Protein thermal stability enhancement by designing salt bridges: a combined computational and experimental study

Salt-bridge mediated conformational dynamics in the figure-of-eight knotted ketol acid reductoisomerase (KARI)

The utility of knotted proteins in biological activities has been ambiguous since their discovery. From their evolutionary significance to their functionality in stabilizing the native protein structure, a unilateral conclusion hasn't been achieved yet. While most studies have been performed to understand the stabilizing effect of the knotted fold on the protein chain, more ideas are yet to emerge regarding the interactions in stabilizing the knot. Using classical molecular dynamics (MD) simulations, we have explored the dynamics of the figure-of-eight knotted domain present in ketol acid reductoisomerase (KARI). Our main focus was on the presence of a salt bridge network evident within the knotted region and its role in shaping the conformational dynamics of the knotted chain. Through the potential of mean forces (PMFs) calculation, we have also marked the specific salt bridges that are pivotal in stabilizing the knotted structure. The correlated motions have been further monitored with the help of principal component analysis (PCA) and dynamic cross-correlation maps (DCCM). Furthermore, mutation of the specific salt bridges led to a change in their conformational stability, vindicating their importance.

Unveiling potential PET degrading eukaryotes through in silico bioprospecting of PETases

This study addresses the environmental problem of PET plastic through in silico bioprospecting for the identification and experimental validation of novel PET degrading eukaryotes through the in silico bioprospectingI of PETases, employing a methodology that combines Hidden Markov Models (HMMs), clustering techniques, molecular docking, and dynamic simulations. A total of 424 putative PETase sequences were identified from 219 eukaryotic organisms, highlighting six sequences with low affinity energies. The Aspergillus luchuensis sequence showed the lowest Gibbs free energy and exhibited stability at different temperatures in molecular dynamics assays. Experimental validation, through a plate clearance assay and HPLC, confirmed PETase activity in three wild-type fungal strains, with A. luchuensis showing the highest efficiency. The results obtained demonstrate the effectiveness of combining computational and experimental approaches as proof of concept to discover and validate eukaryotes with PET-degrading capabilities opening new perspectives for the sustainable management of this type of waste and contributing to its environmental mitigation.

Experimental modification in thermal stability of oligomers by alanine substitution and site saturation mutagenesis of interfacial residues

For certain industrial applications, the stability of protein oligomers is important. In this study, we demonstrated an efficient method to improve the thermal stability of oligomers using the trimeric protein chloramphenicol acetyltransferase (CAT) as the model. We substituted all interfacial residues of CAT with alanine to detect residues critical for oligomer stability. Mutation of six of the forty-nine interfacial residues enhanced oligomer thermal stability. Site saturation mutagenesis was performed on these six residues to optimize the side chains. About 15% of mutations enhanced thermal stability by more than 0.5 °C and most did not disrupt activity of CAT. Certain combinations of mutations further improved thermal stability and resistance against heat treatment. The quadruple mutant, H17V/N34S/F134A/D157C, retained the same activity as the wild-type after heat treatment at 9 °C higher temperature than the wild-type CAT. Furthermore, combinations with only alanine substitutions also improved thermal stability, suggesting the method we developed can be used for rapid modification of industrially important proteins.

Conserved Residues Lys64 and Glu78 at the Subunit Surface of Tau Glutathione Transferase in Rice Affect Structure and Enzymatic Properties

Glutathione transferases (GSTs) are a superfamily of dimeric proteins associated with the detoxification of various reactive electrophiles and responsive to a multitude of stressors. We individually substituted Lys64 and Glu78 with Ala using site-directed mutagenesis to understand the role of subunit interactions in the structure and enzymatic properties of a rice GST (OsGSTU17). The wild-type OsGSTU17 lost the conserved hydrogen bond between subunits in tau class GSTs due to conserved Tyr92 replaced with Phe92, but still exhibited high substrate activities, and thermal stability remained in its dimeric structure. The significant decrease in thermal stability and obvious changes in the structure of mutant K64A implied that conserved Lys64 might play an essential role in the structural stability of tau class GSTs. The mutant E78A, supposed to be deprived of hydrogen and salt bonds between subunits, appeared in the soluble form of dimers, even though its tertiary structure altered and stability declined dramatically. These results suggest that the hydrogen and ionic bonds provided by conserved residues are not as important for OsGSTU17 dimerization and enzymatic properties. These results further supplement our understanding of the relationship between the structure and function of GSTs and provide a theoretical basis for improving crop resistance through targeted modification of GSTs.

Computation-Based Design of Salt Bridges in PETase for Enhanced Thermostability and Performance for PET Degradation

Polyethylene terephthalate (PET) is one of the most widely used plastics, and the accumulation of PET poses a great threat to the environment. IsPETase can degrade PET rapidly at moderate temperatures, but its application is greatly limited by the low stability. Herein, molecular dynamics (MD) simulations combined with a sequence alignment strategy were adopted to introduce salt bridges into the flexible region of IsPETase to improve its thermal stability. In the designed variants, the Tm values of IsPETaseI168R/S188D and IsPETaseI168R/S188E were 7.4 and 8.7 °C higher than that of the wild type, respectively. The release of products degraded by IsPETaseI168R/S188E was 4.3 times that of the wild type. Tertiary structure characterization demonstrated that the structure of the variants IsPETaseI168R/S188D and IsPETaseI168R/S188E became more compact. Extensive MD simulations verified that a stable salt bridge was formed between the residue R168 and D186 in IsPETaseI168R/S188D , while in IsPETaseI168R/S188E an R168-D186-E188 salt bridge network was observed. These results confirmed that the proposed computation-based salt bridge design strategy could efficiently generate variants with enhanced thermal stability for the long-term degradation of PET, which would be helpful for the design of enzymes with improved stability.

