Rational Design of Disulfide Bonds Increases Thermostability of a Mesophilic 1,3-1,4-β-Glucanase from Bacillus terquilensis

Comprehensive Understanding of Laboratory Evolution for Food Enzymes: From Design to Screening Innovations

In the field of food processing, enzymes play a pivotal role in improving product quality and flavor, and extending shelf life. However, the exposure of traditional food enzymes to high temperatures during processing often leads to a decrease in activity or even inactivation, limiting the effectiveness of their application under high-temperature conditions. Therefore, the modification of thermostability and activity of enzymes to adapt to extreme conditions through protein engineering has become a key way to improve the efficiency and economic benefits of industrial production. Directed evolution and semirational design strategies in the laboratory have proven to be broadly applicable frameworks for biochemical researchers in the food field, including those who are beginners. In this review, we systematically summarize semirational design strategies and high-throughput screening strategies, and introduce some intuitive computer simulation software to improve the thermostability and enzyme activity of food enzymes. The application of these strategies and techniques provides a comprehensive guide for the optimization of food enzymes. In addition, the latest hot topics of genetically engineered food enzymes in the field of application are discussed.

Thermal Characterization and Interaction of the Subunits from the Multimeric Bacteriophage Endolysin PlyC

Bacteriophage endolysins degrade the bacterial peptidoglycan and are considered enzymatic alternatives to small-molecule antibiotics. In particular, the multimeric streptococcal endolysin PlyC has appealing antibacterial properties. However, a comprehensive thermal analysis of PlyC is lacking, which is necessary for evaluating its long-term stability and downstream therapeutic potential. Biochemical and kinetic-based methods were used in combination with differential scanning calorimetry to investigate the structural, kinetic, and thermodynamic stability of PlyC and its various subunits and domains. The PlyC holoenzyme structure is irreversibly compromised due to partial unfolding and aggregation at 46 °C. Unfolding of the catalytic subunit, PlyCA, instigates this event, resulting in the kinetic inactivation of the endolysin. In contrast to PlyCA, the PlyCB octamer (the cell wall-binding domain) is thermostable, denaturing at ~75 °C. The isolation of PlyCA or PlyCB alone altered their thermal properties. Contrary to the holoenzyme, PlyCA alone unfolds uncooperatively and is thermodynamically destabilized, whereas the PlyCB octamer reversibly dissociates into monomers and forms an intermediate state at 74 °C in phosphate-buffered saline with each subunit subsequently denaturing at 92 °C. Adding folded PlyCA to an intermediate state PlyCB, followed by cooling, allowed for in vitro reconstitution of the active holoenzyme.

Purification, characterization, immobilization and applications of an enzybiotic β-1,3-1,4-glucanase produced from halotolerant marine Halomonas meridiana ES021

Extracellular β-1,3-1,4-glucanase-producing strain Halomonas meridiana ES021 was isolated from Gabal El-Zeit off shore, Red Sea, Egypt. The Extracellular enzyme was partially purified by precipitation with 75% acetone followed by anion exchange chromatography on DEAE-cellulose, where a single protein band was determined with molecular mass of approximately 72 kDa. The Km value was 0.62 mg β-1,3-1,4-glucan/mL and Vmax value was 7936 U/mg protein. The maximum activity for the purified enzyme was observed at 40 °C, pH 5.0, and after 10 min of the reaction. β-1,3-1,4-glucanase showed strong antibacterial effect against Bacillus subtilis, Streptococcus agalactiae and Vibrio damsela. It also showed antifungal effect against Penicillium sp. followed by Aspergillus niger. No toxicity was observed when tested on Artemia salina. Semi-purified β-1,3-1,4-glucanase was noticed to be effective in clarification of different juices at different pH values and different time intervals. The maximum clarification yields were 51.61% and 66.67% on mango juice at 40 °C and pH 5.3 for 2 and 4 h, respectively. To our knowledge, this is the first report of β-1,3-1,4-glucanase enzyme from halotolerant Halomonas species.

