Protein activation entropy

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

From Table 8-1 it is evident that the protein provides a catalytic effect lowering the barriers by about 10 kJ/mol with respect to the unperturbed condition. Moreover, the free-energy barriers at the two temperatures are, within the noise, almost identical suggesting low activation entropies. 

In this complex, there are two optically active sites. Spinach plastocyanin is a type I copper protein, in which two reactive sites have been identified on its surface, at least. The electron transfer reaction occurs with significantly large stereoselectivity the ratio of the observed reaction rate constant (k /k ) is 1.6 to 2.0. The difference in the activation enthalpy, AAH a, is 3.0 kJ mol-1, and the difference in the activation entropy, AS (a-a) is 15 J mol-1 K-1. This means that the stereoselectivity arises from the entropy term. 

S.R. Tzeng, C.G. Kalodimos, Protein activity regidation by conformation entropy. Nature 488 (2012) 236-240. 

The activation entropy of the denaturation process is lowered by the isotopic substitution of the solvent (AS (D20) < AS (H20)). It is difficult to see how this could be done other than through solvent effects, since the protein configurational entropy can hardly be affected by replacement of hydrogen by deuterium inside the macromolecule. 

Tzeng SR, Kalodimos CG (2012) Protein activity tegulation by ctnttinmational entropy. Nature 488 236... 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

As already mentioned, a continual inflow of energy is necessary to maintain the stationary state of a living system. It is mostly chemical energy which is injected into the system, for example by activated amino acids in protein biosynthesis (see Sect. 5.3) or by nucleoside triphosphates in nucleic acid synthesis. Energy flow is always accompanied by entropy production (dS/dt), which is composed of two contributions ... 

---