Comparing the effect on protein stability of methionine oxidation versus mutagenesis: steps toward engineering oxidative resistance in proteins.

The biological activity of some proteins is known to be sensitive to oxidative damage caused by a variety of oxidants. The model protein staphylococcal nuclease was used to explore the effect on protein structural stability of oxidizing methionine to the sulfoxide form. These effects were compared with the effects of substituting methionines with isoleucine and leucine, a potential strategy for stabilizing proteins against oxidative damage. Wild-type nuclease and various mutants were oxidized with hydrogen peroxide. Stabilities of both oxidized and unoxidized proteins were determined by guanidine hydrochloride denaturation. Oxidation destabilized the wild-type protein by over 4 kcal/mol. This large loss of stability supports the idea that in some cases loss of biological activity is linked to disruption of the protein native state. Comparison of mutant protein's stability losses upon oxidation showed that methionines 65 and 98 had a much greater destabilizing effect when oxidized than methionines 26 or 32. While substitution of methionine 98 carried as great an energetic penalty as oxidation, substitution at position 65 was less disruptive than oxidation. Thus a simple substitution mutagenesis strategy to protect a protein against oxidative destabilization is practical for some methionine residues.     

Multiple methionine substitutions are tolerated in T4 lysozyme and have coupled effects on folding and stability

In order to further explore the tolerance of proteins to amino acid substitutions within the interior, a series of core residues was replaced by methionine within the C-terminal domain of T4 lysozyme. By replacing leucine, isoleucine, valine and phenylalanine residues a total of 10 methionines could be introduced, which corresponds to a third of the residues that are buried in this domain. As more methionines are incorporated the protein gradually loses stability. This is attributed in part to a reduction in hydrophobic stabilization, in part to the increased entropic cost of localizing the long, flexible methionine sidechains, and in part to steric clashes. The changes in structure of the mutants relative to the wildtype protein are modest but tend to increase in an additive fashion as more methionines are included. In the most extreme case, namely the 10-methionine mutant, much of the C-terminal domain remains quite similar to wildtype (root-mean-square backbone shifts of 0.56 Å), while the F and G helices undergo rotations of approximately 20° and center-of-mass shifts of approximately 1.4 Å. For up to six methionine substitutions the changes in stability are additive. Beyond this point, however, the multiple mutants are somewhat more stable than suggested from the sum of their constituents, especially for those including the replacement Val111→Met. This is interpreted in terms of the larger structural changes associated with this substitution. The substituted sidechains in the mutant structures have somewhat higher crystallographic thermal factors than their counterparts in WT*. Nevertheless, the interiors of the mutant proteins retain a well-defined structure with little suggestion of molten-globule characteristics. Lysozymes in which selenomethionine has been incorporated rather than methionine tend to have increased stability. At the same time they also fold faster. This provides further evidence that, at the rate-limiting step in folding, the structure of the C-terminal domain of T4 lysozyme is similar to that of the fully folded protein.     

Energetics of side chain packing in staphylococcal nuclease assessed by exchange of valines, isoleucines, and leucines

To examine the importance of side chain packing to protein stability, each of the 11 leucines in staphylococcal nuclease was substituted with isoleucine and valine. The nine valines were substituted with leucine and isoleucine, while the five isoleucines, previously substituted with valine, were substituted with leucine and methionine. These substitutions conserve the hydrophobic character of these side chains but alter side chain geometry and, in some cases, size. In addition, eight threonine residues, previously substituted with valine, were substituted with isoleucine to test the importance of packing at sites normally not occupied by a hydrophobic residue. The stabilities of these 58 mutant proteins were measured by guanidine hydrochloride denaturation. To the best of our knowledge, this is the largest library of single packing mutants yet characterized. As expected, repacking stability effects are tied to the degree of side chain burial. The average energetic cost of moving a single buried methyl group was 0.9 kcal/mol, albeit with a standard deviation of 0.8 kcal/mol. This average is actually slightly greater than the value of 0.7-0.8 kcal/mol estimated for the hydrophobic transfer energy of a methylene from octanol to water. These results appear to indicate that van der Waals interactions gained from optimal packing are at least as important in stabilizing the native state of proteins as hydrophobic transfer effects.     

Substitution with selenomethionine can enhance the stability of methionine-rich proteins.

