Structural basis of thermostability. Analysis of stabilizing mutations in subtilisin BPN'

Effect of cysteine mutation at Ca2+ coordinating residues to the autolysis, folding and hydrophobicity of full length and mature Rand protease: molecular dynamics simulation and essential dynamics

Rand protease is a serine protease that shared common characteristics with members of the MEROPS S8 subtilisin family. It is thermostable, highly stable in organic solvent and broad in specificity. Many structures of homologous protein solved by X-ray crystallography and NMR have been deposited to Protein Data Bank (PDB) which allowed this study to rely on structure prediction by deep learning to build three-dimensional (3D) structure of full length and mature Rand protease (flRP and mRP). In silico cysteine mutation to 7 predicted high affinity Ca2+ coordinating residues were introduced, and the mutants were subjected to molecular dynamics simulation to study its effect on flRP and mRP. MD simulation showed a marked increase in flexibility of the pro-peptide segment indicating the impact of single cysteine substitution at high affinity Ca2+ coordinating residues to autolysis of flRP. MD simulation for mRP reaffirmed the role of Ca2+ coordinating sites in providing stability to Rand protease. In addition, these residues also affect the autolysis, folding and hydrophobicity of RP. Essential dynamics observed large contribution of the first few eigenvectors of flRP, mRP and their high affinity Ca2+ coordinating residues mutants to the TMSF values which indicates that these values account for a large portion of the overall atomic fluctuations. These results have given a more comprehensive understanding on the role of cysteine substituted Ca2+ coordinating surface loop to the structure of flRP and mRP which are important in contributing to the structural stability of subtilisin.Communicated by Ramaswamy H. Sarma.

Counteraction of stability-activity trade-off of Nattokinase through flexible region shifting

The widespread trade-off between stability and activity severely limits enzyme evolution. Although some progresses have been made to overcome this limitation, the counteraction mechanism for enzyme stability-activity trade-off remains obscure. Here, we clarified the counteraction mechanism of the Nattokinase stability-activity trade-off. A combinatorial mutant M4 was obtained by multi-strategy engineering, exhibiting a 20.7-fold improved half-life; meanwhile, the catalytic efficiency was doubled. Molecular dynamics simulation revealed that an obvious flexible region shifting in the structure of mutant M4 was occurred. The flexible region shifting which contributed to maintain the global structural flexibility, was considered to be the key factor for counteracting the stability-activity trade-off. Further analysis illustrated that the flexible region shifting was driven by region dynamical networks reshaping. This work provided deep insight into the counteraction mechanism of enzyme stability-activity trade-off, suggesting that flexible region shifting would be an effective strategy for enzyme evolution through computational protein engineering.

From thiol-subtilisin to omniligase: Design and structure of a broadly applicable peptide ligase

Omniligase-1 is a broadly applicable enzyme for peptide bond formation between an activated acyl donor peptide and a non-protected acyl acceptor peptide. The enzyme is derived from an earlier subtilisin variant called peptiligase by several rounds of protein engineering aimed at increasing synthetic yields and substrate range. To examine the contribution of individual mutations on S/H ratio and substrate scope in peptide synthesis, we selected peptiligase variant M222P/L217H as a starting enzyme and introduced successive mutations. Mutation A225N in the S1′ pocket and F189W of the S2′ pocket increased the synthesis to hydrolysis (S/H) ratio and overall coupling efficiency, whereas the I107V mutation was added to S4 pocket to increase the reaction rate. The final omniligase variants appeared to have a very broad substrate range, coupling more than 250 peptides in a 400-member library of acyl acceptors, as indicated by a high-throughput FRET assay. Crystal structures and computational modelling could rationalize the exceptional properties of omniligase-1 in peptide synthesis

A highly efficient protein degradation system in Bacillus sp. CN2: a functional-degradomics study