Stability of collagen heterotrimer with same charge pattern and different charged residue identities

Salt bridges are important factors in maintaining the stability of proteins, and their contribution to protein folding has received much attention. Although the interaction energies, or stabilizing contributions, of individual salt bridges have been measured in various proteins, a systematic assessment of various types of salt bridges in a relatively uniform environment is still a valuable analysis. Here, we used a collagen heterotrimer as a host-guest platform to construct 48 heterotrimers with the same charge pattern. A variety of salt bridges were formed between the oppositely charged residues Lys, Arg, Asp, and Glu. The melting temperature (Tm) of the heterotrimers was measured with circular dichroism. The atomic structures of 10 salt bridges were shown in three x-ray crystals of heterotrimer. Molecular dynamics simulation based on the crystal structures indicated that strong, intermediate, and weak salt bridges have distinctive N-O distances. A linear regression model was used to predict the stability of heterotrimers with high accuracy (R2 = 0.93). We developed an online database to help readers understand how a salt bridge stabilizes collagen. This work will help us better understand the stabilizing mechanism of salt bridges in collagen folding and provide a new strategy to design collagen heterotrimers.

Recombinant pseudorabies virus (PRV) expressing stabilized E2 of classical swine fever virus (CSFV) protects against both PRV and CSFV

Pseudorabies (PR) and classical swine fever (CSF) are economically important infectious diseases of pigs. Most pig farms in China are immunized against these two diseases. Here, we describe a stabilized E2 protein as an immunogen inserted into the PRV genome as a bivalent live virus-vectored vaccine. The E2 protein has 48 variant sites, there are 2-5 candidate amino acids per variant site, and the relative energy contribution of each amino acid to E2 energy was calculated. Combined substitutions of amino acids at the neighbor variant site (neighbor substitution) were performed to obtain the E2 protein sequence with the lowest energy (stabilized E2). Multiple amino acid substitutions at 48 variant sites were performed, and the results were consistent with neighbor substitutions. The stabilized E2 sequence was obtained, and its energy decreased by 22 Rosetta Energy Units (REUs) compared with the original sequence. After the recombinant PRV expressing stabilized E2 of CSFV was constructed, the secretion efficiency of stabilized E2 was increased by 2.97 times, and the thermal stability was increased by 10.5 times. Immunization of mice resulted in a 2-fold increase in antibody production, and a balanced antibody level against subtype 1.1 and subtype 2.1d E2 was achieved. In rabbits immunized, the lethal challenge of PRV-ZJ and the fever response induced by CSFV could be prevented simultaneously. These findings suggest that rPRV-muta/287aaE2 is a promising bivalent vaccine against CSFV and PRV infections.

Engineering non-conserved salt bridges in GH11 xylanase from Bacillus pumilus SSP34 for improved thermal stability: an in-silico evaluation

GH11 xylanases are commercially important enzymes for degradation of xylan fibers. We have identified the presence of nine non-conserved and five conserved salt bridges in GH11 xylanase from Bacillus pumilus SSP34. We have designed two sets of mutants viz., (1) substitution mutants in which non-conserved charged amino acid residues have been replaced with appropriate hydrophobic residues based on side chain occupancy and hydrophobicity and (2) deletion mutants where non-conserved charged residues have been deleted. The stability of the mutants has been evaluated in-silico by analyzing the contributions of non-covalent interactions like hydrophobic interaction clusters and salt bridges. The stability of the resultant mutants was evaluated using parameters such as radius of gyration, solvent accessible surface area, root mean square deviation, root mean square fluctuations and protein unfolding measurements using molecular dynamic simulations. The deletion of certain charged residues resulted in mutants having lowered radius of gyration and decreased surface areas. However, RMSD and RMSF measurements indicated lowered stability in comparison to substitution mutants. Of the substitution mutants, the SBM 3 was the most stable mutant as indicated by Rg, SASA, RMSF and simulated protein unfolding measurements. The major contributing factors for improved stability could be strengthening of hydrophobic interactions in the GH11 xylanase from B. pumilus. These in-silico stability measurements of salt bridge mutants may lead to better design of GH11 xylanases for commercial applications.Communicated by Ramaswamy H. Sarma.

Analysis of conformational stability of interacting residues in protein binding interfaces

After approximately 60 years of work, the protein folding problem has recently seen rapid advancement thanks to the inventions of AlphaFold and RoseTTAFold, which are machine-learning algorithms capable of reliably predicting protein structures from their sequences. A key component in their success was the inclusion of pairwise interaction information between residues. As research focus shifts towards developing algorithms to design and engineer binding proteins, it is likely that knowledge of interaction features at protein interfaces can improve predictions. Here, 574 protein complexes were analyzed to identify the stability features of their pairwise interactions, revealing that interactions between pre-stabilized residues are a selected feature in protein binding interfaces. In a retrospective analysis of 475 de novo designed binding proteins with an experimental success rate of 19%, inclusion of pairwise interaction pre-stabilization parameters increased the frequency of identifying experimentally successful binders to 40%.

Protein engineering of NADH pyrophosphatase for efficient biocatalytic production of reduced nicotinamide mononucleotide

Introduction: NADH pyrophosphatase, a hydrolase catalyzing the phosphate bond of NADH to reduced nicotinamide mononucleotide, has potential applications in the food, cosmetic and pharmaceutical industry. Methods: Here, we investigated the effects of vector screening, promoter and RBS strategies on NADH pyrophosphatase expression and protein engineering on its enzymatic activity and thermal stability. Results: In this study, we describe a NADH pyrophosphatase derived from Escherichia coli (EcNudc). Strategies focusing on expression regulation including screening vectors, optimizing promoters and ribosome binding sites were utilized to enhance the productivity of EcNudc (1.8 U/mL). Moreover, protein engineering was adopted to further improve the catalytic properties of EcNudc, achieving 3.3-fold higher activity and 3.6-fold greater thermostability at 50°C. Furthermore, fermentation for the combined mutant R148A-H149E (EcNudc-M) production in a 7 L fermenter was implemented and the enzyme activity of EcNudc-M reached 33.0 U/mL. Finally, the EcNudc-M was applied in the catalysis of NADH with the highest NMNH yield of 16.65 g/L. Discussion: In conclusion, we constructed a commercially available genetically engineered strain with high activity and thermal stability of NADH pyrophosphatase, laying a broad foundation for the biocatalytic industrial production of NMNH and expand its application range.

