Contribution of hydrogen bonds to the conformational stability of human lysozyme: calorimetry and X-ray analysis of six Ser --> Ala mutants.

To further examine the contribution of hydrogen bonds to the conformational stability of the human lysozyme, six Ser to Ala mutants were constructed. The thermodynamic parameters for denaturation of these six Ser mutant proteins were investigated by differential scanning calorimetry (DSC), and the crystal structures were determined by X-ray analysis. The denaturation Gibbs energy (DeltaG) of the Ser mutant proteins was changed from 2.0 to -5.7 kJ/mol, compared to that of the wild-type protein. With an analysis in which some factors that affected the stability due to mutation were considered, the contribution of hydrogen bonds to the stability (Delta DeltaGHB) was extracted on the basis of the structures of the mutant proteins. The results showed that hydrogen bonds between protein atoms and between a protein atom and a water bound with the protein molecule favorably contribute to the protein stability. The net contribution of one intramolecular hydrogen bond to protein stability (DeltaGHB) was 8.9 +/- 2.6 kJ/mol on average. However, the contribution to the protein stability of hydrogen bonds between a protein atom and a bound water molecule was smaller than that for a bond between protein atoms.     

Role of surface hydrophobic residues in the conformational stability of human lysozyme at three different positions

To evaluate the contribution of the amino acid residues on the surface of a protein to its stability, a series of hydrophobic mutant human lysozymes (Val to Gly, Ala, Leu, Ile, Met, and Phe) modified at three different positions on the surface, which are located in the alpha-helix (Val 110), the beta-sheet (Val 2), and the loop (Val 74), were constructed. Their thermodynamic parameters of denaturation and crystal structures were examined by calorimetry and by X-ray crystallography at 100 K, respectively. Differences in the denaturation Gibbs energy change between the wild-type and the hydrophobic mutant proteins ranged from 4.6 to -9.6 kJ/mol, 2.7 to -1.5 kJ/mol, and 3.6 to -0.2 kJ/mol at positions 2, 74, and 110, respectively. The identical substitution at different positions and different substitutions at the same position resulted in different degrees of stabilization. Changes in the stability of the mutant proteins could be evaluated by a unique equation considering the conformational changes due to the substitutions [Funahashi et al. (1999) Protein Eng. 12, 841-850]. For this calculation, secondary structural propensities were newly considered. However, some mutant proteins were not adapted to the equation. The hydration structures around the mutation sites of the exceptional mutant proteins were affected due to the substitutions. The stability changes in the exceptional mutant proteins could be explained by the formation or destruction of the hydration structures. These results suggest that the hydration structure mediated via hydrogen bonds covering the protein surface plays an important role in the conformational stability of the protein.     

Positive contribution of hydration structure on the surface of human lysozyme to the conformational stability

Water molecules make a hydration structure with the network of hydrogen bonds, covering on the surface of proteins. To quantitatively estimate the contribution of the hydration structure to protein stability, a series of hydrophilic mutant human lysozymes (Val to Ser, Tyr, Asp, Asn, and Arg) modified at three different positions on the surface, which are located in the alpha-helix (Val-110), the beta-sheet (Val-2), and the loop (Val-74), were constructed. Their thermodynamic parameters of denaturation and crystal structures were examined by calorimetry and by x-ray crystallography at 100 K, respectively. The introduced polar residues made hydrogen bonds with protein atoms and/or water molecules, sometimes changing the hydration structure around the mutation site. Changes in the stability of the mutant proteins can be evaluated by a unique equation that considers the conformational changes resulting from the substitutions. Using this analysis, the relationship between the changes in the stabilities and the hydration structures for mutant human lysozymes substituted on the surface could be quantitatively estimated. The analysis indicated that the hydration structure on protein surface plays an important role in determining the conformational stability of the protein.     

Contribution of amino acid substitutions at two different interior positions to the conformational stability of human lysozyme.

