Elasticity of globular proteins. The relation between mechanics, thermodynamics and mobility.

An analysis of elasticity of lysozyme and myoglobin crystals in terms of thermodynamics has revealed a direct relation between entropy and enthalpy of deformation and delta S* and delta H* terms in the standard free energy change in proteins, delta G(o), (K.P. Murphy, P.L. Privalov, S.J. Gill (1990) Science 247, 559-561), so that at any temperature (between the glass-transition and denaturation temperatures) free energy of deformation is proportional to the hydration independent part of delta G(o). Both energies are characterized with large enthalpy-entropy compensation and tend to zero at the same temperature, Tm = (delta H*/delta S*) = 353 +/- 20 K. Large positive entropy contribution to deformation energy causes large linear decrease in protein elasticity, and increase in thermal mobility of protein atoms with temperature. Being plotted in inverse coordinates, temperature dependence of the mean-square amplitudes, obtained in neutron and mossbauer experiments as well as in molecular dynamic simulations, gives the same 353 +/- 10 K for the temperature, where the amplitudes tend to infinity. Mechanism explaining large positive entropy contribution in deformation energy of native protein molecules presumably involves emergence of more room for motion of protein side-chain groups squeezed between alpha-helices and other rigid skeleton elements, when precise packing of atoms in native protein molecule is distorted as a result of deformation.