Enhancement of the polyethylene terephthalate and mono-(2-hydroxyethyl) terephthalate degradation activity of Ideonella sakaiensis PETase by an electrostatic interaction-based strategy

The serious environmental pollution that came up with the continuously growing demand for polyethylene terephthalate (PET) has attracted global concern. The IsPETase which has shown the highest PET degradation activity under ambient temperature is a promising enzyme for PET biodegradation, while poor thermostability limited its practical application. Herein, an electrostatic interaction-based strategy was applied for rational design of IsPETase towards enhanced thermostability. The IsPETaseI139R variant displayed the highest Tm value of 56.4 °C and 3.6-times higher PET degradation activity. Molecular simulations demonstrated that the introduction of salt bridges stabilized the local structures, resulting in robust thermostability. Meanwhile, the IsPETaseS92K/D157E/R251A not only exhibited higher thermostability but also showed a 1.74-fold kcat increase towards mono-(2-hydroxyethyl) terephthalate, which ultimately achieved PET depolymerization to complete monomer TPA. Collectively, the electrostatic interaction-based strategy, together with the derived IsPETase variants, could help promote the bio-recycle of PET, reducing the severe global burden of PET waste.

A Statistical Analysis of the Sequence and Structure of Thermophilic and Non-Thermophilic Proteins

Thermophilic proteins have various practical applications in theoretical research and in industry. In recent years, the demand for thermophilic proteins on an industrial scale has been increasing; therefore, the engineering of thermophilic proteins has become a hot direction in the field of protein engineering. However, the exact mechanism of thermostability of proteins is not yet known, for engineering thermophilic proteins knowing the basis of thermostability is necessary. In order to understand the basis of the thermostability in proteins, we have made a statistical analysis of the sequences, secondary structures, hydrogen bonds, salt bridges, DHA (Donor-Hydrogen-Accepter) angles, and bond lengths of ten pairs of thermophilic proteins and their non-thermophilic orthologous. Our findings suggest that polar amino acids contribute to thermostability in proteins by forming hydrogen bonds and salt bridges which provide resistance against protein denaturation. Short bond length and a wider DHA angle provide greater bond stability in thermophilic proteins. Moreover, the increased frequency of aromatic amino acids in thermophilic proteins contributes to thermal stability by forming more aromatic interactions. Additionally, the coil, helix, and loop in the secondary structure also contribute to thermostability.

Improvement of FGF7 Thermal Stability by Introduction of Mutations in Close Vicinity to Disulfide Bond and Surface Salt Bridge

Fibroblast Growth Factor 7 (FGF7), a growth factor specific to epithelial cells, has attracted attention as a therapeutic protein. However, FGF7 has a limitation in its use due to low protein stability. Here, the mutations were designed to increase the stability of FGF7 by analyzing its 3D structure and sequence of other FGFs. Palifermin, N-terminal truncated FGF7 is known to have improved stability and was used as control protein in our study. The K126 and K178 were substituted into glutamate to form salt bridge with the neighboring residue R175 respectively and A120C mutation was introduced in close vicinity to disulfide bond between C133 and C137. The data of Circular Dichroism (CD) showed that all mutant proteins tested had higher Tm value than Palifermin and Tm of A120C/K126E/K178E FGF7 mutant protein was 15.24 °C higher than that of Palifermin. The results of cell proliferation activity and soluble protein analyzed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) after 37 °C or 45 °C incubation exhibited that the stability of A120C mutant protein and A120C-including mutant proteins was improved. These results suggest that the mutation of amino acid in close vicinity to disulfide bond and the salt bridge at the surface of FGF7 enhanced thermal stability and make FGF7 more useful for pharmaceutical and cosmetical application.

Protein Mutations and Stability, a Link with Disease: The Case Study of Frataxin

Protein mutations may lead to pathologies by causing protein misfunction or propensity to degradation. For this reason, several studies have been performed over the years to determine the capability of proteins to retain their native conformation under stress condition as well as factors to explain protein stabilization and the mechanisms behind unfolding. In this review, we explore the paradigmatic example of frataxin, an iron binding protein involved in Fe–S cluster biogenesis, and whose impairment causes a neurodegenerative disease called Friedreich’s Ataxia (FRDA). We summarize what is known about most common point mutations identified so far in heterozygous FRDA patients, their effects on frataxin structure and function and the consequences of its binding with partners.

OUP accepted manuscript

Motivation

Thermal properties of proteins are of great importance for a number of theoretical and practical implications. Predicting the thermal stability of a protein is a difficult and still scarcely addressed task.

Results

Here, we introduce Thermometer, a webserver to assess the thermal stability of a protein using structural information. Thermometer is implemented as a publicly available, user-friendly interface.

Availability and implementation

Our server can be found at the following link (all major browser supported): http://service.tartaglialab.com/new_submission/thermometer_file.

Supplementary information

Supplementary data are available at Bioinformatics online.

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.

Surface salt bridges contribute to the extreme thermal stability of an FN3-like domain from a thermophilic bacterium

This study uses differential scanning calorimetry, X-ray crystallography, and molecular dynamics simulations to investigate the structural basis for the high thermal stability (melting temperature 97.5°C) of a FN3-like protein domain from thermophilic bacteria Thermoanaerobacter tengcongensis (FN3tt). FN3tt adopts a typical FN3 fold with a three-stranded beta sheet packing against a four-stranded beta sheet. We identified three solvent exposed arginine residues (R23, R25, and R72), which stabilize the protein through salt bridge interactions with glutamic acid residues on adjacent strands. Alanine mutation of the three arginine residues reduced melting temperature by up to 22°C. Crystal structures of the wild type (WT) and a thermally destabilized (∆Tm -19.7°C) triple mutant (R23L/R25T/R72I) were found to be nearly identical, suggesting that the destabilization is due to interactions of the arginine residues. Molecular dynamics simulations showed that the salt bridge interactions in the WT were stable and provided a dynamical explanation for the cooperativity observed between R23 and R25 based on calorimetry measurements. In addition, folding free energy changes computed using free energy perturbation molecular dynamics simulations showed high correlation with melting temperature changes. This work is another example of surface salt bridges contributing to the enhanced thermal stability of thermophilic proteins. The molecular dynamics simulation methods employed in this study may be broadly useful for in silico surface charge engineering of proteins.

