Methyl group dynamics as a probe of the protein dynamical transition

An experimentally representative in-silico protocol for dynamical studies of lyophilised and weakly hydrated amorphous proteins

Characterization of biopolymers in both dry and weakly hydrated amorphous states has implications for the pharmaceutical industry since it provides understanding of the effect of lyophilisation on stability and biological activity. Atomistic Molecular Dynamics (MD) simulations probe structural and dynamical features related to system functionality. However, while simulations in homogenous aqueous environments are routine, dehydrated model assemblies are a challenge with systems investigated in-silico needing careful consideration; simulated systems potentially differing markedly despite seemingly negligible changes in procedure. Here we propose an in-silico protocol to model proteins in lyophilised and weakly hydrated amorphous states that is both more experimentally representative and routinely applicable. Since the outputs from MD align directly with those accessed by neutron scattering, the efficacy of the simulation protocol proposed is shown by validating against experimental neutron data for apoferritin and insulin. This work also highlights that without cooperative experimental and simulative data, development of simulative procedures using MD alone would prove most challenging.

Energy condensation and dipole alignment in protein dynamics

The possibility that distant biomolecules in a cell interact via electromagnetic (e.m.) radiation was proposed many years ago to explain the high rate of encounters of partners in some enzymatic reactions. The results of two recent experiments designed to test the propensity of protein bovine serum albumin (BSA) to interact via e.m. radiation with other proteins were interpreted in a theoretical framework based on three main assumptions: (i) in order to experience this kind of interaction the protein must be in an out-of-equilibrium state; (ii) in this state there is a condensation of energy in low-frequency vibrational modes; and (iii) the hydration layers of water around the protein sustain the energy condensation. In the present paper we present the results of molecular dynamics simulations of BSA in four states: at equilibrium and out-of-equilibrium in water, and at room and high temperature in vacuum. By comparing physical properties of the system in the four states, our simulations provide a qualitative and quantitative assessment of the three assumptions on which the theoretical framework is based. Our results confirm the assumptions of the theoretical model showing energy condensation at low frequency and electretlike alignment between the protein's and the water's dipoles; they also allow a quantitative estimate of the contribution of the out-of-equilibrium state and of the water to the observed behavior of the protein. In particular, it has been found that in the out-of-equilibrium state the amplitude of the oscillation of the protein's dipole moment greatly increases, thereby enhancing a possible absorption or emission of e.m. radiation. The analysis of BSA's dynamics outlined in the present paper provides a procedure for checking the propensity of a biomolecule to interact via e.m. radiation with its biochemical partners.

Modeling of protein hydration dynamics is supported by THz spectroscopy of highly diluted solutions

In this investigation, we report the effect on the microscopic dynamics and interactions of the cytokine interferon gamma (IFN-γ) and antibodies to IFN-γ (anti-IFN-γ) and to the interferon gamma receptor 1 (anti-IFNGR1) prepared in highly dilute (HD) solutions of initial proteins. THz spectroscopy measurements have been conducted as a means to analyze and characterize the collective dynamics of the HD samples. MD simulations have also been performed that have successfully reproduced the observed signatures from experimental measurement. Using this joint experimental-computational approach we determine that the HD process associated with the preparation of the highly diluted samples used in this investigation induces a dynamical transition that results in collective changes in the hydrogen-bond network of the solvent. The dynamical transition in the solvent is triggered by changes in the mobility and hydrogen-bonding interactions of the surface molecules in the HD samples and is characterized by dynamical heterogeneity. We have uncovered that the reorganization of the sample surface residue dynamics at the solvent-protein interface leads to both structural and kinetic heterogeneous dynamics that ultimately create interactions that enhance the binding probability of the antigen binding site. Our results indicate that the modified interfacial dynamics of anti-IFN-γ and anti-IFGNR1 that we probe experimentally are directly associated with alterations in the complementarity regions of the distinct antibodies that designate both antigen-antibody affinity and recognition.

