Solvent dependence of dynamic transitions in protein solutions

Abstract

A transition as a function of increasing temperature from harmonic to anharmonic dynamics has been observed in globular proteins by using spectroscopic, scattering, and computer simulation techniques. We present here results of a dynamic neutron scattering analysis of the solvent dependence of the picosecond-time scale dynamic transition behavior of solutions of a simple single-subunit enzyme, xylanase. The protein is examined in powder form, in D 2O, and in four two-component perdeuterated single-phase cryosolvents in which it is active and stable. The scattering profiles of the mixed solvent systems in the absence of protein are also determined. The general features of the dynamic transition behavior of the protein solutions follow those of the solvents. The dynamic transition in all of the mixed cryosolvent-protein systems is much more gradual than in pure D 2O, consistent with a distribution of energy barriers. The differences between the dynamic behaviors of the various cryosolvent protein solutions themselves are remarkably small. The results are consistent with a picture in which the picosecond-time scale atomic dynamics respond strongly to melting of pure water solvent but are relatively invariant in cryosolvents of differing compositions and melting points. X-ray diffraction, dynamic neutron scattering, and various spectroscopies have demonstrated a quantitative change in the nature of internal motions of proteins, at Ϸ200-220 K (1-8). Below this transition, the internal motions are essentially harmonic whereas above it anharmonic dynamics contribute and, at physiological temperatures, dominate the internal fluctuations. The anharmonic motions may involve confined continuous diffusion (6, 9) and͞or jump diffusion between potential energy wells associated with ''conformational substates'' of slightly different structure in which proteins are trapped below the transition (2, 10-12).Correlations have been made between some protein functions (such as ligand binding, electron transfer, and proton pumping) and the presence of equilibrium anharmonic motion (8,(13)(14)(15)(16)(17). Recent x-ray diffraction work on carbon monoxy-myoglobin showed that, below 180 K, photodissociated ligands migrate to specific sites within an internal cavity of an essentially immobilized, frozen protein, from which they subsequently rebind by thermally activated barrier crossing (18). On photodissociation above 180 K, ligands escape from the distal pocket, aided by protein fluctuations that transiently open exit channels.The increased flexibility conferred by anharmonic dynamics may indeed be required for some proteins to rearrange their structures to achieve functional configurations. However, the forms and time scales of the anharmonic motions required for function are in general unknown. In particular, it was recently shown that, in cases of enzyme activity in a cryosolvent, the rate-limiting step is independent of the Ϸ220 K dynamic transition (19,20). Furthermore, it was demonstrated that dynamic transition temperatures in a ...

From shell to cell: neutron scattering studies of biological water dynamics and coupling to activity

“…The term 'slaving' has been used to express that water can impose its dynamical fingerprint on a protein 11,12 . Cryo-temperature dependent neutron scattering experiments revealed the solvent dependence of dynamical transitions in soluble proteins [13][14][15][16][17][18][19][20][21] and in RNA 22 and provided insights into the coupling between them. Molecular dynamics simulations suggested the onset of water translational diffusion to be at the origin of the dynamical transition 23,24 .…”

Temperature and timescale dependence of protein dynamics in methanol : water mixtures

“…31,32 Neutron scattering experiments have shown that dry protein samples exhibit little motion at 300 K over that expected from a harmonic solid. 33,34 The presence of highly-viscous solvents such as trehalose has been shown to abolish the dynamical transition. 35 In the present work we investigate the dependence of the dynamical transition on solvent composition.…”

The dynamic transition in proteins may have a simple explanation

“…The variation of dynamics with solvent conditions is therefore in itself a relevant area of study. 23 We focus here on the possible dependence of the dynamical transition on the timescale of the motions probed. For such measurements, neutron scattering is a particularly useful technique, as the timescale of the motions measured depends on the resolution of the instrument used.…”