To elucidate correlative relationships between structural change and thermodynamic stability in proteins, a series of mutant human lysozymes modified at two buried positions (Ile56 and Ile59) were examined. Their thermodynamic parameters of denaturation and crystal structures were studied by calorimetry and X-ray crystallography. The mutants at positions 56 and 59 exhibited different responses to a series of amino acid substitutions. The changes in stability due to substitutions showed a linear correlation with changes in hydrophobicity of substituted residues, having different slopes at each mutation site. However, the stability of each mutant was found to be represented by a unique equation involving physical properties calculated from mutant structures. By fitting present and previous stability data for mutant human lysozymes substituted at various positions to the equation, the magnitudes of the hydrophobicity of a carbon atom and the hydrophobicity of nitrogen and neutral oxygen atoms were found to be 0.178 and -0.013 kJ/mol.A(2), respectively. It was also found that the contribution of a hydrogen bond with a length of 3.0 A to protein stability was 5.1 kJ/mol and the entropy loss of newly introduction of a water molecules was 7.8 kJ/mol.     

A new scale for side-chain contribution to protein stability based on the empirical stability analysis of mutant proteins

The hydrophobicity scales for amino acid side chains based on the transfer Gibbs energy (DeltaG(trans)) of amino acids from non-aqueous phases to water have been widely used to estimate the contribution of buried side chains to the conformational stability of proteins. In this paper, we propose a new scale for the side-chain contribution to protein stability, which is derived from data on protein denaturation experiments using systematic and comprehensive mutant proteins. In the experiments, the contribution of some physical properties were quantitatively determined as parameters in a unique equation representing the stability change (DeltaDeltaG) of mutant proteins as a function of the structural changes due to the mutations. These parameters are able conveniently to provide a scale for the side-chain contribution to protein stability. This new scale also has the advantage over the previously reported hydrophobicity scales of residues with the contributions of hydrogen bonds or secondary structural propensity. It may find practical application in algorithms for the prediction of protein structures.     

Contribution of polar groups in the interior of a protein to the conformational stability

It has been generally believed that polar residues are usually located on the surface of protein structures. However, there are many polar groups in the interior of the structures in reality. To evaluate the contribution of such buried polar groups to the conformational stability of a protein, nonpolar to polar mutations (L8T, A9S, A32S, I56T, I59T, I59S, A92S, V93T, A96S, V99T, and V100T) in the interior of a human lysozyme were examined. The thermodynamic parameters for denaturation were determined using a differential scanning calorimeter, and the crystal structures were analyzed by X-ray crystallography. If a polar group had a heavy energy cost to be buried, a mutant protein would be remarkably destabilized. However, the stability (Delta G) of the Ala to Ser and Val to Thr mutant human lysozymes was comparable to that of the wild-type protein, suggesting a low-energy penalty of buried polar groups. The structural analysis showed that all polar side chains introduced in the mutant proteins were able to find their hydrogen bond partners, which are ubiquitous in protein structures. The empirical structure-based calculation of stability change (Delta Delta G) [Takano et al. (1999) Biochemistry 38, 12698--12708] revealed that the mutant proteins decreased the hydrophobic effect contributing to the stability (Delta G(HP)), but this destabilization was recovered by the hydrogen bonds newly introduced. The present study shows the favorable contribution of polar groups with hydrogen bonds in the interior of protein molecules to the conformational stability.     

Contribution of hydrogen bonding to protein stability estimated from isotope effects

An unresolved issue in structural biology concerns the relative contribution of H bonds to protein stability. We use the small molecules 4-acetamidobenzoic acid and N-acetylanthranilic acid as model compounds to relate the energetic contribution from hydrogen bonds (H bonds) to the deuterium/hydrogen amide isotope effect. N-Acetylanthranilic acid models carbonyl-amide H bonds formed during protein folding; 4-acetamidobenzoic acid models the unfolded state in which the amide H bonds to water. NMR is used to measure shifts in the pK(a) of the ionizable carboxyl group when the amides of the compounds are either protonated or deuterated. From the pK(a) shift, we obtain a quantitative scale factor: SF = partial partial differential(DeltaG(HB))/partial partial differential(RT ln Phi), where DeltaG(HB) is the change in free energy of an H bond upon isotope substitution and Phi is the fractionation factor. Isotope effect data also are reported for a small globular protein, lambda repressor, using the "C(m) experiment". The protein's isotope effect, which reports on the shape of the energy well, is converted to H-bonding free energy by applying the scale factor. We estimate that amide-related H bonds (amide-carbonyl and amide-water) contribute favorably to protein stability by approximately 30-50 kcal/mol in lambda repressor, GCN4 coiled coil, and cytochrome c but unfavorably by approximately 6 kcal/mol in ubiquitin. The results indicate that H-bond strength varies from one protein to another and presumably at different sites within the same protein.     

