Importance of surrounding residues for protein stability of partially buried mutations

For understanding the factors influencing protein stability, we have analyzed the relationship between changes in protein stability caused by partially buried mutations and changes in 48 physico-chemical, energetic and conformational properties of amino acid residues. Multiple regression equations were derived to predict the stability of protein mutants and the efficiency of the method has been verified with both back-check and jack-knife tests. We observed a good agreement between experimental and computed stabilities. Further, we have analyzed the effect of sequence window length from 1 to 12 residues on each side of the mutated residue to include the sequence information for predicting protein stability and we found that the preferred window length for obtaining the highest correlation is different for each secondary structure; the preferred window length for helical, strand and coil mutations are, respectively, 0, 9 and 4 residues on both sides of the mutant residues. However, all the secondary structures have significant correlation for a window length of one residue on each side of the mutant position, implying the role of short-range interactions. Extraction of surrounding residue information for various distances (3 to 20A) around the mutant position showed the highest correlation at 8A, 6A and 7A, respectively, for mutations in helical, strand and coil segments. Overall, the information about the surrounding residues within the sphere of 7 to 8A, may explain better the stability in all subsets of partially buried mutations implying that this distance is sufficient to accommodate the residues influenced by major intramolecular interactions for the stability of protein structures.     

Relationship Between Amino Acid Properties and Protein Stability: Buried Mutations

In order to understand the mechanism of protein stability and to develop a simple method for predicting mutation-induced stability changes, we analyzed the relationship between stability changes caused by buried mutations and changes in 48 amino acid properties. As expected from the importance of hydrophobicity, properties reflecting hydrophobicity are strongly correlated with the stability of proteins. We found that subgroup classification based on secondary structure increased correlations significantly, and mutations within -strand segments correlated better than did those in -helical segments, which may result from stronger hydrophobicity of the -strands. Multiple regression analyses incorporating combinations of three properties from among all possible combinations of the 48 properties increased the correlation coefficient to 0.88 and by an average of 13% for all data sets. Analyzing the stability of tryptophan synthase mutants with Glu49 replaced by all other residues except Arg revealed that combining buriedness, solvent-accessible surface area for denatured protein, and unfolding Gibbs free energy change increased the correlation to 0.95. Consideration of sequence and structural information (neighboring residues in sequence and in space) did not significantly strengthen the correlations in buried mutations, suggesting that nonspecific interactions dominate in the interior of proteins.     

Revisiting “reverse hydrophobic effect”: Applicable only to coil mutations at the surface

In a seminal paper, Pakula and Sauer (Nature, 1990, 344, 363–364) demonstrated that the increase in side‐chain hydrophobicity has a reverse relationship with protein stability. We have addressed this problem with several examples of mutants that span at different locations in protein structure based on secondary structure and solvent accessibility. We confirmed that the stability change upon single coil mutation at exposed region is reversely correlated with hydrophobicity with a single exception. In addition, we found the existence of such relationship in partially buried coil mutants. The stability of exposed helical mutants is governed by conformational properties. In buried and partially buried helical and strand mutants properties reflecting hydrophobicity have direct relationship with stability, whereas an opposite relationship was obtained with entropy and flexibility. The structural analysis of partially buried/exposed mutants showed that the surrounding residues are important for the stability change upon mutation. These results provide insights to understand the general behavior for the stability of proteins upon amino acid substitutions. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 591–599, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Molecular dynamics simulation of the unfolding of individual bacteriorhodopsin helices in sodium dodecyl sulfate micelles