A novel protease-producing Bacillus sp. CN2 isolated from chicken manure composts exhibited a relatively high proteolytic specific activity. The strain CN2 degradome consisted of at least 149 proteases and homolog candidates, which were distributed into 4 aspartic, 30 cysteine, 55 metallo, 56 serine, and 4 threonine proteases. Extracellular proteolytic activity was almost completely inhibited by PMSF (phenylmethylsulfonyl fluoride) rather than o-P, E-64, or pepstatin A, suggesting that strain CN2 primarily secreted serine protease. More importantly, analysis of the extracellular proteome of strain CN2 revealed the presence of a highly efficient protein degradation system. Three serine proteases of the S8 family with different active site architectures firstly fragmented protein substrates which were then degraded to smaller peptides by a M4 metalloendopeptidase that prefers to degrade hydrophobic peptides and by a S13 carboxypeptidase. Those enzymes acted synergistically to degrade intact substrate proteins outside the cell. Furthermore, highly expressed sequence-specific intracellular aminopeptidases from multiple families (M20, M29, and M42) accurately degraded peptides into oligopeptides or amino acids, thus realizing the rapid acquisition and utilization of nitrogen sources. In this paper, a systematic study of the functional-degradome provided a new perspective for understanding the complexity of the protease hydrolysis system of Bacillus, and laid a solid foundation for further studying the precise degradation of proteins with the cooperative action of different family proteases. KEY POINTS: • Bacillus sp. CN2 has relatively high proteolytic specific activity. • Bacillus sp. CN2 harbors a highly efficient protein degradation system. • The site-specific endopeptidases were secreted extracellular, while the sequence-specific aminopeptidases played a role in the cell.

Design of a substrate-tailored peptiligase variant for the efficient synthesis of thymosin-α1

The synthesis of thymosin-α₁, an acetylated 28 amino acid long therapeutic peptide, via conventional chemical methods is exceptionally challenging. The enzymatic coupling of unprotected peptide segments in water offers great potential for a more efficient synthesis of peptides that are difficult to synthesize. Based on the design of a highly engineered peptide ligase, we developed a fully convergent chemo-enzymatic peptide synthesis (CEPS) process for the production of thymosin-α₁via a 14-mer + 14-mer segment condensation strategy. Using structure-inspired enzyme engineering, the thiol-subtilisin variant peptiligase was tailored to recognize the respective 14-mer thymosin-α₁ segments in order to create a clearly improved biocatalyst, termed thymoligase. Thymoligase catalyzes peptide bond formation between both segments with a very high efficiency (>94% yield) and is expected to be well applicable to many other ligations in which residues with similar characteristics (e.g. Arg and Glu) are present in the respective positions P1 and P1'. The crystal structure of thymoligase was determined and shown to be in good agreement with the model used for the engineering studies. The combination of the solid phase peptide synthesis (SPPS) of the 14-mer segments and their thymoligase-catalyzed ligation on a gram scale resulted in a significantly increased, two-fold higher overall yield (55%) of thymosin-α₁ compared to those typical of existing industrial processes.

Enhanced extracellular recombinant keratinase activity in Bacillus subtilis SCK6 through signal peptide optimization and site-directed mutagenesis

The extracellular recombinant keratinase activity in Bacillus subtilis SCK6 was enhanced by signal peptide optimization and site-directed mutagenesis.

Understanding Interactions of Surfactants and Enzymes: Impact of Individual Surfactants on Stability and Wash Performance of Protease Enzyme in Detergents

Enzymes and surfactants are both essential ingredients that determine the performance of modern laundry detergents. We have conducted an investigation of the interaction of surfactants and enzymes under laundry detergent application conditions in order to understand the influence of individual ingredients and to optimize detergent performance. We can show that for a given protease enzyme, individual surfactants in a constant detergent matrix have a significant impact on relevant stability and performance parameter. While certain anionic surfactants like e.g. linear alkylbenzene sulfonate show strong protease inactivation, nonionic surfactants did only show slight inactivation over time. On the other hand, proteolytic performance of protease on test stains was most driven by fatty alcohol ether sulfate. Knowledge about the impact of individual surfactants on proteases will enable the best choice of ingredients for mixed surfactant systems with optimized enzyme performance and stability.

A single mutation Gln142Lys doubles the catalytic activity of VPR, a cold adapted subtilisin-like serine proteinase

Structural comparisons of the cold adapted subtilase VPR and its thermophilic homologue, aqualysin I (AQUI) indicated the presence of additional salt bridges in the latter. Few of those appear to contribute significantly to thermal stability of AQUI. This includes a putative salt bridge between residues Lys142 and Glu172 as its deletion did not have any significant effect on its stability or activity (Jónsdóttir et al. (2014)). Insertion of this putative salt bridge into the structure of VPR, in a double mutant (VPRΔC_Q142K/S172E), however was detrimental to the stability of the enzyme. Incorporation of either the Q142K or S172E mutations into VPR, were found to significantly affect the catalytic properties of the enzyme. The single mutation Q142K was highly effective, as it increased the kcat and kcat/Km more than twofold. When the Q142K mutation was inserted into a thermostabilized, but a low activity mutant of VPR (VPRΔC_N3P/I5P), the activity increased about tenfold in terms of kcat and kcat/Km, while retaining the stability of the mutant. Molecular dynamics simulations of the single mutants were carried out to provide structural rationale for these experimental observations. Based on root mean square fluctuation (RMSF) profiles, the two mutants were more flexible in certain regions of the structure and the Q142K mutant had the highest overall flexibility of the three enzymes. The results suggest that weakening of specific H-bonds resulting from the mutations may be propagated over some distance giving rise to higher flexibility in the active site regions of the enzyme, causing higher catalytic activity in the mutants.