The Mutational Robustness of the Genetic Code and Codon Usage in Environmental Context: A Non-Extremophilic Preference?

The genetic code was evolved, to some extent, to minimize the effects of mutations. The effects of mutations depend on the amino acid repertoire, the structure of the genetic code and frequencies of amino acids in proteomes. The amino acid compositions of proteins and corresponding codon usages are still under selection, which allows us to ask what kind of environment the standard genetic code is adapted to. Using simple computational models and comprehensive datasets comprising genomic and environmental data from all three domains of Life, we estimate the expected severity of non-synonymous genomic mutations in proteins, measured by the change in amino acid physicochemical properties. We show that the fidelity in these physicochemical properties is expected to deteriorate with extremophilic codon usages, especially in thermophiles. These findings suggest that the genetic code performs better under non-extremophilic conditions, which not only explains the low substitution rates encountered in halophiles and thermophiles but the revealed relationship between the genetic code and habitat allows us to ponder on earlier phases in the history of Life.

Composition-dependent energetic contribution of complex salt bridges to collagen stability

Complex salt bridges, on which three or more charged residues interplay simultaneously, cannot be considered as addition of individual salt bridges. This is still an intriguing problem in protein folding and stability. Here, we used an obligated ABC-type collagen heterotrimer as a platform to study the relationship between energetic contributions and conformational details of three-body complex salt bridges anchored by positively charged residues, K and R. Eight complex salt bridges were constructed by engineering point mutations in the heterotrimer. The circular dichroism measurements showed that the K-anchored complex salt bridges were stronger than the R-anchored ones. The molecular dynamics simulation revealed that both types of salt bridges had distinct dynamic features. The energetic contribution of K-anchored salt bridges was mainly determined by strong single bridges. In the R-anchored complex salt bridges, both side-chain electrostatic interactions and side-chain-backbone hydrogen bonding were involved. An empirical equation was proposed to predict the energetic contributions with high accuracy (R² = 0.93). This work could help us take insights into the mechanisms of composition-dependent behaviors of the complex salt bridges on protein surface.

Phylogenetic analysis of mutational robustness based on codon usage supports that the standard genetic code does not prefer extreme environments

The mutational robustness of the genetic code is rarely discussed in the context of biological diversity, such as codon usage and related factors, often considered as independent of the actual organism's proteome. Here we put the living beings back to picture and use distortion as a metric of mutational robustness. Distortion estimates the expected severities of non-synonymous mutations measuring it by amino acid physicochemical properties and weighting for codon usage. Using the biological variance of codon frequencies, we interpret the mutational robustness of the standard genetic code with regards to their corresponding environments and genomic compositions (GC-content). Employing phylogenetic analyses, we show that coding fidelity in physicochemical properties can deteriorate with codon usages adapted to extreme environments and these putative effects are not the artefacts of phylogenetic bias. High temperature environments select for codon usages with decreased mutational robustness of hydrophobic, volumetric, and isoelectric properties. Selection at high saline concentrations also leads to reduced fidelity in polar and isoelectric patterns. These show that the genetic code performs best with mesophilic codon usages, strengthening the view that LUCA or its ancestors preferred lower temperature environments. Taxonomic implications, such as rooting the tree of life, are also discussed.

The desirable salt bridges in amylases: Distribution, configuration and location

The α-amylases are the most widely used industrial enzymes, and are particularly useful as liquifying enzymes in industrial processes based upon starch. Since starch liquefication is carried out at evaluated temperatures, typically above 60 °C, there is substantial demand for thermostable α -amylases. Most naturally occurring α -amylases exhibit moderate thermostability, so substantial effort has been invested in attempts to increase their thermostability. One structural feature that has the potential to increase protein thermostability is the introduction of salt bridges. However, not every salt bridge contributes to protein thermostability. The salt bridges in amylases have their characteristics in terms of distribution, configuration and location. The summary of these features helps to introduce new salt bridges based on the characteristics. This review focuses on salt bridges of α-amylases, both naturally present and introduced using mutagenesis. Its aim is to provide a bird's eye view of distribution, configuration, location of desirable salt bridges.

Structural Insights into a Novel Esterase from the East Pacific Rise and Its Improved Thermostability by a Semirational Design

Lipolytic enzymes are essential biocatalysts in food processing as well as pharmaceutical and pesticide industries, catalyzing the cleavage of ester bonds in a variety of acyl chain substrates. Here, we report the crystal structure of an esterase from the deep-sea hydrothermal vent of the East Pacific Rise (EprEst). The X-ray structure of EprEst in complex with the ligand, acetate, has been determined at 2.03 Å resolution. The structure reveals a unique spatial arrangement and orientation of the helix cap domain and α/β hydrolase domain, which form a substrate pocket with preference for short-chain acyl groups. Molecular docking analysis further demonstrated that the active site pocket could accommodate p-nitrophenyl (pNP) carboxyl ligands of varying lengths (≤6 C atoms), with pNP-butyrate ester predicted to have the highest binding affinity. Additionally, the semirational design was conducted to improve the thermostability of EprEst by enzyme engineering based on the established structure and multiple sequence alignment. A mutation, K114P, introduced in the hinge region of the esterase, which displayed increased thermostability and enzyme activity. Collectively, the structural and functional data obtained herein could be used as basis for further protein engineering to ultimately expand the scope of industrial applications of marine-derived lipolytic enzymes.

Power of Biocatalysis for Organic Synthesis

Biocatalysis, using defined enzymes for organic transformations, has become a common tool in organic synthesis, which is also frequently applied in industry. The generally high activity and outstanding stereo-, regio-, and chemoselectivity observed in many biotransformations are the result of a precise control of the reaction in the active site of the biocatalyst. This control is achieved by exact positioning of the reagents relative to each other in a fine-tuned 3D environment, by specific activating interactions between reagents and the protein, and by subtle movements of the catalyst. Enzyme engineering enables one to adapt the catalyst to the desired reaction and process. A well-filled biocatalytic toolbox is ready to be used for various reactions. Providing nonnatural reagents and conditions and evolving biocatalysts enables one to play with the myriad of options for creating novel transformations and thereby opening new, short pathways to desired target molecules. Combining several biocatalysts in one pot to perform several reactions concurrently increases the efficiency of biocatalysis even further.