We report molecular dynamics simulations of the trends in the changes in secondary structure of the seven individual helices of bacteriorhodopsin when inserted into sodium dodecyl sulfate (SDS) micelles, and their dependence on the amino acid sequence. The results indicate that the partitioning of the helices in the micelles and their stability are dependent on the hydrophobicity of the transmembrane segments. Helices A, B, and E are stable and retain their initial secondary structure throughout the 100 ns simulation time. In contrast, helices C, D, F, and G show structural perturbations within the first 10 ns. The instabilities are localized near charged residues within the transmembrane segments. The overall structural instability of the helix is correlated with its partitioning to the surface of the micelle and its interaction with polar groups there. The in silico experiments were performed to complement the in vitro experiments that examined the partial denaturation of bacteriorhodopsin in SDS described in the preceding article (DOI 10.1021/bi201769z ). The simulations are consistent with the trends revealed by the experimental results but strongly underestimate the extent of helix to extended coil transformation. The reason may be either that the sampling time was not sufficiently long or, more interestingly, that interhelix residue interactions play a role in the unfolding of the helices.     

Importance of mutant position in Ramachandran plot for predicting protein stability of surface mutations

Understanding the mechanisms by which mutations affect protein stability is one of the most important problems in molecular biology. In this work, we analyzed the relationship between changes in protein stability caused by surface mutations and changes in 49 physicochemical, energetic, and conformational properties of amino acid residues. We found that the hydration entropy was the major contributor to the stability of surface mutations in helical segments; other properties responsible for size and volume of molecule also correlated significantly with stability. Classification of coil mutations based on their locations in the (phi-psi) map improved the correlation significantly, demonstrating the existence of a relationship between stability and strain energy, which indicates that the role of strain energy is very important for the stability of surface mutations. We observed that the inclusion of sequence and structural information raised the correlation, indicating the influence of surrounding residues on the stability of surface mutations. Further, we examined the previously reported "inverse relationship" between stability and hydrophobicity, and observed that the inverse hydrophobic effect was generally applicable only to coil mutations. The present study leads to a simple method for predicting protein stability changes caused by amino acid substitutions, which will be useful for protein engineering in designing novel proteins with increased stability and altered function.     

NMR characterization of residual structure in the denatured state of protein L

Triple-resonance NMR experiments were used to assign the (13)C(alpha), (13)C(beta), (15)N and NH resonances for all the residues in the denatured state of a destabilized protein L variant in 2 M guanidine. The chemical shifts of most resonances were very close to their random coil values. Significant deviations were observed for G22, L38 and K39; increasing the denaturant concentration shifted the chemical shifts of these residues towards theory random coil values. Medium-range nuclear Overhauser enhancements were detected in segments corresponding to the turn between the first two strands, the end of the second strand through the turn between the second strand and the helix, and the turn between the helix and the third strand in 3D H(1), N(15)-HSQC-NOESY-HSQC experiments on perdeuterated samples. Longer-range interactions were probed by measuring the paramagnetic relaxation enhancement produced by nitroxide spin labels introduced via cysteine residues at five sites around the molecule. Damped oscillations in the magnitude of the paramagnetic relaxation enhancement as a function of distance along the sequence suggested native-like chain reversals in the same three turn regions. The more extensive interactions within the region corresponding to the first beta-turn than in the region corresponding to the second beta-turn suggests that the asymmetry in the folding reaction evident in previous studies of the protein L folding transition state is already established in the denatured state.     

Stabilization of proteins by enhancement of inter-residue hydrophobic contacts: lessons of T4 lysozyme and barnase