The availability of a series of phage T4 lysozymes with up to 14 methionine residues incorporated within the protein has made it possible to systematically compare the effect on protein stability of selenomethionine relative to methionine. Wild-type lysozyme contains two fully buried methionine residues plus three more on the surface. The substitution of these methionine residues with selenomethionine slightly stabilizes the protein. As more and more methionine residues are substituted into the protein, there is a progressive loss of stability. This is, however, increasingly offset in the selenomethionine variants, ultimately resulting in a differential increase in melting temperature of about 7 degrees C. This increase, corresponding to about 0.25 kcal/mol per substitution, is in reasonable agreement with the difference in the solvent transfer free energy between the two amino acids.     

Tolerance of point substitution of methionine for isoleucine in hen egg white lysozyme

X-ray structure determination of proteins by using the multiple-wavelength anomalous dispersion method targeting selenomethionine is now widely employed. Isoleucine was examined for the second choice of the substitution of methionine next to leucine. We performed a systematic mutational study of the substitutions of methionine for isoleucine. All mutated lysozymes were less stable than the wild-type by about 1 kcal/mol and it is suggested that this instability was caused by the change in residual hydrophobicity from isoleucine to methionine. The X-ray structures of all mutant lysozymes were very similar to that of the wild-type. In addition, both the accessible surface areas and the conformation of the side chain of methionine in all mutant lysozymes were similar to those of the side chain at the respective isoleucine in the wild-type. Therefore, it is suggested that the mutation from isoleucine to methionine in a protein can be considered as a "safe" substitution.     

Oxidation of buried cysteines is slow and an insignificant factor in the structural destabilization of staphylococcal nuclease caused by H<Subscript>2</Subscript>O<Subscript>2</Subscript> exposure

Summary. The oxidation of buried cysteine or methionine residues can destroy the enzyme activity of a protein by disrupting structure. Engineering in such an oxidatively triggered switch for enzyme activity would only be useful if the effects of substitution are relatively minor, while the effects of the oxidized side chain upon structure are significant and the oxidation relatively easy. To assess the feasibility of this strategy for controlling enzyme activity, the effects of such substitutions and their oxidation were studied in a well characterized model protein, staphylococcal nuclease. Stability and enzyme activity of the oxidized proteins was assessed and compared to the stability and enzyme activity of the unoxidized proteins. Cysteines were found to be generally well tolerated in buried positions but these mutants were not more destabilized than wild-type when oxidized. This shows that buried cysteines are difficult enough to oxidize that this is not likely to be a useful protein engineering strategy or a commonly used regulatory modification. Similar effects were observed for methionine.     

Structural and thermodynamic analysis of the packing of two alpha-helices in bacteriophage T4 lysozyme

Packing interactions in bacteriophage T4 lysozyme were explored by determining the structural and thermodynamic effects of substitutions for Ala98 and neighboring residues. Ala98 is buried in the core of T4 lysozyme in the interface between two alpha-helices. The Ala98 to Val (A98V) replacement is a temperature-sensitive lesion that lowers the denaturation temperature of the protein by 15 degrees C (pH 3.0, delta delta G = -4.9 kcal/mol) and causes atoms within the two helices to move apart by up to 0.7 A. Additional structural shifts also occur throughout the C-terminal domain. In an attempt to compensate for the A98V replacement, substitutions were made for Val149 and Thr152, which make contact with residue 98. Site-directed mutagenesis was used to construct the multiple mutants A98V/T152S, A98V/V149C/T152S and the control mutants T152S, V149C and A98V/V149I/T152S. These proteins were crystallized, and their high-resolution X-ray crystal structures were determined. None of the second-site substitutions completely alleviates the destabilization or the structural changes caused by A98V. The changes in stability caused by the different mutations are not additive, reflecting both direct interactions between the sites and structural differences among the mutants. As an example, when Thr152 in wild-type lysozyme is replaced with serine, the protein is destabilized by 2.6 kcal/mol. Except for a small movement of Val94 toward the cavity created by removal of the methyl group, the structure of the T152S mutant is very similar to wild-type T4 lysozyme. In contrast, the same Thr152 to Ser replacement in the A98V background causes almost no change in stability. Although the structure of A98V/T152S remains similar to A98V, the combination of T152S with A98V allows relaxation of some of the strain introduced by the Ala98 to Val replacement. These studies show that removal of methyl groups by mutation can be stabilizing (Val98----Ala), neutral (Thr152----Ser in A98V) or destabilizing (Val149----Cys, Thr152----Ser). Such diverse thermodynamic effects are not accounted for by changes in buried surface area or free energies of transfer of wild-type and mutant side-chains. In general, the changes in protein stability caused by a mutation depend not only on changes in the free energy of transfer associated with the substitution, but also on the structural context within which the mutation occurs and on the ability of the surrounding structure to relax in response to the substitution.(ABSTRACT TRUNCATED AT 400 WORDS)     