New insights into the microscopic interactions associated with the physical mechanism of action of highly diluted biologics

In this investigation, we report the effect on the microscopic dynamics and interactions of the cytokine interferon gamma (IFN-γ) and antibodies to IFN-γ (anti-IFN-γ) and to the interferon gamma receptor 1 (anti-IFNGR1) prepared in exceptionally dilute solutions of initial proteins. Using both THz spectroscopy and molecular dynamics simulations we have uncovered that the high dilution method of sample preparation results in the reorganization of the sample surface residue dynamics at the solvent–protein interface that leads to both structural and kinetic heterogeneous dynamics that ultimately create interactions that enhance the binding probability of the antigen binding site. Our results indicate that the modified interfacial dynamics of anti-IFN-γ and anti-IFGNR1 that we probe experimentally are directly associated with alterations in the complementarity regions of the distinct antibodies that designate both antigen–antibody affinity and recognition.

NMR-Based Methods for Protein Analysis

Nuclear magnetic resonance (NMR) spectroscopy is a well-established method for analyzing protein structure, interaction, and dynamics at atomic resolution and in various sample states including solution state, solid state, and membranous environment. Thanks to rapid NMR methodology development, the past decade has witnessed a growing number of protein NMR studies in complex systems ranging from membrane mimetics to living cells, which pushes the research frontier further toward physiological environments and offers unique insights in elucidating protein functional mechanisms. In particular, in-cell NMR has become a method of choice for bridging the huge gap between structural biology and cell biology. Herein, we review the recent developments and applications of NMR methods for protein analysis in close-to-physiological environments, with special emphasis on in-cell protein structural determination and the analysis of protein dynamics, both difficult to be accessed by traditional methods.

Molecular Dynamics Simulations of a Powder Model of the Intrinsically Disordered Protein Tau

The tau protein, whose aggregates are involved in Alzheimer's disease, is an intrinsically disordered protein (IDP) that regulates microtubule activity in neurons. An IDP lacks a single, well-defined structure and, rather, constantly exchanges among multiple conformations. In order to study IDP dynamics, the combination of experimental techniques, such as neutron scattering, and computational techniques, such as molecular dynamics (MD) simulations, is a powerful approach. Amorphous hydrated powder samples have been very useful for studying protein internal dynamics experimentally, e.g., using neutron scattering. Thus, there is demand for realistic in silico models of hydrated protein powders. Here we present an MD simulation analysis of a powder hydrated at 0.4 g water/g protein of the IDP tau in the temperature range 20-300 K. By comparing with neutron scattering data, we identify the protein-water interface as the predominant feature determining IDP dynamics. The so-called protein dynamical transition is shown to be attenuated, but not suppressed, in the parts of the protein that are not exposed to the solvent. In addition, we find similarities in the mean-squared displacements of the core of a globular protein and "dry" clusters formed by the IDP in hydrated powders. Thus, the ps to ns dynamics of proteins in hydrated powders originate mainly from those residues in contact with solvent. We propose that by measuring the dynamics of protein assemblies, such as aggregates, one might assess qualitatively their state of hydration.

Slowdown of Interhelical Motions Induces a Glass Transition in RNA

RNA function depends crucially on the details of its dynamics. The simplest RNA dynamical unit is a two-way interhelical junction. Here, for such a unit--the transactivation response RNA element--we present evidence from molecular dynamics simulations, supported by nuclear magnetic resonance relaxation experiments, for a dynamical transition near 230 K. This glass transition arises from the freezing out of collective interhelical motional modes. The motions, resolved with site-specificity, are dynamically heterogeneous and exhibit non-Arrhenius relaxation. The microscopic origin of the glass transition is a low-dimensional, slow manifold consisting largely of the Euler angles describing interhelical reorientation. Principal component analysis over a range of temperatures covering the glass transition shows that the abrupt slowdown of motion finds its explanation in a localization transition that traps probability density into several disconnected conformational pools over the low-dimensional energy landscape. Upon temperature increase, the probability density pools then flood a larger basin, akin to a lakes-to-sea transition. Simulations on transactivation response RNA are also used to backcalculate inelastic neutron scattering data that match previous inelastic neutron scattering measurements on larger and more complex RNA structures and which, upon normalization, give temperature-dependent fluctuation profiles that overlap onto a glass transition curve that is quasi-universal over a range of systems and techniques.

Painting biological low-frequency vibrational modes from small peptides to proteins

We use experiments and simulation to investigate the validity of different model systems used to study the low-frequency vibrations of proteins.