Rational design of a disulfide bridge increases the thermostability of microbial transglutaminase

Microbial transglutaminase (MTG) has numerous industrial applications in the food and pharmaceutical sectors. Unfortunately, the thermostability of MTG is too low to tolerate the desired conditions used in many of these commercial processes. In a previous study, we used protein engineering to improve the thermostability of MTG. Specifically, we generated a T7C/E58C mutant of MTG from Streptomyces mobaraensis that displayed enhanced resistance to thermal inactivation. In this study, a rational structure-based approach was adopted to introduce a disulfide bridge to further increase the thermostability of MTG. In all, four new mutants, each containing a novel disulfide bond, were engineered. Of these four mutants, D3C/G283C showed the most promising thermostability with a significantly higher ∆T₅₀ (defined as the temperature of incubation at which 50% of the initial activity remains) of + 9 °C by comparison to wild-type MTG. Indeed, D3C/G283C combined enhanced thermostability with a 2.1-fold increased half-life at 65 °C compared with the wild-type enzyme. By structure-based rational design, we were able to create an MTG variant which might be useful for expanding the scope of application in food. KEY POINTS: • Microbial transglutaminase (MTG) is an enzyme used in many food applications • The applicability of MTG to various industrial processes other than the food sector is being investigated • Improvement of thermostability was confirmed for the disulfide bridge mutant D3C/G283C.

Construction of L-Asparaginase Stable Mutation for the Application in Food Acrylamide Mitigation

Acrylamide, a II A carcinogen, widely exists in fried and baked foods. L-asparaginase can inhibit acrylamide formation in foods, and enzymatic stability is the key to its application. In this study, the Escherichia coli L-asparaginase (ECA) stable variant, D60W/L211R/L310R, was obtained with molecular dynamics (MD) simulation, saturation mutation, and combinatorial mutation, the half-life of which increased to 110 min from 60 min at 50 °C. Furthermore, the working temperature (maintaining the activity above 80%) of mutation expanded from 31 °C–43 °C to 35 °C–55 °C, and the relative activity of mutation increased to 82% from 65% at a pH range of 6–10. On treating 60 U/mL and 100 U/g flour L-asparaginase stable mutant (D60W/L211R/L310R) under uncontrolled temperature and pH, the acrylamide content of potato chips and bread was reduced by 66.9% and 51.7%, which was 27% and 49.9% higher than that of the wild type, respectively. These results demonstrated that the mutation could be of great potential to reduce food acrylamide formation in practical applications.

Design, expression and functional assessment of novel engineered serratiopeptidase analogs with enhanced protease activity and thermal stability

Serratiopeptidase is a bacterial protease that has been used medicinally in variety of applications. Though, some drawbacks like sensitivity to environmental conditions and low penetration into cells limited its usage as a potent pharmaceutical agent. This study aimed to produce four novel truncated serratiopeptidase analogs with different lengths and possessing one disulfide bridge, in order to enhance protease activity and thermal stability of this enzyme. Mutagenesis and truncation were performed using specific primers by conventional and overlap PCR. The recombinant proteins were expressed in E. coli cells then purified and their protease activity and stability were checked at different pH and temperatures in comparison to the native form of the enzyme, Serra473. Enzyme activity assay showed that T306 [12-302 ss] was not further active which could be due to the large truncation. However, T344 [8-339 ss], T380 [8-339 ss] and T380 [12-302 ss] proteins showed higher proteolytic activity comparing to Serra473. These analogs were active at temperatures of 25-90 °C and pH 6-9.5. Interestingly, remaining enzyme activity of T344 [8-339 ss], T380 [8-339 ss] and T380 [12-302 ss] forms at 90 °C calculated as 87, 83 and 86 percent, respectively, comparing to the activity at room temperature. However, residual activity at the same conditions was 50% for the full length enzyme. Formation of disulfide bond in engineered serratiopeptidases could be the main reason for higher thermal stability compared to Serra473. Thermostability of T344 [8-339 ss], as the most thermostable designed serratiopeptidase, was additionally confirmed using differential scanning calorimetry.

Enhanced Thermostability of D-Psicose 3-Epimerase from Clostridium bolteae through Rational Design and Engineering of New Disulfide Bridges

D-psicose 3-epimerase (DPEase) catalyzes the isomerization of D-fructose to D-psicose (aka D-allulose, a low-calorie sweetener), but its industrial application has been restricted by the poor thermostability of the naturally available enzymes. Computational rational design of disulfide bridges was used to select potential sites in the protein structure of DPEase from Clostridium bolteae to engineer new disulfide bridges. Three mutants were engineered successfully with new disulfide bridges in different locations, increasing their optimum catalytic temperature from 55 to 65 °C, greatly improving their thermal stability and extending their half-lives (t_1/2) at 55 °C from 0.37 h to 4−4.5 h, thereby greatly enhancing their potential for industrial application. Molecular dynamics simulation and spatial configuration analysis revealed that introduction of a disulfide bridge modified the protein hydrogen–bond network, rigidified both the local and overall structures of the mutants and decreased the entropy of unfolded protein, thereby enhancing the thermostability of DPEase.