Structural and functional characterization of peptidyl-tRNA hydrolase from Klebsiella pneumoniae

Klebsiella pneumoniae is a member of the ESKAPE panel of pathogens that are top priority to tackle AMR. Bacterial peptidyl tRNA hydrolase (Pth), an essential, ubiquitous enzyme, hydrolyzes the peptidyl-tRNAs that accumulate in the cytoplasm because of premature termination of translation. Pth cleaves the ester bond between 2' or 3' hydroxyl of the ribose in the tRNA and C-terminal carboxylate of the peptide, thereby making free tRNA available for repeated cycles of protein synthesis and preventing cell death by alleviating tRNA starvation. Pth structures have been determined in peptide-bound or peptide-free states. In peptide-bound state, highly conserved residues F67, N69 and N115 adopt a conformation that is conducive to their interaction with peptide moiety of the substrate. While, in peptide-free state, these residues move away from the catalytic center, perhaps, in order to facilitate release of hydrolysed peptide. Here, we present a novel X-ray crystal structure of Pth from Klebsiella pneumoniae (KpPth), at 1.89 Å resolution, in which out of the two molecules in the asymmetric unit, one reflects the peptide-bound while the other reflects peptide-free conformation of the conserved catalytic site residues. Each molecule of the protein has canonical structure with seven stranded β-sheet structure surrounded by six α-helices. MD simulations indicate that both the forms converge over 500 ns simulation to structures with wider opening of the crevice at peptide-binding end. In solution, KpPth is monomeric and its 2D-HSQC spectrum displays a single set of well dispersed peaks. Further, KpPth was demonstrated to be enzymatically active on BODIPY-Lys-tRNA^Lys3.

Revisiting the scope and applications of food enzymes from extremophiles

Microorganisms from extreme environments tend to undergo various adaptations due to environmental conditions such as extreme pH, temperature, salinity, heavy metals, and solvents. Thus, they produce enzymes with unique properties and high specificity, making them useful industrially, particularly in the food industries. Despite these enzymes' remarkable properties, only a few instances can be reported for actual exploitation in the food industry. This review's objectives are to highlight the properties of these enzymes and their prospects in the food industry. First, an introduction to extremophilic organisms is presented, followed by the categories and application of food enzymes from extremophiles. Then, the unique structural features of extremozymes are shown. This review also covers the prospective applications of extremozymes in the food industry in a broader sense, including degradation of toxins, deconstruction of polymers into monomers, and catalysis of multistep processes. Finally, the challenges in bioprocessing of extremozymes and applications in food are presented. PRACTICAL APPLICATIONS: Enzymes are important players in food processing and preservation. Extremozymes, by their nature, are ideal for a broad range of food processing applications, particularly those that require process conditions of extreme pH, temperature, and salinity. As the global food industry grows, so too will grow the need to research and develop food products that are diverse, safe, healthy, and nutritious. There is also the need to produce food in a sustainable way that generates less waste or maximizes waste valorization. We anticipate that extremozymes can meet some of the research and development needs of the food industry.

A Novel Thermostable Cytochrome P450 from Sequence-Based Metagenomics of Binh Chau Hot Spring as a Promising Catalyst for Testosterone Conversion

Biotechnological applications of cytochromes P450 show difficulties, such as low activity, thermal and/or solvent instability, narrow substrate specificity and redox partner dependence. In an attempt to overcome these limitations, an exploitation of novel thermophilic P450 enzymes from nature via uncultured approaches is desirable due to their great advantages that can resolve nearly all mentioned impediments. From the metagenomics library of the Binh Chau hot spring, an open reading frame (ORF) encoding a thermostable cytochrome P450—designated as P450-T3—which shared 66.6% amino acid sequence identity with CYP109C2 of Sorangium cellulosum So ce56 was selected for further identification and characterization. The ORF was synthesized artificially and heterologously expressed in Escherichia coli C43(DE3) using the pET17b system. The purified enzyme had a molecular weight of approximately 43 kDa. The melting temperature of the purified enzyme was 76.2 °C and its apparent half-life at 60 °C was 38.7 min. Redox partner screening revealed that P450-T3 was reduced well by the mammalian AdR-Adx4-108 and the yeast Arh1-Etp1 redox partners. Lauric acid, palmitic acid, embelin, retinoic acid (all-trans) and retinoic acid (13-cis) demonstrated binding to P450-T3. Interestingly, P450-T3 also bound and converted testosterone. Overall, P450-T3 might become a good candidate for biocatalytic applications on a larger scale.

A thermophilic and thermostable xylanase from Caldicoprobacter algeriensis: Recombinant expression, characterization and application in paper biobleaching

A novel xylanase gene xynBCA, encoding a polypeptide of 439 residues (XynBCA), was cloned from Caldicoprobacter algeriensis genome and recombinantly expressed in Escherichia coli BL21(DE3). The amino acid sequence analysis showed that XynBCA belongs to the glycoside hydrolase family 10. The purified recombinant enzyme has a monomeric structure of 52 kDa. It is active and stable in a wide range of pH from 3 to 10 with a maximum activity at 6.5. Interestingly, XynBCA was highly thermoactive with an optimum temperature of 80 °C, thermostable with a half-life of 20 min at 80 °C. The specific activity was 117 U mg^-1, while the K_m and V_max were 1.247 mg ml^-1, and 114.7 μmol min^-1 mg^-1, respectively. The investigation of XynBCA in kraft pulp biobleaching experiments showed effectiveness in releasing reducing sugars and chromophores, with best achievements at 100 U g^-1 of pulp and 1 h of incubation. The comparative molecular modeling studies with the less thermostable Xylanase B from Clostridium stercorarium, revealed extra charged residues at the surface of XynBCA potentially participating in the formation of intermolecular hydrogen bonds with solvent molecules or generating salt bridges, therefore contributing to the higher thermal stability.