Although the hydrophobic interactions are considered as the main contributors to the protein stability, not much examples of protein stabilization by rational increasing of this type of interactions still can be found in literature. This is partly due to the lack of proper theoretical "measure" of hydrophobic interactions and their changes upon mutations. In the present paper the molecular hydrophobicity potential approach is used to assess how the changes in type and the strength of inter-residue contacts upon single amino acid mutations are correlated with the changes in thermodynamic stability of T4 lysozyme and barnase mutants, and which factors affect these correlations. Mutations changing unfavorable hydrophilic-to-hydrophobic contacts into favorable hydrophobic were found to enhance the thermodynamic stability in more than 81 % of cases, if these mutations do not create steric bumps and do not involve proline residues and hydrogen-bonded side-chains. Mutations increasing hydrophobic contributions (according to molecular hydrophobicity potential formalism) lead to increase of thermodynamic stability in more than 94% of cases for certain type of mutations (i.e., mutations not involving charged residues, Pro and residues with side-chain hydrogen bonds, when these mutations do not introduce steric bumps and do not involve strongly exposed residues and residues situated at helix N- and C-cap positions). For this type of mutations the correlation was found between the change in hydrophobic contributions of mutated residues deltaCphob and thermodynamic parameters deltaTm (change in melting temperature) and deltadeltaG (change in free energy of unfolding). Although the correlation coefficients were larger if the experimental structures of mutants were used for the calculations (correlation coefficients r(exp) deltaC,deltaT = .85 and r(exp) deltaC,deltadeltaG = 0.87) than if the modeled structures were used instead (r(mod) deltaC,deltaT = 0.74 and r(mod)deltaC,deltadeltaG = 0.76), the modelled structures of mutants in the vast majority of cases can be used for qualitative predictition of the protein stabilization. Basing on the analysis of mutations increasing hydrophobic contributions in T4 lysozyme the substitution matrix was derived, which can be used to decide which new residue should be put instead the old one to increase the stability of protein. The estimation shows that the number of potential mutation sites for enhancement of hydrophobic interactions in T4 lysozyme is quite large, and only approximately 10 per cent of them were studied thus far. Basing on the current analysis of T4 lysozyme and barnase mutations the algorithm for increasing of protein stability via increasing of hydrophobic interactions for the proteins with known spatial structure is proposed.     

Analyses of the Sequence and Structural Properties Corresponding to Pentapeptide and Large Palindromes in Proteins

The analyses of 3967 representative proteins selected from the Protein Data Bank revealed the presence of 2803 pentapeptide and large palindrome sequences with known secondary structure conformation. These represent 2014 unique palindrome sequences. 60% palindromes are not associated with any regular secondary structure and 28% are in helix conformation, 11% in strand conformation and 1% in the coil conformation. The average solvent accessibility values are in the range between 0–155.28 Ų suggesting that the palindromes in proteins can be either buried, exposed to the solvent or share an intermittent property. The number of residue neighborhood contacts defined by interactions ≤ 3.2 Ǻ is in the range between 0–29 residues. Palindromes of the same length in helix, strand and coil conformation are associated with different amino acid residue preferences at the individual positions. Nearly, 20% palindromes interact with catalytic/active site residues, ligand or metal ions in proteins and may therefore be important for function in the corresponding protein. The average hydrophobicity values for the pentapeptide and large palindromes range between -4.3 to +4.32 and the number of palindromes is almost equally distributed between the negative and positive hydrophobicity values. The palindromes represent 107 different protein families and the hydrolases, transferases, oxidoreductases and lyases contain relatively large number of palindromes.     

Complementation of buried lysine and surface polar residues in a designed heterodimeric coiled coil

The coiled coil is an attractive target for protein design. The helices of coiled coils are characterized by a heptad repeat of residues denoted a to g. Residues at positions a and d form the interhelical interface and are usually hydrophobic. An established strategy to confer structural uniqueness to two-stranded coiled coils is the use of buried polar Asn residues at position a, which imparts dimerization and conformational specificity at the expense of stability. Here we show that polar interactions involving buried position-a Lys residues that can interact favorably only with surface e' or g' Glu residues also impart structural uniqueness to a designed heterodimeric coiled coil with the nativelike properties of sigmoidal thermal and urea-induced unfolding transitions, slow hydrogen exchange and lack of ANS binding. The position-a Lys residues do not, however, confer a single preference for helix orientation, likely reflecting the ability of Lys at position a to from favorable interactions with g' or e' Glu residues in the parallel and antiparallel orientations, respectively. The Lys-Glu polar interaction is less destabilizing than the Asn-Asn a-->a' interaction, presumably reflecting a higher desolvation penalty associated with the completely buried polar position-a groups. Our results extend the range of approaches for two-stranded coiled-coil design and illustrate the role of complementing polar groups associated with buried and surface positions of proteins in protein folding and design.     