On the unyielding hydrophobic core of villin headpiece

Villin headpiece (HP67) is a small, autonomously-folding domain that has become a model system for understanding the fundamental tenets governing protein folding. In this communication, we explore the role that Leu61 plays in the structure and stability of the construct. Deletion of Leu61 results in a completely unfolded protein that cannot be expressed in Escherichia coli. Omission of only the aliphatic leucine side chain (HP67 L61G) perturbed neither the backbone conformation nor the orientation of local hydrophobic side chains. As a result, a large, solvent-exposed hydrophobic pocket, a negative replica of the leucine side-chain, was created on the surface. The loss of the hydrophobic interface between leucine 61 and the hydrophobic pocket destabilized the construct by ~3.3 kcal/mol. Insertion of a single glycine residue immediately before Leu61 (HP67 L61[GL]) was also highly destabilizing and had the effect of altering the backbone conformation (α-helix to π-helix) in order to precisely preserve the wild-type position and conformation of all hydrophobic residues, including Leu61. In addition to demonstrating that the hydrophobic side-chain of Leu61 is critically important for the stability of villin headpiece, our results are consistent with the notion that the precise interactions present within the hydrophobic core, rather than the hydrogen bonds that define the secondary structure, specify a protein's fold.     

A single disulfide bond restores thermodynamic and proteolytic stability to an extensively mutated protein

The potential for engineering stable proteins with multiple amino acid substitutions was explored. Eleven lysine, five methionine, two tryptophan, one glycine, and three threonine substitutions were simultaneously made in barley chymotrypsin inhibitor-2 (CI-2) to substantially improve the essential amino acid content of the protein. These substitutions were chosen based on the three-dimensional structure of CI-2 and an alignment of homologous sequences. The initial engineered protein folded into a wild-type-like structure, but had a free energy of unfolding of only 2.2 kcal/mol, considerably less than the wild-type value of 7.5 kcal/mol. Restoration of the lysine mutation at position 67 to the wild-type arginine increased the free energy of unfolding to 3.1 kcal/mol. Subsequent cysteine substitutions at positions 22 and 82 resulted in disulfide bond formation and a protein with nearly wild-type thermodynamic stability (7.0 kcal/mol). None of the engineered proteins retained inhibitory activity against chymotrypsin or elastase, and all had substantially reduced inhibitory activity against subtilisin. The proteolytic stabilities of the proteins correlated with their thermodynamic stabilities. Reduction of the disulfide bond resulted in substantial loss of both thermodynamic and proteolytic stabilities, confirming that the disulfide bond, and not merely the cysteine substitutions, was responsible for the increased stability. We conclude that it is possible to replace over a third of the residues in CI-2 with minimal disruption of stability and structural integrity.     

Determination of protein secondary structure and solvent accessibility using site-directed fluorescence labeling. Studies of T4 lysozyme using the fluorescent probe monobromobimane

We report an investigation of how much protein structural information could be obtained using a site-directed fluorescence labeling (SDFL) strategy. In our experiments, we used 21 consecutive single-cysteine substitution mutants in T4 lysozyme (residues T115-K135), located in a helix-turn-helix motif. The mutants were labeled with the fluorescent probe monobromobimane and subjected to an array of fluorescence measurements. Thermal stability measurements show that introduction of the label is substantially perturbing only when it is located at buried residue sites. At buried sites (solvent surface accessibility of <40 A(2)), the destabilizations are between 3 and 5.5 kcal/mol, whereas at more exposed sites, DeltaDeltaG values of < or = 1.5 kcal/mol are obtained. Of all the fluorescence parameters that were explored (excitation lambda(max), emission lambda(max), fluorescence lifetime, quantum yield, and steady-state anisotropy), the emission lambda(max) and the steady-state anisotropy values most accurately reflect the solvent surface accessibility at each site as calculated from the crystal structure of cysteine-less T4 lysozyme. The parameters we identify allow the classification of each site as buried, partially buried, or exposed. We find that the variations in these parameters as a function of residue number reflect the sequence-specific secondary structure, the determination of which is a key step for modeling a protein of unknown structure.     