Structure of a switchable subtilisin complexed with a substrate and with the activator azide

An engineered variant of the protease subtilisin from Bacillus amyloliquefaciens, in which the D32A mutation renders the enzyme's activity dependent on the presence of certain small anions such as fluoride or azide, has been produced. This modified enzyme has applications as an azide or fluoride-triggered expression-purification tool. We report activity measurements showing that the enzyme is activated more than 3000-fold by azide and describe the 1.8 A resolution structure of an inactive form (by replacing the catalytic nucleophile Ser 221 with alanine) of the protease, in complex with azide and with a substrate that spans the active site. Both enzyme and substrate have been engineered to increase their stability and the affinity of their interaction. The substrate is based on a stabilized subtilisin prodomain, extended across the active site by the addition of four residues at its C-terminus. In the crystal structure, the substrate is well-ordered across the active site, and the azide anion is observed bound adjacent to Ala 32. The structures of the substrate complex in three different crystals (anion-free, fluoride-soaked, and azide-soaked) are compared. These structures provide extensive information for understanding subtilisin's substrate binding and catalytic mechanism, and for the development of biotechnology tools based on anion-activated proteolysis. The mechanism of anion-dependent proteolysis appears to be a slight modification of the accepted charge-relay mechanism for serine proteases.

PIK3CA somatic mutations in breast cancer: Mechanistic insights from Langevin dynamics simulations

Somatic mutations in PIK3CA (phosphatidylinositol-3 kinase, catalytic subunit, alpha isoform) are reported in breast and other human cancers to concentrate at hotspots within its kinase and helical domains. Most of these mutations cause kinase gain of function in vitro and are associated with oncogenicity in vivo. However, little is known about the mechanisms driving tumor development. We have performed computational structural studies on a homology model of wildtype PIK3CA plus recurrent H1047R, H1047L, and P539R mutations, located in the kinase and helical domains, respectively. The time evolution of the structures show that H1047R/L mutants exhibit a larger area of the catalytic cleft between the kinase N- and C-lobes compared with the wildtype that could facilitate the entrance of substrates. This larger area might yield enhanced substrate-to-product turnover associated with oncogenicity. In addition, the H1047R/L mutants display increased kinase activation loop mobility, compared with the wildtype. The P539R mutant forms more hydrogen bonds and salt-bridge interactions than the wildtype, properties that are associated with enhanced thermostability. Mutant-specific differences in the catalytic cleft and activation loop behavior suggest that structure-based mutant-specific inhibitors can be designed for PIK3CA-positive breast cancers.

The crystal structures of the psychrophilic subtilisin S41 and the mesophilic subtilisin Sph reveal the same calcium-loaded state

We determine and compare the crystal structure of two proteases belonging to the subtilisin superfamily: S41, a cold-adapted serine protease produced by Antarctic bacilli, at 1.4 A resolution and Sph, a mesophilic serine protease produced by Bacillus sphaericus, at 0.8 A resolution. The purpose of this comparison was to find out whether multiple calcium ion binding is a molecular factor responsible for the adaptation of S41 to extreme low temperatures. We find that these two subtilisins have the same subtilisin fold with a root mean square between the two structures of 0.54 A. The final models for S41 and Sph include a calcium-loaded state of five ions bound to each of these two subtilisin molecules. None of these calcium-binding sites correlate with the high affinity known binding site (site A) found for other subtilisins. Structural analysis of the five calcium-binding sites found in these two crystal structures indicate that three of the binding sites have two side chains of an acidic residue coordinating the calcium ion, whereas the other two binding sites have either a main-chain carbonyl, or only one acidic residue side chain coordinating the calcium ion. Thus, we conclude that three of the sites are of high affinity toward calcium ions, whereas the other two are of low affinity. Because Sph is a mesophilic subtilisin and S41 is a psychrophilic subtilisin, but both crystal structures were found to bind five calcium ions, we suggest that multiple calcium ion binding is not responsible for the adaptation of S41 to low temperatures.