Role of water in protein folding, oligomerization, amyloidosis and miniprotein

The essential involvement of water in most fundamental extra-cellular and intracellular processes of proteins is critically reviewed and evaluated in this article. The role of water in protein behavior displays structural ambivalence; it can protect the disordered peptide-chain by hydration or helps the globular chain-folding, but promotes also the protein aggregation, as well (see: diseases). A variety of amyloid diseases begins as benign protein monomers but develops then into toxic amyloid aggregates of fibrils. Our incomplete knowledge of this process emphasizes the essential need to reveal the principles of governing this oligomerization. To understand the biophysical basis of the simpler in vitro amyloid formation may help to decipher also the in vivo way. Nevertheless, to ignore the central role of the water's effect among these events means to receive an uncompleted picture of the true phenomenon. Therefore this review represents a stopgap role, because the most published studies--with a few exceptions--have been neglected the crucial importance of water in the protein research. The following questions are discussed from the water's viewpoint: (i) interactions between water and proteins, (ii) protein hydration/dehydration, (iii) folding of proteins and miniproteins, (iv) peptide/protein oligomerization, and (v) amyloidosis.

On the Coupling between the Collective Dynamics of Proteins and Their Hydration Water

Picosecond time scale dynamics of hydrated proteins has been connected with the onset of biological activity as it coincides with solvent-solute hydrogen bond rearrangements and amino acid rotational relaxation time scales. The presence and fluctuations of protein hydration water (PHW) largely influence protein motions that are believed to be slaved to those of the solvent, yet to date, how protein and hydration water dynamics are coupled remains unclear. Here, we provide a significant advance in characterizing this coupling; we present the first full study of both the longitudinal and transverse coherent collective motions in a protein-solvent system. The data show unexpectedly the presence in the water dynamics of collective modes belonging to the protein. The properties of these modes, in particular, their propagation velocities and amplitudes, indicate a strengthening of the interactions and a higher rigidity of the network of solvent molecules close to the protein surface. Accordingly, the present study presents the most compelling and clear evidence of a very strong dynamical coupling between a protein and its hydration water, previously suggested by studies using various experimental techniques.

Three entropic classes of side chain in a globular protein

The relationship between the NMR methyl group axial order parameter and the side chain conformational entropy is investigated in inhibitor-bound and apo human HIV protease using molecular dynamics simulation. Three distinct entropic classes of methyl-bearing side chains, determined by the topological distance of the methyl group from the protein backbone (i.e., the number of χ-bonds between the Cα and the carbon of the CH3 group), are revealed by atomistic trajectory analyses performed in the local frame of reference of individual methyl probes. The results demonstrate that topologically equivalent methyl groups experience similar nonbonded microenvironments regardless of the type of residues to which they are attached. Similarly, methyl groups that belong to the same side chain but that are not topologically equivalent exhibit different thermodynamic and dynamic properties. The two-parameter classification (based upon entropy and methyl axial order parameter) of side chains described here permits improved estimates of the conformational entropies of proteins from NMR motional parameters.

Derivation of mean-square displacements for protein dynamics from elastic incoherent neutron scattering

The derivation of mean-square displacements from elastic incoherent neutron scattering (EINS) of proteins is examined, with the aid of experiments on camphor-bound cytochrome P450cam and complementary molecular dynamics simulations. It is shown that a q(4) correction to the elastic incoherent structure factor (where q is the scattering vector) can be simply used to reliably estimate from the experiment both the average mean-square atomic displacement, <Δr(2)> of the nonexchanged hydrogen atoms in the protein and its variance, σ(2). The molecular dynamics simulation results are in broad agreement with the experimentally derived <Δr(2)> and σ(2) derived from EINS on instruments at two different energy resolutions, corresponding to dynamics on the ∼100 ps and ∼1 ns time scales. Significant dynamical heterogeneity is found to arise from methyl-group rotations. The easy-to-apply q(4) correction extends the information extracted from elastic incoherent neutron scattering experiments and should be of wide applicability.