Selection of a buried salt bridge by phage display

The α-helical coiled coil is a valuable folding motif for protein design and engineering. By means of phage display technology, we selected a capable binding partner for one strand of a coiled coil bearing a charged amino acid in a central hydrophobic core position. This procedure resulted in a novel coiled coil pair featuring an opposed Glu-Lys pair arranged staggered within the hydrophobic core of a coiled coil structure. Structural investigation of the selected coiled coil dimer by CD spectroscopy and MD simulations suggest that a buried salt bridge within the hydrophobic core enables the specific dimerization of two peptides.     

Structural consequences of an amino acid deletion in the B1 domain of protein G

We describe the NMR structure of a deletion mutant of the B1 IgG-binding domain from Group G Streptococcus. The deletion occurs within the last beta-strand of the protein, where it may potentially have a deleterious effect on the stability of the protein if the protein were not able to conformationally adjust to the perturbation. In particular, the deletion changes the registry of the final three residues in the sheet, forcing a polar Thr to be buried in the interior of the protein and exposing a hydrophobic Val to solvent. The deletion could also potentially create a large cavity in the beta-sheet and force the alpha- and gamma-carboxylates of the C-terminal Glu residue into a partially buried region of the sheet. The structure of the mutant illustrates how the conformation of the protein adjusts to the deletion, thereby mitigating some of the potentially deleterious consequences. Although the elements of secondary structure are retained between the mutant and the wt domain, there are multiple small adjustments in the segments connecting secondary structure elements. In particular, a hydrogen bond between the Glu57 carboxylates and two main chain amides is introduced that alters the conformation in the loop connecting the helix to strand 3. In addition, to minimize hydrophobic surface exposure, the turn connecting strands 1 and 2 folds toward the core so that the molecular volume is decreased.     

Prediction of protein mutant stability using classification and regression tool

Prediction of protein stability upon amino acid substitutions is an important problem in molecular biology and the solving of which would help for designing stable mutants. In this work, we have analyzed the stability of protein mutants using two different datasets of 1396 and 2204 mutants obtained from ProTherm database, respectively for free energy change due to thermal (DeltaDeltaG) and denaturant denaturations (DeltaDeltaG(H(2)O)). We have used a set of 48 physical, chemical energetic and conformational properties of amino acid residues and computed the difference of amino acid properties for each mutant in both sets of data. These differences in amino acid properties have been related to protein stability (DeltaDeltaG and DeltaDeltaG(H(2)O)) and are used to train with classification and regression tool for predicting the stability of protein mutants. Further, we have tested the method with 4 fold, 5 fold and 10 fold cross validation procedures. We found that the physical properties, shape and flexibility are important determinants of protein stability. The classification of mutants based on secondary structure (helix, strand, turn and coil) and solvent accessibility (buried, partially buried, partially exposed and exposed) distinguished the stabilizing/destabilizing mutants at an average accuracy of 81% and 80%, respectively for DeltaDeltaG and DeltaDeltaG(H(2)O). The correlation between the experimental and predicted stability change is 0.61 for DeltaDeltaG and 0.44 for DeltaDeltaG(H(2)O). Further, the free energy change due to the replacement of amino acid residue has been predicted within an average error of 1.08 kcal/mol and 1.37 kcal/mol for thermal and chemical denaturation, respectively. The relative importance of secondary structure and solvent accessibility, and the influence of the dataset on prediction of protein mutant stability have been discussed.     