You cannot oxidize what you cannot reach: Oxidative susceptibility of buried methionine residues

Oxidation of protein methionines to methionine sulfoxides can result in protein structural alterations with a wide variety of biological implications. Factors that determine susceptibility to oxidation are not well understood. The recent JBC Editors Pick by Walker et al. applied proteomic methodologies to show that the oxidative susceptibility of buried methionine residues is strongly correlated with folding stability of the contextual peptide. Proteome-wide analysis of oxidation-susceptible methionines promises to answer open questions about the biological functions of reversible methionine oxidation.     

Contribution of the hydrophobic effect to globular protein stability.

The decrease in conformational stability, delta(delta G), has been measured for 72 aliphatic side-chain mutants from four proteins in which a larger side-chain is replaced by a smaller side-chain so that steric effects are minimal. When these delta(delta G) values are corrected to the same accessibility, namely 100% buried, then the following -delta(delta G) values per -CH2- group (in kcal/mol) are obtained: Ile----Val (1.26), Ala (1.26), Gly (1.26); Leu----Ala (1.16), Gly (1.21); Val----Ala (1.23), Gly (1.53). The average of these values is 1.27(+/- 0.07) kcal/mol. The 72 individual values range from 0 to 2.4 kcal/mol with an average value of 1.27(+/- 0.51) (standard deviation) kcal/mol. When the delta Gtr values from n-octanol to water are corrected for the difference in volume between the solutes and the solvents, the average value for the same substitutions is 1.25(+/- 0.05) kcal/mol. This suggests that proteins gain 1.3(+/- 0.5) kcal/mol in stability for each -CH2- group buried in folding, and, furthermore, that the volume corrected delta Gtr values for n-octanol for the amino acid side-chains provide good estimates of the contribution of the hydrophobic effect to globular protein stability.     

Fluorous effect in proteins: de novo design and characterization of a four-alpha-helix bundle protein containing hexafluoroleucine

Several studies have demonstrated that proteins incorporating fluorinated analogues of hydrophobic amino acids such as leucine and valine into their hydrophobic cores exhibit increased stability toward thermal denaturation and unfolding by guanidinium chloride. However, estimates for the increase in the thermodynamic stability of a protein (DeltaDeltaG(unfold)) afforded by the substitution of a hydrophobic amino acid with its fluorinated analogue vary quite significantly. To address this, we have designed a peptide that adopts an antiparallel four-helix bundle structure in which the hydrophobic core is packed with leucine, and investigated the effects of substituting the central two layers of the core with L-5,5,5,5',5',5'-hexafluoroleucine (hFLeu). We find that DeltaDeltaG(unfold) is increased by 0.3 kcal/mol per hFLeu residue. This is in good agreement with the predicted increase in DeltaDeltaG(unfold) of 0.4 kcal/mol per residue arising from the increased hydrophobicity of the hFLeu side chain, which we determined experimentally from partitioning measurements on hFLeu and leucine. The increased stability of this fluorinated protein may therefore be ascribed to simple hydrophobic effects, rather than specific "fluorous" interactions between the hFLeu residues.     

Substitution of conserved methionines by leucines in chloroplast small heat shock protein results in loss of redox-response but retained chaperone-like activity

During evolution of land plants, a specific motif occurred in the N-terminal domain of the chloroplast-localized small heat shock protein, Hsp21: a sequence with highly conserved methionines, which is predicted to form an amphipathic alpha-helix with the methionines situated along one side. The functional role of these conserved methionines is not understood. We have found previously that treatment, which causes methionine sulfoxidation in Hsp21, also leads to structural changes and loss of chaperone-like activity. Here, mutants of Arabidopsis thaliana Hsp21 protein were created by site-directed mutagenesis, whereby conserved methionines were substituted by oxidation-resistant leucines. Mutants lacking the only cysteine in Hsp21 were also created. Protein analyses by nondenaturing electrophoresis, size exclusion chromatography, and circular dichroism proved that sulfoxidation of the four highly conserved methionines (M49, M52, M55, and M59) is responsible for the oxidation-induced conformational changes in the Hsp21 oligomer. In contrast, the chaperone-like activity was not ultimately dependent on the methionines, because it was retained after methionine-to-leucine substitution. The functional role of the conserved methionines in Hsp21 may be to offer a possibility for redox control of chaperone-like activity and oligomeric structure dynamics.     