Computational Design of Oligosaccharide-Producing Levansucrase from Bacillus licheniformis RN-01 to Increase Its Stability at High Temperature

Levan-type fructooligosaccharides (LFOs) and levan can potentially be used as ingredients in prebiotics, skincare products, and antitumor agents. The Y246S mutant of Bacillus licheniformis RN-01 levansucrase (oligosaccharide-producing levansucrase, OPL) was reported to productively synthesize LFOs; however, OPL's thermostability is low at high temperatures. To enhance OPL structural stability, this study employed molecular dynamics (AMBER) to identify a highly flexible region, as measured by its average root-mean-square fluctuation (RMSF) value, on the OPL surface and computational protein design (Rosetta) to rigidify and increase favorable interactions to increase its structural stability. AMBER identified region nine (residues 277-317) as a highly flexible region that was selected for design because it has the highest number of residues and the second-highest average RMSF, and it is farthest from the active site. Rosetta designed 14 mutants with the best ΔΔG value in each position, where three mutants have better ΔG than OPL. To determine whether their flexibilities and stabilities are lower than those of OPL, all 14 designed mutants were simulated at high temperature (500 K), and we found that K296E, G309S, and A310W mutants were predicted to be more stable and could retain their native structures better than OPL. Our results suggest that enhanced structural stabilities of these mutants are caused by increased hydrogen bond strengths of the designed residues and their neighboring residues. This study designed K296E, G309S, and A310W mutants of OPL with high potential for stability improvement, and they could potentially be used for the effective production of LFOs.

Cysteine Engineering of an Endo-polygalacturonase from Talaromyces leycettanus JCM 12802 to Improve Its Thermostability

Thermostable enzymes have many advantages for industrial applications. Therefore, in this study, computer-aided design technology was used to improve the thermostability of a highly active endo-polygalacturonase from Talaromyces leycettanus JCM12802 at an optimal temperature of 70 °C. The melting temperature and specific activity of the obtained mutant T316C/G344C were increased by 10 °C and 36.5%, respectively, compared with the wild-type enzyme. The crystal structure of the T316C/G344C mutant showed no formation of a disulfide bond between the introduced cysteines, indicating a different mechanism than the conventional mechanism underlying improved enzyme thermostability. The cysteine substitutions directly formed a new alkyl hydrophobic interaction and caused conformational changes in the side chains of the adjacent residues Asn315 and Thr343, which in turn caused a local reconstruction of hydrogen bonds. This method greatly improved the thermostability of the enzyme without affecting its activity; thus, our findings are of great significance for both theoretical research and practical applications.

Extra disulfide and ionic salt bridge improves the thermostability of lignin peroxidase H8 under acidic condition

The development of a lignin peroxidase (LiP) that is thermostable even under acidic pH conditions is a main issue for efficient enzymatic lignin degradation due to reduced repolymerization of free phenolic products at acidic pH (< 3). Native LiP under mild conditions (half-life (t_1/2) of 8.2 days at pH 6) exhibits a marked decline in thermostability under acidic conditions (t_1/2 of only 14 min at pH 2.5). Thus, improving the thermostability of LiP in acidic environments is required for effective lignin depolymerization in practical applications. Here, we show the improved thermostability of a synthetic LiPH8 variant (S49C/A67C/H239E, PDB: 6ISS) capable of strengthening the helix-loop interactions under acidic conditions. This variant retained excellent thermostability at pH 2.5 with a 10-fold increase in t_1/2 (2.52 h at 25 °C) compared with that of the native enzyme. X-ray crystallography analysis showed that the recombinant LiPH8 variant is the only unique lignin peroxidase containing five disulfide bridges, and the helix-loop interactions of the synthetic disulfide bridge and ionic salt bridge in its structure are responsible for stabilizing the Ca²⁺-binding region and heme environment, resulting in an increase in overall structural resistance against acidic conditions. Our work will allow the design of biocatalysts for ligninolytic enzyme engineering and for efficient biocatalytic degradation of plant biomass in lignocellulose biorefineries.