Insights on protein thermal stability: a graph representation of molecular interactions

Motivation

Understanding the molecular mechanisms of thermal stability is a challenge in protein biology. Indeed, knowing the temperature at which proteins are stable has important theoretical implications, which are intimately linked with properties of the native fold, and a wide range of potential applications from drug design to the optimization of enzyme activity.

Results

Here, we present a novel graph-theoretical framework to assess thermal stability based on the structure without any a priori information. In this approach we describe proteins as energy-weighted graphs and compare them using ensembles of interaction networks. Investigating the position of specific interactions within the 3D native structure, we developed a parameter-free network descriptor that permits to distinguish thermostable and mesostable proteins with an accuracy of 76% and area under the receiver operating characteristic curve of 78%.

Availability and implementation

Code is available upon request to edoardo.milanetti@uniroma1.it

Supplementary information

Supplementary data are available at Bioinformatics online.

On the Thermal Stability of O6-Methylguanine-DNA Methyltransferase from Archaeon Pyrococcus kodakaraensis by Molecular Dynamics Simulations

We have employed molecular dynamics simulations to analyze the thermal stability of the O⁶-methylguanine-DNA methyltransferase (MGMT) protein, both hyperthermophilic archaeon Pyrococcus kodakaraensis (Pk-MGMT) and its mesophilic homologue pair, obtained from enterobacterium Escherichia coli (AdaC). This theoretical study was done at three different temperatures: 302, 371, and 450 K. The molecular dynamics has been performed in explicit aqueous solvent during a period of time of 95 ns, including periodic boundary conditions and constant pressure. The same procedure has been used for both proteins, and each simulation has been carried out by triplicate. Hence, we performed 18 simulations. In this way, we have done different analyses to explore the factors that may affect the thermal stability of Pk-MGMT. The structural behavior was analyzed using indicators such as root-mean-square deviation, radius of gyration, solvent-accessible surface area, hydrogen bonds, native contacts, secondary structure, and salt bridge formation. The results showed that when the temperature increases, the global atomic fluctuations increase too, which suggests that both proteins lose thermal stability, but as expected, this fact is highlighted in AdaC. Moreover, the contacts of the native state in AdaC are considerably lower than those found in Pk-MGMT at 450 K. Also, the structural studies showed that conserved and nonconserved salt bridges kept close contacts with the Pk-MGMT protein at high temperatures. These interaction types act as molecular staples and are mainly responsible to provide thermostability to the hyperthermophilic protein.

Robust ω-Transaminases by Computational Stabilization of the Subunit Interface

Transaminases are attractive catalysts for the production of enantiopure amines. However, the poor stability of these enzymes often limits their application in biocatalysis. Here, we used a framework for enzyme stability engineering by computational library design (FRESCO) to stabilize the homodimeric PLP fold type I ω-transaminase from Pseudomonas jessenii. A large number of surface-located point mutations and mutations predicted to stabilize the subunit interface were examined. Experimental screening revealed that 10 surface mutations out of 172 tested were indeed stabilizing (6% success), whereas testing 34 interface mutations gave 19 hits (56% success). Both the extent of stabilization and the spatial distribution of stabilizing mutations showed that the subunit interface was critical for stability. After mutations were combined, 2 very stable variants with 4 and 6 mutations were obtained, which in comparison to wild type (T _m ^app = 62 °C) displayed T _m ^app values of 80 and 85 °C, respectively. These two variants were also 5-fold more active at their optimum temperatures and tolerated high concentrations of isopropylamine and cosolvents. This allowed conversion of 100 mM acetophenone to (S)-1-phenylethylamine (>99% enantiomeric excess) with high yield (92%, in comparison to 24% with the wild-type transaminase). Crystal structures mostly confirmed the expected structural changes and revealed that the most stabilizing mutation, I154V, featured a rarely described stabilization mechanism: namely, removal of steric strain. The results show that computational interface redesign can be a rapid and powerful strategy for transaminase stabilization.

Lysosomal protein thermal stability does not correlate with cellular half-life: global observations and a case study of tripeptidyl-peptidase 1

Late-infantile neuronal ceroid lipofuscinosis (LINCL) is a neurodegenerative lysosomal storage disorder caused by mutations in the gene encoding the protease tripeptidyl-peptidase 1 (TPP1). Progression of LINCL can be slowed or halted by enzyme replacement therapy, where recombinant human TPP1 is administered to patients. In this study, we utilized protein engineering techniques to increase the stability of recombinant TPP1 with the rationale that this may lengthen its lysosomal half-life, potentially increasing the potency of the therapeutic protein. Utilizing multiple structure-based methods that have been shown to increase the stability of other proteins, we have generated and evaluated over 70 TPP1 variants. The most effective mutation, R465G, increased the melting temperature of TPP1 from 55.6°C to 64.4°C and increased its enzymatic half-life at 60°C from 5.4 min to 21.9 min. However, the intracellular half-life of R465G and all other variants tested in cultured LINCL patient-derived lymphoblasts was similar to that of WT TPP1. These results provide structure/function insights into TPP1 and indicate that improving in vitro thermal stability alone is insufficient to generate TPP1 variants with improved physiological stability. This conclusion is supported by a proteome-wide analysis that indicates that lysosomal proteins have higher melting temperatures but also higher turnover rates than proteins of other organelles. These results have implications for similar efforts where protein engineering approaches, which are frequently evaluated in vitro, may be considered for improving the physiological properties of proteins, particularly those that function in the lysosomal environment.

Simulated Epidemics in 3D Protein Structures to Detect Functional Properties

The outcome of an epidemic is closely related to the network of interactions between individuals. Likewise, protein functions depend on the 3D arrangement of their residues and the underlying energetic interaction network. Borrowing ideas from the theoretical framework that has been developed to address the spreading of real diseases, we study for the first time the diffusion of a fictitious epidemic inside the protein nonbonded interaction network, aiming to study network features and properties. Our approach allows us to probe the overall stability and the capability of propagating information in complex 3D structures, proving to be very efficient in addressing different problems, from the assessment of thermal stability to the identification of functional sites.