Role of non-glycine residues in left-handed helical conformation for the conformational stability of human lysozyme.

To understand the role of non-Gly residues in the left-handed helical conformation for the conformational stability of a protein, the non-Gly to Gly and Ala mutations at six left-handed residues (R21, Y38, R50, Q58, H78, and N118) of the human lysozyme were examined. The thermodynamic parameters for denaturation were determined using a differential scanning calorimeter, and the crystal structures were analyzed by X-ray crystallography. If a left-handed non-Gly had an unfavorable steric interaction between the side-chain Cbeta and backbone, the Gly mutation would be expected to stabilize more than the Ala mutation at the same position. For the mutant human lysozymes, however, there were few differences in the denaturation Gibbs energy (DeltaG) between the Gly and Ala mutants, except for the substitution at position 58. Analysis of the changes in stability (DeltaDeltaG) based on the structures of the wild-type and mutant proteins showed that the experimental DeltaDeltaG value of Q58G was approximately 7 kJ/mol higher than the estimated value without consideration of any local steric interaction. These results indicate that only Q58G increased the stability by elimination of local constraints. The residue 58 is located at the most rigid position in the left-handed non-Gly residues and is involved in its enzymatic function. It can be concluded that the left-handed non-Gly residues do not always have unfavorable strain energies as compared with Gly at the same position.     

Tyrosine hydrogen bonds make a large contribution to protein stability

The aim of this study was to gain a better understanding of the contribution of hydrogen bonds by tyrosine -OH groups to protein stability. The amino acid sequences of RNases Sa and Sa3 are 69 % identical and each contains eight Tyr residues with seven at equivalent structural positions. We have measured the stability of the 16 tyrosine to phenylalanine mutants. For two equivalent mutants, the stability increases by 0.3 kcal/mol (RNase Sa Y30F) and 0.5 kcal/mol (RNase Sa3 Y33F) (1 kcal=4.184 kJ). For all of the other mutants, the stability decreases with the greatest decrease being 3.6 kcal/mol for RNase Sa Y52F. Seven of the 16 tyrosine residues form intramolecular hydrogen bonds and the average decrease in stability for these is 2.0(+/-1.0) kcal/mol. For the nine tyrosine residues that do not form intramolecular hydrogen bonds, the average decrease in stability is 0.4(+/-0.6) kcal/mol. Thus, most tyrosine -OH groups contribute favorably to protein stability even if they do not form intramolecular hydrogen bonds. Generally, the stability changes for equivalent positions in the two proteins are remarkably similar. Crystal structures were determined for two of the tyrosine to phenylalanine mutants of RNase Sa: Y80F (1.2 A), and Y86F (1.7 A). The structures are very similar to that of wild-type RNase Sa, and the hydrogen bonding partners of the tyrosine residues always form intermolecular hydrogen bonds to water in the mutants. These results provide further evidence that the hydrogen bonding and van der Waals interactions of polar groups in the tightly packed interior of folded proteins are more favorable than similar interactions with water in the unfolded protein, and that polar group burial makes a substantial contribution to protein stability.     

Contribution of Thr29 to the thermodynamic stability of goat alpha-lactalbumin as determined by experimental and theoretical approaches

The Thr29 residue in the hydrophobic core of goat alpha-lactalbumin (alpha-LA) was substituted with Val (Thr29Val) and Ile (Thr29Ile) to investigate the contribution of Thr29 to the thermodynamic stability of the protein. We carried out protein stability measurements, X-ray crystallographic analyses, and free energy calculations based on molecular dynamics simulation. The equilibrium unfolding transitions induced by guanidine hydrochloride demonstrated that the Thr29Val and Thr29Ile mutants were, respectively, 1.9 and 3.2 kcal/mol more stable than the wild-type protein (WT). The overall structures of the mutants were almost identical to that of WT, in spite of the disruption of the hydrogen bonding between the side-chain O-H group of Thr29 and the main-chain C=O group of Glu25. To analyze the stabilization mechanism of the mutants, we performed free energy calculations. The calculated free energy differences were in good agreement with the experimental values. The stabilization of the mutants was mainly caused by solvation loss in the denatured state. Furthermore, the O-H group of Thr29 favorably interacts with the C=O group of Glu25 to form hydrogen bonds and, simultaneously, unfavorably interacts electrostatically with the main-chain C=O group of Thr29. The difference in the free energy profile of the unfolding path between WT and the Thr29Ile mutant is discussed in light of our experimental and theoretical results.     