Structural stability of wild type and mutated alpha-keratin fragments: molecular dynamics and free energy calculations

The objective of this study is to investigate the influence of point mutations on the structural stability of coiled coil fragments of the human hair intermediate filament by molecular dynamics simulations and free energy calculations. Mutations in the helix termination motif of human hair keratin gene hHb6 seem to be connected to the hereditary hair dystrophy Monilethrix. The most common mutations reported are Glu413Lys and Glu413Asp, located at the C-terminal end of the coiled coil 2B rod domain of the IF. According to our simulations, significant conformational changes of the side chains at the mutation and neighboring sites occur due to the Glu413Lys mutation. Furthermore, the differences in electrostatic interactions cause a large change in free energy during transformation of Glu413 to Lys calculated by the thermodynamic integration approach. It is speculated that the structural rearrangement necessary to adapt the interactions in the mutated coiled coil leads to changes in the IF assembly or its stability. The second mutation, Glu413Asp, only leads to a small value of the calculated free energy difference that is within the error limits of the simulations. Thus, it has to be concluded that this mutation does not affect the coiled coil stability.     

Role of medium- and long-range interactions to the stability of the mutants of T4 lysozyme

Inter-residue interactions play an important role to the folding and stability of protein molecules. In this work, we analyze the role of medium- and long-range interactions to the stability of T4 lysozyme mutants. We found that, in buried mutations, the increase in long-range contacts upon mutations destabilizes the protein, whereas, in surface mutations, the increase in long-range contacts increases the stability, indicating the importance of surrounding polar residues to the stability of surface mutations. Further, the increase in medium-range contacts decreases the stability of buried and surface mutations and a direct relationship is observed between the increase of medium-range contacts and increase in stability for partially buried/exposed mutations. Moreover, the relationship between amino acid properties and stability of T4 lysozyme mutants at positions Ile3, Phe53, and Leu99 showed that the effect of medium- and long-range contacts is less for buried mutations and the inter-residue contacts have significant correlation with the stability of partially buried mutations.     

Global secondary structure packing angle bias in proteins

While studies of secondary structure interactions have focused on local interacting features, there is a need for a more global characterization of packing-induced aligned packing of secondary structures. This study presents an analysis of the distribution of globally sampled secondary structures within selected subunits of a selected set of multimeric proteins. Comparisons are made between the distribution of the cosines of angles between triplets of linear segments associated to secondary structures and a theoretically obtained distribution for triplets of random uniformly distributed unit vectors. We show that, among all pairs of helix or strand segments, planar configurations appear more frequently than expected for uniformly distributed vectors, and alignment is strongly preferred compared to that expected for uniformly distributed vector triplets. Among all secondary structure triplets, pairs of angle cosines between helix strand segments deviate from uniformity corresponding to alignment and anti-alignment. Furthermore, among all helix or strand segments, including non-interacting secondary structures, the distribution of a single angle cosine indicates a strong preference for alignment and anti-alignment. Selection for interactive triplets shows results consistent with prior studies. Lastly, angle pairs are not statistically independent, indicating that alignment between two helix or strand segments is more likely if another helix or strand is aligned with either of the first two helices or strands. Selection for interactive segment triplets shows results consistent with prior studies.     

Average assignment method for predicting the stability of protein mutants

Prediction of protein stability upon amino acid substitutions is an important problem in molecular biology and it will be helpful for designing stable mutants. In this work, we have analyzed the stability of protein mutants using three different data sets of 1791, 1396, and 2204 mutants, respectively, for thermal stability (DeltaTm), free energy change due to thermal (DeltaDeltaG), and denaturant denaturations (DeltaDeltaGH2O), obtained from the ProTherm database. We have classified the mutants into 380 possible substitutions and assigned the stability of each mutant using the information obtained with similar type of mutations. We observed that this assignment could distinguish the stabilizing and destabilizing mutants to an accuracy of 70-80% at different measures of stability. Further, we have classified the mutants based on secondary structure and solvent accessibility (ASA) and observed that the classification significantly improved the accuracy of prediction. The classification of mutants based on helix, strand, and coil distinguished the stabilizing/destabilizing mutants at an average accuracy of 82% and the correlation is 0.56; information about the location of residues at the interior, partially buried, and surface regions of a protein correctly identified the stabilizing/destabilizing residues at an average accuracy of 81% and the correlation is 0.59. The nine subclassifications based on three secondary structures and solvent accessibilities improved the accuracy of assigning stabilizing/destabilizing mutants to an accuracy of 84-89% for the three data sets. Further, the present method is able to predict the free energy change (DeltaDeltaG) upon mutations within a deviation of 0.64 kcal/mol. We suggest that this method could be used for predicting the stability of protein mutants.     

