Molecular origin and hydration dependence of protein anharmonicity: an elastic neutron scattering study

Abstract

Two main onsets of anharmonicity are present in protein dynamics. Neutron scattering on protein hydrated powders revealed a first onset at about 150 K and a second one at about 230 K (the so called dynamical transition). In order to assess the molecular origin of protein anharmonicity, we study different homomeric polypeptides by incoherent elastic neutron scattering, thus disentangling the contribution of different molecular groups in proteins. We show that methyl group rotations are the main contributors to the low temperature onset. Concerning the dynamical transition, we show that it also occurs in absence of side chains; however, the presence and mobility of side chains substantially increases the fluctuations amplitude without influencing the transition temperature. We also investigate the role of hydration on the anharmonic contributions. Our study shows that methyl group rotations are unaffected by hydration and confirms that the dynamical transition is suppressed in dry samples. In hydrated samples, while the pure backbone contribution does not depend on the hydration h at h > or = 0.2, in the presence of side chains the anharmonic fluctuations involved in the dynamical transition are enhanced by increasing the water content.

Physical Origin of Anharmonic Dynamics in Proteins: New Insights From Resolution-Dependent Neutron Scattering on Homomeric Polypeptides

“…These models propose physical pictures partially alternative, demonstrating that the question has not been definitively settled yet: (i) and (ii) ascribe the PDT to a temperature dependent relaxation time crossing the instrumental time-scale; (iii) and (iv) interpret the PDT as a change in thermodynamics and structure of the proteinwater system. All these hypotheses share the relevant role attributed to the hydration water dynamics: in fact, there is clear experimental evidence that the PDT occurs only in the presence of a sufficient amount of hydration water [1,4]. A key issue is whether and how the onset temperature and amplitude of the detected anharmonic MSDs depend on the time scale probed by neutron spectrometers, i.e., on their energy resolution.…”

“…Molecular origin, physical nature and biological relevance of these ''transitions'' are still matter of discussion. The first one is attributed mainly to thermally activated motions of CH 3 methyl groups [1,[3][4][5][6] (for this reason hereon called methyl groups activation, MGA). The second one, called ''protein dynamical transition'' (PDT), has been first interpreted as a glasslike transition [2] directly correlated to the onset of biological activity [7], but this view has been later challenged.…”

Protein dynamical transition at 110 K

“…It has been reported that some proteins exhibit a rapid increase in the mean square atomic displacement below 150 K, but it was attributed to the onset of local structural motions (such as rotational motions of methyl group) rather than the global protein molecular motions (14,16,28,39). Fig.…”

“…Proteins below the transition temperature are in a glassy state with little conformational flexibility and no appreciable biological function; above the transition temperature flexibility is restored and the protein becomes biologically active (2-4). This protein dynamical transition has been extensively studied by measuring the mean square atomic displacement, hx 2 i, of protein atoms as a function of temperature with various methods, including Mössbauer and terahertz time domain spectroscopies (5-7), X-ray crystallography (4, 8-11), neutron scattering (2,(12)(13)(14)(15)(16)(17)(18)(19), and molecular dynamics simulations (20)(21)(22)(23)(24)(25).…”

“…Proteins below the transition temperature are in a glassy state with little conformational flexibility and no appreciable biological function; above the transition temperature flexibility is restored and the protein becomes biologically active (2-4). This protein dynamical transition has been extensively studied by measuring the mean square atomic displacement, hx 2 i, of protein atoms as a function of temperature with various methods, including Mössbauer and terahertz time domain spectroscopies (5-7), X-ray crystallography (4, 8-11), neutron scattering (2,(12)(13)(14)(15)(16)(17)(18)(19), and molecular dynamics simulations (20)(21)(22)(23)(24)(25).It is believed that the protein dynamical transition involves a strong coupling to the hydration water (10,26,27), as a protein dehydrated below a critical level (water/protein mass ratio of ∼0.2) does not show the dynamical transition (13,14,28). Several mechanisms have been proposed to explain the microscopic origin of protein dynamical transition, including α and β fluctuations in the bulk solvent and the hydration shell (29, 30), liquid-glass transition of hydration water (31), a frequency window scenario (32, 33), and a fragile to strong transition of the hydration water (15,20).…”