Improving the kinetic stability of a hyperthermostable β-mannanase by a rationally combined strategy

Feasible and easily accessible methods for the rational design of enzyme engineering strategies remain to be established. Thus, a new rationally combined strategy based on disulfide bond engineering and HotSpot Wizard 3.0 was proposed and experimentally demonstrated to be effective using a hyperthermostable β-mannanase. Ten of 42 mutants showed prominent enhancement of kinetic stability with 26.4%-39.9% increases in t_1/2 (75 °C) compared with the parent enzyme ManAKH. The best mutant, D273-V308, showed apparent increases in both optimal temperature (5 °C) and T₅₀ (6.8 °C), as well as advanced catalytic efficiency. The low rate of inactive mutants and the high rate of positive mutants indicated that newly introduced screening factors (distance from catalytic residues, Gibbs free energy term, molecular simulation, and visual inspections) greatly enhance the design of thermostable β-mannanase. Moreover, these findings further advance the industrial application of β-mannanase (ManAK) in food and food-related applications.

Biotransformation of lignocellulosic biomass into industrially relevant products with the aid of fungi-derived lignocellulolytic enzymes

Lignocellulosic material has drawn significant attention among the scientific community due to its year-round availability as a renewable resource for industrial consumption. Being an economic substrate alternative, various industries are reevaluating processes to incorporate derived compounds from these materials. Varieties of fungi and bacteria have the ability to depolymerize lignocellulosic biomass by synthesizing degrading enzymes. Owing to catalytic activity stability and high yields of conversion, lignocellulolytic enzymes derived from fungi currently have a high spectrum of industrial applications. Moreover, these materials are cost effective, eco-friendly and nontoxic while having a low energy input. Techno-economic analysis for current enzyme production technologies indicates that synthetic production is not commercially viable. Instead, the economic projection of the use of naturally-produced ligninolytic enzymes is promising. This approach may improve the economic feasibility of the process by lowering substrate expenses and increasing lignocellulosic by-product's added value. The present review will discuss the classification and enzymatic degradation pathways of lignocellulolytic biomass as well as the potential and current industrial applications of the involved fungal enzymes.

Degradation of ochratoxin A by supernatant and ochratoxinase of Aspergillus niger W-35 isolated from cereals

Ochratoxin A (OTA) is a mycotoxin produced by Aspergillus spp. and Penicillium spp. and poses a threat to food safety. Biodegradation may be a promising strategy for reducing the OTA contamination in the future. In this study, Aspergillus niger strain W-35 was isolated from cereals and studied for its ability to degrade OTA. Results showed that the supernatant of W-35 could degrade OTA both in vitro and in commercial feeds after incubation at 37 °C for 12 h by 78.0 and 37.0%, respectively. Ochratoxin α (OTα) was assayed as a degradation product by HPLC-FLD. Furthermore, an enzyme specific for OTA degradation (ochratoxinase, OTase) obtained from W-35 was successfully expressed in Escherichia coli BL21, and degraded OTA at a rate of 85.1% for 12 h. These results indicated that this OTA degradation is enzymatic and that the responsible enzyme is extracellular OTase. Reliable degradation of OTA has the potential for wide-ranging applications in the food and feed industries.

Improving the thermostability and activity of Paenibacillus pasadenensis chitinase through semi-rational design

Chitinase is a promising biocatalyst for chitin biotransformation in the field of recalcitrant biomass degradation. Excellent catalytic performance is conducive to its commercial utilization. In this work, sequence- and structure-based semi-rational design was performed to evolve the thermostability and activity of a previously identified chitinase PpChi1 from Paenibacillus pasadenensis CS0611. After combinational mutagenesis, the mutant S244C-I319C/T259P with disulfide bond introduction and proline substitution exhibited higher specific activity at higher temperature, 26.3-fold in half-life value at 50 °C, and a 7.9 °C rise in half-inactivation temperature T_1/2^15min compared to the wild-type enzyme. The optimal reaction temperature of the mutant was shifted from 45 °C to 52.5 °C. Molecular dynamic simulation and structure analysis confirmed that these improvements of the mutant were attributed to its stabilized folding form, possibly caused by the decreased entropy of unfolding. This work gives an initial insight into the effect of conserved proline residues in thermostable chitinases and proposes a feasible approach for improving chitinase thermostability to facilitate its application in chitin hydrolysis to valuable oligosaccharides.