Computation of non-covalent interactions in TNF proteins and interleukins

The roles played by the non-covalent interactions have been investigated for a set of six TNF proteins and nine Interleukins. The stabilizing residues have been identified by a consensus approach using the concepts of available surface area, medium and long-range interactions and conservation of amino acid residues. The cation-pi interactions have been computed based on a geometric approach such as distance and energy criteria. We identified an average of 1 energetically significant cation-pi interactions in every 94 residues in TNF proteins and 1 in every 62 residues in Interleukins. In TNF proteins, the cationic groups Lys preferred to be in helix while Arg preferred to be in strand regions while in Interleukins the Arg residues preferred to be in helix and Lys preferred to be in strand regions. From the available surface area calculations, we found that, almost all the cation and pi residues in TNF proteins and Interleukins were either in buried or partially buried regions and none of them in the exposed regions. Medium and long-range interactions were predominant in both TNF proteins and Interleukins. It was observed that the percentage of stabilizing centers were more in TNF proteins as compared to the Interleukins, while the percentage of conserved residues were more in Interleukins than in TNF proteins. In the stabilizing residues Lys was observed to be a stabilizing residue in both TNF proteins and Interleukins. Among the aromatic group, Phe was seen to be a stabilizing residue in both TNF and Interleukins. We suggest that this study on the computation of cation-pi interactions in TNF proteins and Interleukins would be very helpful in further understanding the structure, stability and functional similarity of these proteins.     

Influence of cation–π interactions to the structural stability of prokaryotic and eukaryotic translation elongation factors

We have investigated the role of cation–π interactions on translation elongation factors. In our investigation, an average of four significant cation–π interactions were found, that is, an average of one cation–π interaction per 44 residues in the ten elongation factors were observed. The analysis on the influence of short (<±4), medium (>±4 to <±20) and long (>20) range contacts showed that cation–π interactions are mainly formed by medium and long-range contacts. Arg-Tyr pair was found largest in number but energetic contribution of Arg-Trp pair was found most. Preferred secondary structural conformation analysis of the residues involved in cation–π interaction indicates that the cationic Arg prefers to be in helix and Lys having equal probability for helix and strand, whereas the aromatic Phe and Trp were found mostly in helix while Tyr in strand regions. The cation–π interaction residues involved in these proteins were found highly conserved with 48.86% residues having conservation score of ≥6. Analysis of secondary structure preference of the energetically significant cation–π residues in different solvent accessible range indicates that most of the π residues are found buried or partially buried whereas cationic residues were found mostly at the protein surface. The results presented in this study will be useful for structural stability studies in translation elongation factors.     

The contribution of buried polar groups to the conformational stability of the GCN4 coiled coil

The dimeric interface of the leucine zipper coiled coil from GCN4 has been used to probe the contributions of hydrophobic and hydrogen bonding interactions to protein stability. We have determined the energetics of placing Ile or Asn residues at four buried positions in a two-stranded coiled coil. As expected, Ile is favored over Asn at these buried positions, but not as much as predicted by considering only the hydrophobic effect. It appears that interstrand hydrogen bonds form between the side-chains of the buried Asn residues and these contribute to the conformational stability of the coiled-coil peptides. However, these contributions are highly dependent on the locations of the Asn pairs. The effect of an Ile to Asn mutation is greatest at the N terminus of the peptide and decreases almost twofold as we move the substitution from the N to C-terminal heptads.     

激动素、脱落酸和丙二醛对SOD活性影响及其与SOD构象和疏水性变化间的关系研究

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