Role of a surface tryptophan in defining the structure, stability, and DNA binding of the hyperthermophile protein Sac7d

Sac7d is a small, chromatin protein from Sulfolobus acidocaldarius which induces a sharp kink in DNA with intercalation of valine and methionine side chains. The crystal structure of the protein-DNA complex indicates that a surface tryptophan (W24) plays a key role in DNA binding by hydrogen bonding to the DNA at the kink site. We show here that substitution of the solvent-exposed tryptophan with alanine (W24A) led to a significant loss in not only DNA binding affinity but also protein stability. The W24A substitution proved to be one of the most destabilizing surface substitutions in Sac7d. A global linkage analysis of the pH and salt dependence of stability indicated that the protein stability surface (DeltaG vs temperature, pH, and salt concentration) was lowered overall by 2 kcal/mol (from 0 to 100 degrees C, pH 0 to 7, and 0 to 0.3 M KCl). The lower free energy of unfolding could not be attributed to significant structural perturbations of surface electrostatic interactions. Residual dipolar coupling of partially aligned protein and the NMR solution structure of W24A confirmed that the surface substitution resulted in no significant change in structure. Stabilization of this hyperthermophile protein and its DNA complex by a surface cluster of hydrophobic residues involving W24 and the two intercalating side chains is discussed.     

Nine hydrophobic side chains are key determinants of the thermodynamic stability and oligomerization status of tumour suppressor p53 tetramerization domain.

The contribution of almost each amino acid side chain to the thermodynamic stability of the tetramerization domain (residues 326-353) of human p53 has been quantitated using 25 mutants with single-residue truncations to alanine (or glycine). Truncation of either Leu344 or Leu348 buried at the tetramer interface, but not of any other residue, led to the formation of dimers of moderate stability (8-9 kcal/mol of dimer) instead of tetramers. One-third of the substitutions were moderately destabilizing (<3.9 kcal/mol of tetramer). Truncations of Arg333, Asn345 or Glu349 involved in intermonomer hydrogen bonds, Ala347 at the tetramer interface or Thr329 were more destabilizing (4.1-5.7 kcal/mol). Strongly destabilizing (8.8- 11.7 kcal/mol) substitutions included those of Met340 at the tetramer interface and Phe328, Arg337 and Phe338 involved peripherally in the hydrophobic core. Truncation of any of the three residues involved centrally in the hydrophobic core of each primary dimer either prevented folding (Ile332) or allowed folding only at high protein concentration or low temperature (Leu330 and Phe341). Nine hydrophobic residues per monomer constitute critical determinants for the stability and oligomerization status of this p53 domain.     

Clarifying the influence of core amino acid hydrophobicity, secondary structure propensity, and molecular volume on amyloid-β 16-22 self-assembly

The self-assembly of amyloid peptides is influenced by hydrophobicity, charge, secondary structure propensity, and sterics. Previous experiments have shown that increasing hydrophobicity at the aromatic positions of the amyloid-β 16-22 fragment (Aβ(16-22)) without introducing steric restraints greatly increases the rate of self-assembly and thermodynamically stabilizes the resulting fibrils [Senguen et al., Mol. BioSyst., 2011, DOI: 10.1039/c0mb00080a]. Conversely, when increasing side chain hydrophobicity coincides with an increase in side chain volume, the increase in the rate of self-assembly is offset by a thermodynamic destabilization of the resulting amyloid fibrils when direct cross-strand side chain interactions occur. These findings indicate that steric effects also influence the self-assembly of amyloidogenic peptides. Herein, the aromatic Phe residues at positions 19, 20, and 19,20 of Aβ(16-22) have been systematically replaced by Val, Leu, Ile, or hexafluoroleucine (Hfl) and amyloid formation has been characterized. The Val variants, despite the high β-sheet propensity of Val, were thermodynamically destabilized (ΔΔG = +0.1-0.4 kcal mol(-1)) relative to the wild-type with the double mutant failing to self-assemble at the concentrations studied. Conversely, the Leu and Ile variants formed fibrils at enhanced rates relative to wild-type and exhibited similar, or in some cases enhanced thermodynamic stabilities relative to the wild-type (ΔΔG = 0-0.6 kcal mol(-1)). The more hydrophobic Hfl variants were greatly stabilized (ΔΔG = -0.3-2.1 kcal mol(-1)) relative to the wild-type. These data indicate that hydrophobicity and steric effects both influence peptide self-assembly processes, including nucleation and fibrillization rates and the thermodynamic stability of the resulting fibrils.     