Blocking nonspecific adsorption of native food-borne microorganisms by immunomagnetic beads with iota-carrageenan

We present herein the partitioning characteristics of anti-Salmonella and anti-Escherichia coli O157 immunomagnetic beads (IMB) with respect to the nonspecific adsorption of several nontarget food-borne organisms with and without an assortment of well-known blocking agents, such as casein, which have been shown to be useful in other immunochemical applications. We found several common food-borne organisms that strongly interacted with both types of IMB, especially with anti-Salmonella form (av DeltaG0=-20 +/- 4 kJ mol(-1)) even in the presence of casein [1% (w/v): DeltaG0=-18 +/- 3 kJ mol(-1); DeltaDeltaG0 approximately -2 kJ mol(-1)]. However, when one of the most problematic organisms (a native K12-like E. coli isolate; DeltaG0=-19 +/- 2 kJ mol(-1)) was tested for nonspecific binding in the presence of iota-carrageenan (0.03-0.05%), there was an average decline of ca. 90% in the equilibrium capture efficiency xi (DeltaG0=-11 +/- 4 kJ mol(-1); DeltaDeltaG0 approximately -8 kJ mol(-1)). Other anionic polysaccharides (0.1% kappa-carrageenan and polygalacturonic acid) had no significant effect (av DeltaG0=-19 +/- 1 kJ mol(-1); DeltaDeltaG0 approximately 0 kJ mol(-1)). Varying iota-carrageenan from 0% to 0.02% resulted in xi significantly diminishing from 0.69 (e.g., 69% of the cells captured; DeltaG0=-19 +/- 3 kJ mol(-1)) to 0.05 (DeltaG0=-11 +/- 2 kJ mol(-1); DeltaDeltaG0 approximately -9 kJ mol(-1)) at about 0.03% iota-carrageenan where xi leveled off. An optimum blocking ability was achieved with 0.04% iota-carrageenan suspended in 100 mM phosphate buffer. We also demonstrated that the utilization of iota-carrageenan as a blocking agent causes no great loss in the IMBs capture efficiency with respect to the capture of its target organisms, various salmonellae.     

Effect of foreign N-terminal residues on the conformational stability of human lysozyme.

To minutely understand the effect of foreign N-terminal residues on the conformational stability of human lysozyme, five mutant proteins were constructed: two had Met or Ala in place of the N-terminal Lys residue (K1M and K1A, respectively), and others had one additional residue, Met, Gly or Pro, to the N-terminal Lys residue (Met(-1), Gly(-1) and Pro(-1), respectively). The thermodynamic parameters for denaturation of these mutant proteins were examined by differential scanning calorimetry and were compared with that of the wild-type protein. Three mutants with the extra residue were significantly destabilized: the changes in unfolding Gibbs energy (DeltaDeltaG) were -9.1 to -12.2 kJ.mol-1. However, the stability of two single substitutions at the N-terminal slightly decreased; the DeltaDeltaG values were only -0.5 to -2.5 kJ.mol-1. The results indicate that human lysozyme is destabilized by an expanded N-terminal residue. The crystal structural analyses of K1M, K1A and Gly(-1) revealed that the introduction of a residue at the N-terminal of human lysozyme caused the destruction of hydrogen bond networks with ordered water molecules, resulting in the destabilization of the protein.     

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.     

Estimating the energetic contribution of hydrogen bonding to the stability of Candida methylica formate dehydrogenase by using double mutant cycle

An homology model of Candida methylica formate dehydrogenase (cm FDH) was constructed based on the Pseudomonas sp. 101 formate dehydrogenase (ps FDH) structure. In wild type cm FDH, Thr169 and Thr226 can form hydrogen bonds with each other. We measured the interaction energy between the two threonines independent of other interactions in the proteins by using a so-called double mutant cycle and assessing the protein stability from the concentration of guanidine hydrochloride needed to denature 50% of the molecules. We conclude that the hydrogen bonds stabilize the wild type protein by -4 kcal mol(-1).     