Evolutionary Stability of Salt Bridges Hints Its Contribution to Stability of Proteins

The contribution of newly designed salt bridges to protein stabilization remains controversial even today. In order to solve this problem, we investigated salt bridges from two aspects: spatial distribution and evolutionary characteristics of salt bridges. Firstly, we analyzed spatial distribution of salt bridges in proteins, elucidating the basic requirements of forming salt bridges. Then, from an evolutionary point of view, the evolutionary characteristics of salt bridges as well as their neighboring residues were investigated in our study. The results demonstrate that charged residues appear more frequently than other neutral residues at certain positions of sequence even under evolutionary pressure, which are able to form electrostatic interactions that could increase the evolutionary stability of corresponding amino acid regions, enhancing their importance to stability of proteins. As a corollary, we conjectured that the newly designed salt bridges with more contribution to proteins, not only, are qualified spatial distribution of salt bridges, but also, are needed to further increase the evolutionary stability of corresponding amino acid regions. Based on analysis, the 8 mutations were accordingly constructed in the 1,4-α-glucan branching enzyme (EC 2.4.1.18, GBE) from Geobacillus thermoglucosidans STB02, of which 7 mutations improved thermostability of GBE. The enhanced thermostability of 7 mutations might be a result of additional salt bridges on residue positions that at least one of amino acids positions is conservative, improving their contribution of stabilization to proteins.

A novel thermostable and halophilic thioredoxin reductase from the Red Sea Atlantis II hot brine pool

The highly extreme conditions of the lower convective layer in the Atlantis II (ATII) Deep brine pool of the Red Sea make it an ideal environment for the search for novel enzymes that can function under extreme conditions. In the current study, we isolated a novel sequence of a thioredoxin reductase (TrxR) enzyme from the metagenomic dataset established from the microbial community that resides in the lower convective layer of Atlantis II. The gene was cloned, expressed and characterized for redox activity, halophilicity, and thermal stability. The isolated thioredoxin reductase (ATII-TrxR) was found to belong to the high-molecular-weight class of thioredoxin reductases. A search for conserved domains revealed the presence of an extra domain (Crp) in the enzyme sequence. Characterization studies of ATII-TrxR revealed that the enzyme was halophilic (maintained activity at 4 M NaCl), thermophilic (optimum temperature was 65°C) and thermostable (60% of its activity was retained at 70°C). Additionally, the enzyme utilized NADH in addition to NADPH as an electron donor. In conclusion, a novel thermostable and halophilic thioredoxin reductase has been isolated with a unique sequence that adapts to the harsh conditions of the brine pools making this protein a good candidate for biological research and industrial applications.

Selection and analyses of variants of a designed protein suggest importance of hydrophobicity of partially buried sidechains for protein stability at high temperatures

Computationally designed proteins of high stability provide specimen in addition to natural proteins for the study of sequence-structure stability relationships at the very high end of protein stability spectrum. The melting temperature of E_1r26, a protein we previously designed using the A Backbone-based Amino aCid Usage Survey (ABACUS) sequence design program, is above 110 °C, more than 50 °C higher than that of the natural thioredoxin protein whose backbone (PDB ID 1R26) has been used as the design target. Using an experimental selection approach, we obtained variants of E_1r26 that remain folded but are of reduced stability, including one whose unfolding temperature and denaturing guanidine concentration are similar to those of 1r26. The mutant unfolds with a certain degree of cooperativity. Its structure solved by X-ray crystallography agrees with that of 1r26 by a root mean square deviation of 1.3 Å, adding supports to the accuracy of the ABACUS method. Analyses of intermediate mutants indicate that the substitution of two partially buried hydrophobic residues (isoleucine and leucine) by polar residues (threonine and serine, respectively) are responsible for the dramatic change in the unfolding temperature. It is suggested that the effects of mutations located in rigid secondary structure regions, but not those in loops, may be well predicted through ABACUS mutation energy analysis. The results also suggest that hydrophobic effects involving intermediately buried sidechains can be critically important for protein stability at high temperatures.

Salt Bridge in Ligand-Protein Complexes-Systematic Theoretical and Statistical Investigations

Although the salt bridge is the strongest among all known noncovalent molecular interactions, no comprehensive studies have been conducted to date to examine its role and significance in drug design. Thus, a systematic study of the salt bridge in biological systems is reported herein, with a broad analysis of publicly available data from Protein Data Bank, DrugBank, ChEMBL, and GPCRdb. The results revealed the distance and angular preferences as well as privileged molecular motifs of salt bridges in ligand-receptor complexes, which could be used to design the strongest interactions. Moreover, using quantum chemical calculations at the MP2 level, the energetic, directionality, and spatial variabilities of salt bridges were investigated using simple model systems mimicking salt bridges in a biological environment. Additionally, natural orbitals for chemical valence (NOCV) combined with the extended-transition-state (ETS) bond-energy decomposition method (ETS-NOCV) were analyzed and indicated a strong covalent contribution to the salt bridge interaction. The present results could be useful for implementation in rational drug design protocols.

The state-of-the-art strategies of protein engineering for enzyme stabilization

Enzymes generated by natural recruitment and protein engineering have greatly contribute in various sets of applications. However, their insufficient stability is a bottleneck that limit the rapid development of biocatalysis. Novel approaches based on precise and global structural dissection, advanced gene manipulation, and combination with the multidisciplinary techniques open a new horizon to generate stable enzymes efficiently. Here, we comprehensively introduced emerging advances of protein engineering strategies for enzyme stabilization. Then, we highlighted practical cases to show importance of enzyme stabilization in pharmaceutical and industrial applications. Combining computational enzyme design with molecular evolution will hold considerable promise in this field.

The Goldilocks Approach: A Review of Employing Design of Experiments in Prokaryotic Recombinant Protein Production

The production of high yields of soluble recombinant protein is one of the main objectives of protein biotechnology. Several factors, such as expression system, vector, host, media composition and induction conditions can influence recombinant protein yield. Identifying the most important factors for optimum protein expression may involve significant investment of time and considerable cost. To address this problem, statistical models such as Design of Experiments (DoE) have been used to optimise recombinant protein production. This review examines the application of DoE in the production of recombinant proteins in prokaryotic expression systems with specific emphasis on media composition and culture conditions. The review examines the most commonly used DoE screening and optimisation designs. It provides examples of DoE applied to optimisation of media and culture conditions.