Oxidation of methionine residues in recombinant human interleukin-1 receptor antagonist: implications of conformational stability on protein oxidation kinetics

Oxidation of methionine residues is involved in several biochemical processes and in degradation of therapeutic proteins. The relationship between conformational stability and methionine oxidation in recombinant human interleukin-1 receptor antagonist (rhIL-1ra) was investigated to document how thermodynamics of unfolding affect methionine oxidation in proteins. Conformational stability of rhIL-1ra was monitored by equilibrium urea denaturation, and thermodynamic parameters of unfolding (DeltaGH2O, m, and Cm) were estimated at different temperatures. Methionine oxidation induced by hydrogen peroxide at varying temperatures was monitored during "coincubation" of rhIL-1ra with peptides mimicking specific regions of the reactive methionine residues in the protein. The coincubation study allowed estimation of oxidation rates in protein and peptide at each temperature from which normalized oxidation rate constants and activation energies were calculated. The rate constants for buried Met-11 in the protein were lower than for methionine in the peptide with an associated increase in activation energy. The rate constants and activation energy of solvent exposed methionines in protein and peptide were similar. The results showed that conformational stability, monitored using the Cm value, has an effect on oxidation rates of buried methionines. The rate constant of buried Met-11 correlated well with the Cm value but not DeltaGH2O. No correlation was observed for the oxidation rates of solvent-exposed methionines with any thermodynamic parameters of unfolding. The findings presented have implications in protein engineering, in design of accelerated stability studies for protein formulation development, and in understanding disease conditions involving protein oxidation.     

Extensive modifications for methionine enhancement in the β-barrels do not alter the structural stability of the bean seed storage protein phaseolin

Common beans are widely utilized as a food source, yet are low in the essential amino acid methionine. As an initial step to overcome this defect the methionine content of the primary bean seed storage protein phaseolin was increased by replacing 20 evolutionarily variant hydrophobic residues with methionine and inserting short, methionine-rich sequences into turn and loop regions of the protein structure. Methionine enhancement ranged from 5 to 30 residues. AnEscherichia coli expression system was developed to characterize the structural stability of the mutant proteins. Proteins of expected sizes were obtained for all constructs except for negative controls, which were rapidly degraded inE. coli. Thermal denaturation of the purified proteins demonstrated that both wild-type and mutant phaseolin proteins denatured reversibly at approximately 61°C. In addition, urea denaturation experiments of the wild-type and a mutant protein (with 30 additional methionines) confirmed that the structural stability of the proteins was very similar. Remarkably, these results indicate that the phaseolin protein tolerates extensive modifications, including 20 substitutions and two loop inserts for methionine enhancement in the-barrel and loop structures, with extremely small effects on protein stability.     

Buried water molecules contribute to the conformational stability of a protein

This study sought to attain a better understanding of the contribution of buried water molecules to protein stability. The 3SS human lysozyme lacks one disulfide bond between Cys77 and Cys95 and is significantly destabilized compared with the wild-type human lysozyme (4SS). We examined the structure and stability of the I59A-3SS mutant human lysozyme, in which a cavity is created at the mutation site. The crystal structure of I59A-3SS indicated that there were ordered new water molecules in the cavity created. The stability of I59A-3SS is 5.5 kJ/mol less than that of 3SS. The decreased stability of I59A-3SS (5.5 kJ/mol) is similar to that of Ile to Ala mutants with newly introduced water molecules in other globular proteins (6.3 +/- 2.1 kJ/mol), but is less than that of Ile/Leu to Ala mutants with empty cavities (13.7 +/- 3.1 kJ/mol). This indicates that water molecules partially compensate for the destabilization by decreasing hydrophobic and van der Waals interactions. These results provide further evidence that buried water molecules contribute to protein stability.