Contribution of salt bridges near the surface of a protein to the conformational stability

Salt bridges play important roles in the conformational stability of proteins. However, the effect of a surface salt bridge on the stability remains controversial even today; some reports have shown little contribution of a surface salt bridge to stability, whereas others have shown a favorable contribution. In this study, to elucidate the net contribution of a surface salt bridge to the conformational stability of a protein, systematic mutant human lysozymes, containing one Glu to Gln (E7Q) and five Asp to Asn mutations (D18N, D49N, D67N, D102N, and D120N) at residues where a salt bridge is formed near the surface in the wild-type structure, were examined. The thermodynamic parameters for denaturation between pH 2.0 and 4.8 were determined by use of a differential scanning calorimeter, and the crystal structures were analyzed by X-ray crystallography. The denaturation Gibbs energy (DeltaG) of all mutant proteins was lower than that of the wild-type protein at pH 4, whereas there was little difference between them near pH 2. This is caused by the fact that the Glu and Asp residues are ionized at pH 4 but protonated at pH 2, indicating a favorable contribution of salt bridges to the wild-type structure at pH 4. Each contribution was not equivalent, but we found that the contributions correlate with the solvent inaccessibility of the salt bridges; the salt bridge contribution was small when 100% accessible, while it was about 9 kJ/mol if 100% inaccessible. This conclusion indicates how to reconcile a number of conflicting reports about role of surface salt bridges in protein stability. Furthermore, the effect of salts on surface salt bridges was also examined. In the presence of 0.2 M KCl, the stability at pH 4 decreased, and the differences in stability between the wild-type and mutant proteins were smaller than those in the absence of salts, indicating the compensation to the contribution of salt bridges with salts. Salt bridges with more than 50% accessibility did not contribute to the stability in the presence of 0.2 M KCl.     

Role of amino acid residues in left-handed helical conformation for the conformational stability of a protein

Our previous study of six non-Gly to Gly/Ala mutant human lysozymes in a left-handed helical region showed that only one non-Gly residue at a rigid site had unfavorable strain energy as compared with Gly at the same position (Takano et al., Proteins 2001; 44:233-243). To further examine the role of left-handed residues in the conformational stability of a protein, we constructed ten Gly to Ala mutant human lysozymes. Most Gly residues in human lysozyme are located in the left-handed helix region. The thermodynamic parameters for denaturation and crystal structures were determined by differential scanning calorimetry and X-ray analysis, respectively. The difference in denaturation Gibbs energy (DeltaDeltaG) for the ten Gly to Ala mutants ranged from + 1.9 to -7.5 kJ/mol, indicating that the effect of the mutation depends on the environment of the residue. We confirm that Gly in a left-handed region is more favorable at rigid sites than non-Gly, but there is little difference in energetic cost between Gly and non-Gly at flexible sites. The present results indicate that dihedral angles in the backbone conformation and also the flexibility at the position should be considered for analyses of protein stability, and protein structural determination, prediction, and design.     

Contribution of hydrogen bonds to protein stability

Our goal was to gain a better understanding of the contribution of the burial of polar groups and their hydrogen bonds to the conformational stability of proteins. We measured the change in stability, Δ(ΔG), for a series of hydrogen bonding mutants in four proteins: villin headpiece subdomain (VHP) containing 36 residues, a surface protein from Borrelia burgdorferi (VlsE) containing 341 residues, and two proteins previously studied in our laboratory, ribonucleases Sa (RNase Sa) and T1 (RNase T1). Crystal structures were determined for three of the hydrogen bonding mutants of RNase Sa: S24A, Y51F, and T95A. The structures are very similar to wild type RNase Sa and the hydrogen bonding partners form intermolecular hydrogen bonds to water in all three mutants. We compare our results with previous studies of similar mutants in other proteins and reach the following conclusions. (1) Hydrogen bonds contribute favorably to protein stability. (2) The contribution of hydrogen bonds to protein stability is strongly context dependent. (3) Hydrogen bonds by side chains and peptide groups make similar contributions to protein stability. (4) Polar group burial can make a favorable contribution to protein stability even if the polar groups are not hydrogen bonded. (5) The contribution of hydrogen bonds to protein stability is similar for VHP, a small protein, and VlsE, a large protein.     