Finding the generalized molecular principles of protein thermal stability

Are there any generalized molecular principles of thermal adaptation? Here, integrating the concepts of structural bioinformatics, sequence analysis, and classical knot theory, we develop a robust computational framework that seeks for mechanisms of thermal adaptation by comparing orthologous mesophilic-thermophilic and mesophilic-hyperthermophilic proteins of remarkable structural and topological similarities, and still leads us to context-independent results. A comprehensive analysis of 4741 high-resolution, non-redundant X-ray crystallographic structures collected from 11 hyperthermophilic, 32 thermophilic and 53 mesophilic prokaryotes unravels at least five "nearly universal" signatures of thermal adaptation, irrespective of the enormous sequence, structure, and functional diversity of the proteins compared. A careful investigation further extracts a set of amino acid changes that can potentially enhance protein thermal stability, and remarkably, these mutations are overrepresented in protein crystallization experiments, in disorder-to-order transitions and in engineered thermostable variants of existing mesophilic proteins. These results could be helpful to find a precise, global picture of thermal adaptation.

Rational Design of Alginate Lyase from Microbulbifer sp. Q7 to Improve Thermal Stability

Alginate lyase degrades alginate by the β-elimination mechanism to produce oligosaccharides with special bioactivities. The low thermal stability of alginate lyase limits its industrial application. In this study, introducing the disulfide bonds while using the rational design methodology enhanced the thermal stability of alginate lyase cAlyM from Microbulbifer sp. Q7. Enzyme catalytic sites, secondary structure, spatial configuration, and molecular dynamic simulation were comprehensively analyzed. When compared with cAlyM, the mutants D102C-A300C and G103C-T113C showed an increase by 2.25 and 1.16 h, respectively, in half-life time at 45 °C, in addition to increases by 1.7 °C and 0.4 °C in the melting temperature, respectively. The enzyme-specific activity and k_cat/K_m values of D102C-A300C were 1.8- and 1.5-times higher than those of cAlyM, respectively. The rational design strategy that was used in this study provides a valuable method for improving the thermal stability of the alginate lyase.

Disulfide Bond Engineering of an Endoglucanase from Penicillium verruculosum to Improve Its Thermostability

Endoglucanases (EGLs) are important components of multienzyme cocktails used in the production of a wide variety of fine and bulk chemicals from lignocellulosic feedstocks. However, a low thermostability and the loss of catalytic performance of EGLs at industrially required temperatures limit their commercial applications. A structure-based disulfide bond (DSB) engineering was carried out in order to improve the thermostability of EGLII from Penicillium verruculosum. Based on in silico prediction, two improved enzyme variants, S127C-A165C (DSB2) and Y171C-L201C (DSB3), were obtained. Both engineered enzymes displayed a 15⁻21% increase in specific activity against carboxymethylcellulose and β-glucan compared to the wild-type EGLII (EGLII-wt). After incubation at 70 °C for 2 h, they retained 52⁻58% of their activity, while EGLII-wt retained only 38% of its activity. At 80 °C, the enzyme-engineered forms retained 15⁻22% of their activity after 2 h, whereas EGLII-wt was completely inactivated after the same incubation time. Molecular dynamics simulations revealed that the introduced DSB rigidified a global structure of DSB2 and DSB3 variants, thus enhancing their thermostability. In conclusion, this work provides an insight into DSB protein engineering as a potential rational design strategy that might be applicable for improving the stability of other enzymes for industrial applications.

Crystal structure and catalytic characterization of the dehydrogenase/reductase SDR family member 4 (DHRS4) from Caenorhabditis elegans