Functional domain motions in proteins on the ~1-100 ns timescale: comparison of neutron spin-echo spectroscopy of phosphoglycerate kinase with molecular-dynamics simulation

Protein function often requires large-scale domain motion. An exciting new development in the experimental characterization of domain motions in proteins is the application of neutron spin-echo spectroscopy (NSE). NSE directly probes coherent (i.e., pair correlated) scattering on the ~1-100 ns timescale. Here, we report on all-atom molecular-dynamics (MD) simulation of a protein, phosphoglycerate kinase, from which we calculate small-angle neutron scattering (SANS) and NSE scattering properties. The simulation-derived and experimental-solution SANS results are in excellent agreement. The contributions of translational and rotational whole-molecule diffusion to the simulation-derived NSE and potential problems in their estimation are examined. Principal component analysis identifies types of domain motion that dominate the internal motion's contribution to the NSE signal, with the largest being classic hinge bending. The associated free-energy profiles are quasiharmonic and the frictional properties correspond to highly overdamped motion. The amplitudes of the motions derived by MD are smaller than those derived from the experimental analysis, and possible reasons for this difference are discussed. The MD results confirm that a significant component of the NSE arises from internal dynamics. They also demonstrate that the combination of NSE with MD is potentially useful for determining the forms, potentials of mean force, and time dependence of functional domain motions in proteins.

Wide-line NMR and protein hydration

In this chapter, the reader is introduced to the basics of wide-line NMR, with particular focus on the following: (1) basic theoretical and experimental NMR elements, necessary before switching the spectrometer and designing the experiment, (2) models/theories for the interpretation of measured data, (3) definition of wide-line NMR spectrometry, the description of the measurement and evaluation variants, useful hints for the novice, (4) advice on selecting the solvent, which is not a trivial task, (5) a note of warning that not all data are acceptable in spite of the statistical confidence. Finally, we wrap up the chapter with the results on two proteins (a globular and an intrinsically disordered).

The low-temperature inflection observed in neutron scattering measurements of proteins is due to methyl rotation: direct evidence using isotope labeling and molecular dynamics simulations

There is increasing interest in the contribution of methyl groups to the overall dynamics measured by neutron scattering experiments of proteins. In particular an inflection observed in atomic mean square displacements measured as a function of temperature on high resolution spectrometers (approximately 1 microeV) was explained by the onset of methyl group rotations. By specifically labeling a non-methyl-containing side-chain in a native protein system, the purple membrane, and performing neutron scattering measurements, we here provide direct experimental evidence that the observed inflection is indeed due to methyl group rotations. Molecular dynamics simulations reproduce the experimental data, and their analysis suggests that the apparent transition is due to methyl group rotation entering the finite instrumental resolution of the spectrometer. Methyl group correlation times measured by solid state NMR in the purple membrane, taken from previous work, support the interpretation.

Response of small-scale, methyl rotors to protein-ligand association: a simulation analysis of calmodulin-peptide binding

Changes in the free energy barrier (DeltaE), entropy, and motional parameters associated with the rotation of methyl groups in a protein (calmodulin (CaM)) on binding a ligand (the calmodulin-binding domain of smooth-muscle myosin (smMLCKp)) are investigated using molecular dynamics simulation. In both the bound and uncomplexed forms of CaM, the methyl rotational free energy barriers follow skewed-Gaussian distributions that are not altered significantly upon ligand binding. However, site-specific perturbations are found. Around 11% of the methyl groups in CaM exhibit changes in DeltaE greater than 0.7 kcal/mol on binding. The rotational entropies of the methyl groups exhibit a nonlinear dependence on DeltaE. The relations are examined between motional parameters (the methyl rotational NMR order parameter and the relaxation time) and DeltaE. Low-barrier methyl group rotational order parameters deviate from ideal tetrahedrality by up to approximately 20%. There is a correlation between rotational barrier changes and proximity to the protein-peptide binding interface. Methyl groups that exhibit large changes in DeltaE are found to report on elements in the protein undergoing structural change on binding.