In vivo glycosylation suppresses the aggregation of amyloidogenic hen egg white lysozymes expressed in yeast

The mutant hen egg white lysozymes Ile55Thr and Asp66His, corresponding to human amyloidogenic mutant lysozymes Ile56Thr and Asp67His, respectively, were secreted in Saccharomyces cerevisiae. The amyloidogenic mutants (I55T and D66H) of hen egg white lysozymes were remarkably less soluble than that of the wild-type protein. To enhance the secretion of these mutants, we constructed the glycosylated amyloidogenic lysozymes (I55T/G49N and D66H/G49N) having the N-glycosylation signal sequence (Asn-X-Ser) by the substitution of glycine with asparagine at position 49. The secretion of these glycosylated mutant proteins is greatly increased in S. cerevisiae, compared with that of non-glycosylated type. Both the glycosylated mutants retained about 40% enzymatic activity when incubated at pH 7.4 for 1 h at the physiological temperature of 37 degrees C whereas the non-glycosylated proteins eventually lost all activity under these conditions. These results suggest that the glycosylated chains could mask the beta-strand of amyloidogenic lysozymes from the intermolecular cross-beta-sheet association, thus improving the solubility of amyloidogenic lysozymes.     

Contribution of cation-pi interactions to protein stability

Calculations predict that cation- interactions make an important contribution to protein stability. While there have been some attempts to experimentally measure strengths of cation-pi interactions using peptide model systems, much less experimental data are available for globular proteins. We have attempted to determine the magnitude of cation-pi interactions of Lys with aromatic amino acids in four different proteins (LIVBP, MBP, RBP, and Trx). In each case, Lys was replaced with Gln and Met. In a separate series of experiments, the aromatic amino acid in each cation-pi pair was replaced by Leu. Stabilities of wild-type (WT) and mutant proteins were characterized by both thermal and chemical denaturation. Gln and aromatic --> Leu mutants were consistently less stable than corresponding Met mutants, reflecting the nonisosteric nature of these substitutions. The strength of the cation-pi interaction was assessed by the value of the change in the free energy of unfolding [DeltaDeltaG(degrees) = DeltaG(degrees)(Met) - DeltaG(degrees)(WT)]. This ranged from +1.1 to -1.9 kcal/mol (average value -0.4 kcal/mol) at 298 K and +0.7 to -2.6 kcal/mol (average value -1.1 kcal/mol) at the Tm of each WT. It therefore appears that the strength of cation-pi interactions increases with temperature. In addition, the experimentally measured values are appreciably smaller in magnitude than calculated values with an average difference /DeltaG(degrees)expt - DeltaG(degrees)calc/av of 2.9 kcal/mol. At room temperature, the data indicate that cation-pi interactions are at best weakly stabilizing and in some cases are clearly destabilizing. However, at elevated temperatures, close to typical Tm's, cation-pi interactions are generally stabilizing.     

Effects of heme pocket structure and mobility on cytochrome c stability

Unfolding thermodynamics of a thermophilic cytochrome c552 from Hydrogenobacter thermophilus (Ht cyt c552) and its mesophilic homologue from Pseudomonas aeruginosa (Pa cyt c551) as well as two heme pocket point mutants (Ht-Q64N and Pa-N64Q) are characterized by determination of protein stability curves (plots of unfolding free energy, DeltaG, vs T). These proteins show revealing differences in heme pocket hydrogen bonding and mobility. It previously has been shown that Asn64 in Pa cyt c551 and in Ht-Q64N interacts with the heme axial Met to fix it in a single conformation [Wen, X., and Bren, K. L. (2005) Biochemistry 44, 5225-5233]. In Ht cyt c552 and Pa-N64Q, Gln64 does not interact with the axial Met; in these variants the axial Met samples more than one conformation [Wen, X., and Bren, K. L. (2005) Inorg. Chem. 44, 8587-8593]. Here it is demonstrated that, relative to wild type, Pa-N64Q displays enhanced stability with an increase in unfolding free energy (DeltaDeltaG) of 7.1 kJ/mol and an increase in denaturation temperature (DeltaTm) of 8 degrees C. Correspondingly, Ht-Q64N is less stable than Ht cyt c552, with a DeltaDeltaG of -10 kJ/mol and a DeltaTm of -10 degrees C. Analysis of unfolding thermodynamics indicates that the net changes in stability resulting from the position 64 mutations are primarily attributable to entropic factors. For Pa-N64Q (Ht-Q64N) it is proposed that the favorable release (unfavorable burial) of residue 64 is the dominant factor impacting stability. The mobility of the axial Met also is proposed to contribute. These results provide a specific illustration of how amino acid side chain mobility and burial or release contribute to protein stability.