The human dehydrogenase/reductase SDR family member 4 (DHRS4) is a tetrameric protein that is involved in the metabolism of several aromatic carbonyl compounds, steroids, and bile acids. The only invertebrate DHRS4 that has been characterized to date is that from the model organism Caenorhabditis elegans. We have previously cloned and initially characterized this protein that was recently annotated as DHRS4_CAEEL in the UniProtKB database. Crystallization and X-ray diffraction studies of the full-length DHRS4_CAEEL protein in complex with diacetyl revealed its tetrameric structure and showed that two subunits are connected via an intermolecular disulfide bridge that is formed by N-terminal cysteine residues (Cys5) of each protein chain, which increases the enzymatic activity. A more detailed biochemical and catalytic characterization shows that DHRS4_CAEEL shares some properties with human DHRS4 such as relatively low substrate affinities with aliphatic α-diketones and a preference for aromatic dicarbonyls such as isatin, with a 30-fold lower Km value compared with the human enzyme. Moreover, DHRS4_CAEEL is active with aliphatic aldehydes (e.g. hexanal), while human DHRS4 is not. Dehydrogenase activity with alcohols was only observed with aromatic alcohols. Protein thermal shift assay revealed a stabilizing effect of phosphate buffer that was accompanied by an increase in catalytic activity of more than two-fold. The study of DHRS4 homologs in simple lineages such as C. elegans may contribute to our understanding of the original function of this protein that has been shaped by evolutionary processes in the course of the development from invertebrates to higher mammalian species.

DATABASE

Structural data are available in the PDB under the accession numbers 5OJG and 5OJI.

Evaluation of irreversible protein thermal inactivation caused by breakage of disulphide bonds using methanethiosulphonate

Many extracellular globular proteins have evolved to possess disulphide bonds in their native conformations, which aids in thermodynamic stabilisation. However, disulphide bond breakage by heating leads to irreversible protein denaturation through disulphide-thiol exchange reactions. In this study, we demonstrate that methanethiosulphonate (MTS) specifically suppresses the heat-induced disulphide-thiol exchange reaction, thus improving the heat-resistance of proteins. In the presence of MTS, small globular proteins that contain disulphides can spontaneously refold from heat-denatured states, maintaining wild-type disulphide pairing. Because the disulphide-thiol exchange reaction is triggered by the generation of catalytic amounts of perthiol or thiol, rapid and specific perthiol/thiol protection by MTS reagents prevents irreversible denaturation. Combining MTS reagents with another additive that suppresses chemical modifications, glycinamide, further enhanced protein stabilisation. In the presence of these additives, reliable remnant activities were observed even after autoclaving. However, immunoglobulin G and biotin-binding protein, which are both composed of tetrameric quaternary structures, failed to refold from heat-denatured states, presumably due to chaperon requirements. Elucidation of the chemical modifications involved in irreversible thermoinactivation is useful for the development of preservation buffers with optimum constitutions for specific proteins. In addition, the impact of disulphide bond breakage on the thermoinactivation of proteins can be evaluated using MTS reagents.

Insights from molecular dynamics simulations for computational protein design

A grand challenge in the field of structural biology is to design and engineer proteins that exhibit targeted functions. Although much success on this front has been achieved, design success rates remain low, an ever-present reminder of our limited understanding of the relationship between amino acid sequences and the structures they adopt. In addition to experimental techniques and rational design strategies, computational methods have been employed to aid in the design and engineering of proteins. Molecular dynamics (MD) is one such method that simulates the motions of proteins according to classical dynamics. Here, we review how insights into protein dynamics derived from MD simulations have influenced the design of proteins. One of the greatest strengths of MD is its capacity to reveal information beyond what is available in the static structures deposited in the Protein Data Bank. In this regard simulations can be used to directly guide protein design by providing atomistic details of the dynamic molecular interactions contributing to protein stability and function. MD simulations can also be used as a virtual screening tool to rank, select, identify, and assess potential designs. MD is uniquely poised to inform protein design efforts where the application requires realistic models of protein dynamics and atomic level descriptions of the relationship between dynamics and function. Here, we review cases where MD simulations was used to modulate protein stability and protein function by providing information regarding the conformation(s), conformational transitions, interactions, and dynamics that govern stability and function. In addition, we discuss cases where conformations from protein folding/unfolding simulations have been exploited for protein design, yielding novel outcomes that could not be obtained from static structures.

Physical and molecular bases of protein thermal stability and cold adaptation

The molecular bases of thermal and cold stability and adaptation, which allow proteins to remain folded and functional in the temperature ranges in which their host organisms live and grow, are still only partially elucidated. Indeed, both experimental and computational studies fail to yield a fully precise and global physical picture, essentially because all effects are context-dependent and thus quite intricate to unravel. We present a snapshot of the current state of knowledge of this highly complex and challenging issue, whose resolution would enable large-scale rational protein design.