The impact of hydration water on the dynamics of side chains of hydrophobic peptides: from dry powder to highly concentrated solutions

Elastic and quasielastic neutron scattering experiments are used to investigate the dynamics of side chains in proteins, using hydrophobic peptides, from dry and hydrated powders up to solutions, as models. The changes of the internal dynamics of a prototypical hydrophobic amino acid, N-acetyl-leucine-methylamide, and alanine amino acids are investigated as a function of water/peptide molecular ratio. While previous results have shown that, in concentrated solution, when the hydrophobic side chains are hydrated by a single hydration water layer, the only allowed motions are confined and can be attributed to librational/rotational movements associated with the methyl groups. In the present work we observe a dynamical evolution from dry to highly hydrated powder. We also observe rotational and diffusive motions and a dynamical transition at approximately 250 K for long side chain peptides while for peptides with short side chains, there is no dynamical transition but only rotational motions. With a local measurement of the influence of hydration water dynamics on the amino acid side chains dynamics, we provide unique experimental evidence that the structural and dynamical properties of interfacial water strongly influence the side chain dynamics and the activation of diffusive motions. We also emphasize that the side chain length has a role on the onset of dynamical transition.

Interfacial water at protein surfaces: wide-line NMR and DSC characterization of hydration in ubiquitin solutions

Wide-line 1H-NMR and differential scanning calorimetry measurements were done in aqueous solutions and on lyophilized samples of human ubiquitin between -70 degrees C and +45 degrees C. The measured properties (size, thermal evolution, and wide-line NMR spectra) of the protein-water interfacial region are substantially different in the double-distilled and buffered-water solutions of ubiquitin. The characteristic transition in water mobility is identified as the melting of the nonfreezing/hydrate water. The amount of water in the low-temperature mobile fraction is 0.4 g/g protein for the pure water solution. The amount of mobile water is higher and its temperature dependence more pronounced for the buffered solution. The specific heat of the nonfreezing/hydrate water was evaluated using combined differential scanning calorimetry and NMR data. Considering the interfacial region as an independent phase, the values obtained are 5.0-5.8 J x g(-1) x K(-1), and the magnitudes are higher than that of pure/bulk water (4.2 J x g(-1) x K(-1)). This unexpected discrepancy can only be resolved in principle by assuming that hydrate water is in tight H-bond coupling with the protein matrix. The specific heat for the system composed of the protein molecule and its hydration water is 2.3 J x g(-1) x K(-1). It could be concluded that the protein ubiquitin and its hydrate layer behave as a highly interconnected single phase in a thermodynamic sense.

Transient protein softening during the working cycle of a molecular machine

Proper functioning of proteins usually requires a certain internal flexibility provided by stochastic structural fluctuations on the picosecond time scale. In contrast with conventional steady-state experiments, we report on a novel type of (laser-neutron) pump-probe experiment combining in situ activation of protein function with a time-dependent test of protein dynamics using quasielastic neutron scattering. A "transient protein softening" is shown to occur during the photocycle of bacteriorhodopsin as a direct proof for the functional significance of protein flexibility.

Hydration-dependent dynamical transition in protein: protein interactions at approximately 240 K

Interprotein motions in low and fully hydrated carboxymyoglobin crystals are investigated using molecular dynamics simulation. Below approximately 240 K, the calculated dynamic structure factor exhibits a peak arising from interprotein vibration. Above approximately 240 K, the intermolecular fluctuations of the fully hydrated crystal increase drastically, whereas the low-hydration model exhibits no transition. Autocorrelation function analysis shows the transition to be dominated by the activation of diffusive intermolecular motion. The potential of mean force for the interaction remains quasiharmonic. The results indicate useful experimental avenues on protein:protein interactions to be explored using next-generation neutron sources.

The effect of hydration on protein flexibility in photosystem II of green plants studied by quasielastic neutron scattering

The effect of hydration on protein dynamics in photosystem II (PS II) membrane fragments from spinach has been investigated by using the method of quasielastic neutron scattering (QENS) at room temperature. The QENS data obtained indicate that the protein dynamics is strongly dependent on the extent of hydration. In particular, the hydration-induced activation of localized diffusive protein motions and QA- reoxidation by QB in PS II appear to be correlated in their onset at a hydration value of about 45% relative humidity (r.h.). These findings underline the crucial functional relevance of localized diffusive protein motions on the picosecond-timescale for the reactions of light-induced photosynthetic water splitting under formation of plastoquinol and molecular oxygen in PS II of green plants.