Loss of a conserved salt bridge in bacterial glycosyl hydrolase BgIM-G1 improves substrate binding in temperate environments

Salt bridges are the strongest electrostatic interactions in proteins. They substantially contribute to a protein's structural stability. Thus, mutations of salt bridges are typically selected against. Here, we report on the evolutionary loss of a highly conserved salt bridge in the GH1 family glycosyl hydrolase BglM-G1. BglM-G1's gene was found in the bacterial metagenome of a temperate, seasonally cold marine habitat. In BglM-G1, arginine 75 is replaced by a histidine. While fully retaining β-glucosidase activity, BglM-G1 is less heat stable than an H75R variant, in which the salt bridge was artificially re-introduced. However, the K m toward its substrates was lower in wild type, leading to an overall higher catalytic efficiency. Our results indicate that this loss of the salt bridge leads to higher flexibility in BglM-G1's active site, trading structural stability at high temperatures, a trait not needed in a temperate, seasonally cold habitat, for a more effective catalytic activity.

Role of simple descriptors and applicability domain in predicting change in protein thermostability

The melting temperature (Tm) of a protein is the temperature at which half of the protein population is in a folded state. Therefore, Tm is a measure of the thermostability of a protein. Increasing the Tm of a protein is a critical goal in biotechnology and biomedicine. However, predicting the change in melting temperature (dTm) due to mutations at a single residue is difficult because it depends on an intricate balance of forces. Existing methods for predicting dTm have had similar levels of success using generally complex models. We find that training a machine learning model with a simple set of easy to calculate physicochemical descriptors describing the local environment of the mutation performed as well as more complicated machine learning models and is 2-6 orders of magnitude faster. Importantly, unlike in most previous publications, we perform a blind prospective test on our simple model by designing 96 variants of a protein not in the training set. Results from retrospective and prospective predictions reveal the limited applicability domain of each model. This study highlights the current deficiencies in the available dTm dataset and is a call to the community to systematically design a larger and more diverse experimental dataset of mutants to prospectively predict dTm with greater certainty.

Review: Engineering of thermostable enzymes for industrial applications

The catalytic properties of some selected enzymes have long been exploited to carry out efficient and cost-effective bioconversions in a multitude of research and industrial sectors, such as food, health, cosmetics, agriculture, chemistry, energy, and others. Nonetheless, for several applications, naturally occurring enzymes are not considered to be viable options owing to their limited stability in the required working conditions. Over the years, the quest for novel enzymes with actual potential for biotechnological applications has involved various complementary approaches such as mining enzyme variants from organisms living in extreme conditions (extremophiles), mimicking evolution in the laboratory to develop more stable enzyme variants, and more recently, using rational, computer-assisted enzyme engineering strategies. In this review, we provide an overview of the most relevant enzymes that are used for industrial applications and we discuss the strategies that are adopted to enhance enzyme stability and/or activity, along with some of the most relevant achievements. In all living species, many different enzymes catalyze fundamental chemical reactions with high substrate specificity and rate enhancements. Besides specificity, enzymes also possess many other favorable properties, such as, for instance, cost-effectiveness, good stability under mild pH and temperature conditions, generally low toxicity levels, and ease of termination of activity. As efficient natural biocatalysts, enzymes provide great opportunities to carry out important chemical reactions in several research and industrial settings, ranging from food to pharmaceutical, cosmetic, agricultural, and other crucial economic sectors.

Engineering the Enantioselectivity and Thermostability of a (+)-γ-Lactamase from Microbacterium hydrocarbonoxydans for Kinetic Resolution of Vince Lactam (2-Azabicyclo[2.2.1]hept-5-en-3-one)

To produce promising biocatalysts, natural enzymes often need to be engineered to increase their catalytic performance. In this study, the enantioselectivity and thermostability of a (+)-γ-lactamase from Microbacterium hydrocarbonoxydans as the catalyst in the kinetic resolution of Vince lactam (2-azabicyclo[2.2.1]hept-5-en-3-one) were improved. Enantiomerically pure (-)-Vince lactam is the key synthon in the synthesis of antiviral drugs, such as carbovir and abacavir, which are used to fight against HIV and hepatitis B virus. The work was initialized by using the combinatorial active-site saturation test strategy to engineer the enantioselectivity of the enzyme. The approach resulted in two mutants, Val54Ser and Val54Leu, which catalyzed the hydrolysis of Vince lactam to give (-)-Vince lactam, with 99.2% (enantiomeric ratio [E] > 200) enantiomeric excess (ee) and 99.5% ee (E > 200), respectively. To improve the thermostability of the enzyme, 11 residues with high temperature factors (B-factors) calculated by B-FITTER or high root mean square fluctuation (RMSF) values from the molecular dynamics simulation were selected. Six mutants with increased thermostability were obtained. Finally, the mutants generated with improved enantioselectivity and mutants evolved for enhanced thermostability were combined. Several variants showing (+)-selectivity (E value > 200) and improved thermostability were observed. These engineered enzymes are good candidates to serve as enantioselective catalysts for the preparation of enantiomerically pure Vince lactam.IMPORTANCE Enzymatic kinetic resolution of the racemic Vince lactam using (+)-γ-lactamase is the most often utilized means of resolving the enantiomers for the preparation of carbocyclic nucleoside compounds. The efficiency of the native enzymes could be improved by using protein engineering methods, such as directed evolution and rational design. In our study, two properties (enantioselectivity and thermostability) of a γ-lactamase identified from Microbacterium hydrocarbonoxydans were tackled using a semirational design. The protein engineering was initialized by combinatorial active-site saturation test to improve the enantioselectivity. At the same time, two strategies were applied to identify mutation candidates to enhance the thermostability based on calculations from both a static (B-FITTER based on the crystal structure) and a dynamic (root mean square fluctuation [RMSF] values based on molecular dynamics simulations) way. After combining the mutants, we successfully obtained the final mutants showing better properties in both properties. The engineered (+)-lactamase could be a candidate for the preparation of (-)-Vince lactam.