thermodynamics of protein association reactions: forces contributing to stability.

Reviewing the thermodynamic parameters characterizing self-association and ligand binding of proteins at 25 OC, we find AGO, AHo, AS", and ACpo are often all of negative sign. It is thus not possible to account for the stability of association complexes of proteins on the basis of hydrophobic interactions alone. We present a conceptual model of protein association consisting of two steps: the mutual penetration of hydration layers causing disordering of the solvent followed by further short-range interactions. The net AGO for the complete association process is primarily determined by the positive entropy change accompanying the first step and the negative enthalpy change of the second step. On the basis of the thermochemical behavior of small molecule interactions, we conclude that the strengthening of hydrogen bonds in the I n the past decade, a complete thermodynamic description of the self-association of many proteins and their interactions From the Laboratory of Molecular Biology (P.D.R.) and Laboratory of Nutrition and Endocrinology (S.S.), National Institute of Arthritis, Metabolism and Digestive Diseases, National Institutes of Health, Bethesda, Maryland 20205. Received September 23, 1980. low dielectric macromolecular interior and van der Waals' interactions introduced as a consequence of the hydrophobic effect are the most important factors contributing to the observed negative values of AHo and ASo and hence to the stability of protein association complexes. The X-ray crystallographic structures of these complexes are consonant with this analysis. The tendency for protein association reactions to become entropy dominated and/or entropy-enthalpy assisted at low temperatures and enthalpy dominated at high temperatures (a consequence of the typically negative values of AC,") arises from the diminution of the hydrophobic effect with increasing temperature which is a general property of the solvent, water. with small molecular substrates has become available. Concomitantly, X-ray crystallography has provided a detailed picture of some of these associations, and this has stimulated a number of theoretical studies (Levitt & Warshel, 1975; Gelin & Karplus, 1975; Chothia & Janin, 1975), based upon energetic considerations, to account for these structures. The This article not subject to U S . Copyright. Published 1981 by the American Chemical Society T H E R M O D Y N A M I C S O F P R O T E I N A S S O C I A T I O N V O L . 2 0 , N O . 1 1 , 1 9 8 1 3097 Table I: Thermodynamics of Protein Association' association process A G " ~ AiY A s o , A c p o (kcal mol-') (kcal mol-l) (cal K-I mol-') (cal K-I mol-I) refb trypsin (bovine) + inhibitor (soybean) -14.6 8.6 78 -440 c, d deoxyhemoglobin S gelation -3.4 2.0 18 -200 e, f lysozyme self-association (indefinite) -3.9 -6 . 4 -8.3 g glucagon trimerization -12.1 -3 1 -64 -430 h, i hemoglobin t haptoglobin -11.5 -3 3 -7 3 -940 i a-chymotrypsin dimerization -7.1 -35 -9 5 k, I S-peptide + S-protein (ribonuclease) -13 -40 -90 -1100 m, n All thermodynamic parameters expressed per mole of complex formed except the indefinite association cases of hemoglobin S and lysozyme for which the mole refers to the monomeric protein reacted. Unitary entropy and free energy are given for processes of defined stoichiometry. Standard states are hypothetical 1 M protein, pH at which the reaction was measured. All pHs were close to 7 except for trypsin, pH 5, haptoglobin, pH 5.5, and glucagon, pH 10.5. All data for 25 "C except glucagon, T = 30 "C. ence is to calorimetric work and the second is to X-ray crystallographic structure determination. al. (1974). e Rosset al. (1977). Wishner e t al. (1975). g Banerjee et al. (1975). Johnson et al. (1979). * Sasaki et al. (1975). For each entry, the first referSweet et Baugh & Trowbridge (1972). Lavialle et al. (1974). Shiao & Sturtevant (1969). lVandlen &Tulinsky (1973). Hearn et al. (1971). Wyckoff e t al. (1970). methodology and problems involved in such calculations have been critically reviewed by NBmethy & Scheraga (1977). In this paper we review the thermodynamics of protein association processes for the examples best characterized in terms of their chemistry and structure. From this survey we find that the thermodynamic parameters AGO, Ai? , AS", and ACpo are predominantly of negative sign. This result poses severe difficulties for interpretations of protein association based upon the entropically driven hydrophobic effect. The aim of this paper is to attempt to account for the signs and magnitudes of these thermodynamic parameters for protein association reactions in terms of known molecular forces and the thermochemistry of small molecule interactions.