Protein dynamics and stability: the distribution of atomic fluctuations in thermophilic and mesophilic dihydrofolate reductase derived using elastic incoherent neutron scattering

The temperature dependence of the dynamics of mesophilic and thermophilic dihydrofolate reductase is examined using elastic incoherent neutron scattering. It is demonstrated that the distribution of atomic displacement amplitudes can be derived from the elastic scattering data by assuming a (Weibull) functional form that resembles distributions seen in molecular dynamics simulations. The thermophilic enzyme has a significantly broader distribution than its mesophilic counterpart. Furthermore, although the rate of increase with temperature of the atomic mean-square displacements extracted from the dynamic structure factor is found to be comparable for both enzymes, the amplitudes are found to be slightly larger for the thermophilic enzyme. Therefore, these results imply that the thermophilic enzyme is the more flexible of the two.

Coupling between lysozyme and trehalose dynamics: microscopic insights from molecular-dynamics simulations

We have carried out molecular-dynamics simulations on fully flexible all-atom models of the protein lysozyme immersed in trehalose, an effective biopreservative, with the purpose of exploring the nature and extent of the dynamical coupling between them. Our study shows a strong coupling over a wide range of temperatures. We found that the onset of anharmonic behavior was dictated by changes in the dynamics and relaxation processes in the trehalose glass. The physical origin of protein-trehalose coupling was traced to the hydrogen bonds formed at the interface between the protein and the solvent. Moreover, protein-solvent hydrogen bonding was found to control the structural relaxation of the protein. The dynamics of the protein was found to be heterogeneous; the motions of surface and core atoms had different dependencies on temperature and, in addition, the surface atoms were more sensitive to the dynamics of the solvent than the core atoms. From the solvent perspective we found that the dynamics near the protein surface showed an unexpected enhanced mobility compared to the bulk. These results shed some light on the microscopic origins of the dynamical coupling in protein-solvent systems.

Effects of hydration water on protein methyl group dynamics in solution

Elastic and quasielastic neutron scattering experiments have been used to investigate the dynamics of methyl groups in a protein-model hydrophobic peptide in solution. The results suggest that, when the hydrophobic side chains are hydrated by a single hydration water layer, the only allowed motions are confined and attributed to librational and rotational movement associated with the methyl groups. They provide unique experimental evidence that the structural and dynamical properties of the interfacial water strongly influence the side-chain dynamics and the activation of diffusive motion.

Glass transition in biomolecules and the liquid-liquid critical point of water

Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a "protein glass transition," occurs at the temperature of dynamic crossover in the diffusivity of hydration water and also coincides with the maxima of the isobaric specific heat C_{P} and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.

Role of solvent on protein-matrix coupling in MbCO embedded in water-saccharide systems: a Fourier transform infrared spectroscopy study

Embedding protein in sugar systems of low water content enables one to investigate the protein dynamic-structure function in matrixes whose rigidity is modulated by varying the content of residual water. Accordingly, studying the dynamics and structure thermal evolution of a protein in sugar systems of different hydration constitutes a tool for disentangling solvent rigidity from temperature effects. Furthermore, studies performed using different sugars may give information on how the detailed composition of the surrounding solvent affects the internal protein dynamics and structural evolution. In this work, we compare Fourier transform infrared spectroscopy measurements (300-20 K) on MbCO embedded in trehalose, sucrose, maltose, raffinose, and glucose matrixes of different water content. At all the water contents investigated, the protein-solvent coupling was tighter in trehalose than in the other sugars, thus suggesting a molecular basis for the trehalose peculiarity. These results are in line with the observation that protein-matrix phase separation takes place in lysozyme-lactose, whereas it is absent in lysozyme-trehalose systems; indeed, these behaviors may respectively be due to the lack or presence of suitable water-mediated hydrogen-bond networks, which match the protein surface to the surroundings. The above processes might be at the basis of pattern recognition in crowded living systems; indeed, hydration shells structural and dynamic matching is first needed for successful come together of interacting biomolecules.

Onsets of anharmonicity in protein dynamics

Two onsets of anharmonicity are observed in the dynamics of the protein lysozyme. One at T approximately 100 K appears in all samples regardless of hydration level and is consistent with methyl group rotation. The second, the well-known dynamical transition at T approximately 200-230 K, is only observed at a hydration level h greater than approximately 0.2 and is ascribed to the activation of an additional relaxation process. Its variation with hydration correlates well with variations of catalytic activity suggesting that the relaxation process is directly related to the activation of modes required for protein function.