Isolation, purification and characterization of a surfactants-, laundry detergents- and organic solvents-resistant alkaline protease from Bacillus sp. HR-08

Bacillus sp. HR-08 screened from soil samples of Iran, is capable of producing proteolytic enzymes. 16S rDNA analysis showed that this strain is closely related to Bacillus subtilis, Bacillus licheniformis, Bacillus pumilus, Bacillus mojavensis, and Bacillus atrophaeus. The zymogram analysis of the crude extract revealed the presence of five extracellular proteases. One of the proteases was purified in three steps procedure involving ammonium sulfate precipitation, DEAE-Sepharose ionic exchange and Sephacryl S-200 gel filtration chromatography. The molecular mass of the enzyme on SDS-PAGE was estimated to be 29 kDa. The protease exhibited maximum activity at pH 10.0 and 60 degrees C and was inhibited by PMSF but it was not affected by cysteine inhibitors, suggesting that the enzyme is a serine alkaline protease. Irreversible thermoinactivation of enzyme was examined at 50, 60, and 70 degrees C in the presence of 10 mM CaCl(2). Results showed that the protease activity retains more than 80% and 50% of its initial activity after incubation for 30 min at 60 and 70 degrees C, respectively. This enzyme had good stability in the presence of H(2)O(2), nonionic surfactant, and local detergents and its activity was enhanced in the presence of 20% of dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and isopropanol. The enzyme retained more than 90% of its initial activity after pre-incubation 1 h at room temperature in the presence of 20% of these solvents. Also, activation can be seen for the enzyme at high concentration (50%, v/v) of DMF and DMSO.

Structural insights into cold inactivation of tryptophanase and cold adaptation of subtilisin S41

A wide variety of enzymes can undergo a reversible loss of activity at low temperature, a process that is termed cold inactivation. This phenomenon is found in oligomeric enzymes such as tryptophanase (Trpase) and other pyridoxal phosphate dependent enzymes. On the other hand, cold-adapted, or psychrophilic enzymes, isolated from organisms able to thrive in permanently cold environments, have optimal activity at low temperature, which is associated with low thermal stability. Since cold inactivation may be considered "contradictory" to cold adaptation, we have looked into the amino acid sequences and the crystal structures of two families of enzymes, subtilisin and tryptophanase. Two cold adapted subtilisins, S41 and subtilisin-like protease from Vibrio, were compared to a mesophilic and a thermophilic subtilisins, as well as to four PLP-dependent enzymes in order to understand the specific surface residues, specific interactions, or any other molecular features that may be responsible for the differences in their tolerance to cold temperatures. The comparison between the psychrophilic and the mesophilic subtilisins revealed that the cold adapted subtilisins have a high content of acidic residues mainly found on their surface, making it charged. The analysis of the Trpases showed that they have a high content of hydrophobic residues on their surface. Thus, we suggest that the negatively charged residues on the surface of the subtilisins may be responsible for their cold adaptation, whereas the hydrophobic residues on the surface of monomeric Trpase molecules are responsible for the tetrameric assembly, and may account for their cold inactivation and dissociation.

Engineered disulfide bonds increase active-site local stability and reduce catalytic activity of a cold-adapted alkaline phosphatase

Alkaline phosphatase is an extracellular enzyme that is membrane-bound in eukaryotes but resides in the periplasmic space of bacteria. It normally carries four cysteine residues that form two disulfide bonds, for instance in the APs of Escherichia coli and vertebrates. An AP variant from a Vibrio sp. has only one cysteine residue. This cysteine is second next to the nucleophilic serine in the active site. We have individually modified seven residues to cysteine that are on two loops predicted to be within a 5 A radius. Four of them formed a disulfide bond to the endogenous cysteine. Thermal stability was monitored by circular dichroism and activity measurements. Global stability was similar to the wild-type enzyme. However, a significant increase in heat-stability was observed for the disulfide-containing variants using activity as a measure, together with a large reduction in catalytic rates (k(cat)) and a general decrease in Km values. The results suggest that a high degree of mobility near the active site and in the helix carrying the endogenous cysteine is essential for full catalytic efficiency in the cold-adapted AP.

Iterative saturation mutagenesis (ISM) for rapid directed evolution of functional enzymes

Iterative saturation mutagenesis (ISM) is a new and efficient method for the directed evolution of functional enzymes. It reduces the necessary molecular biological work and the screening effort drastically. It is based on a Cartesian view of the protein structure, performing iterative cycles of saturation mutagenesis at rationally chosen sites in an enzyme, a given site being composed of one, two or three amino acid positions. The basis for choosing these sites depends on the nature of the catalytic property to be improved, e.g., enantioselectivity, substrate acceptance or thermostability. In the case of thermostability, sites showing highest B-factors (available from X-ray data) are chosen. The pronounced increase in thermostability of the lipase from Bacillus subtilis (Lip A) as a result of applying ISM is illustrated here.

Engineering proteinase K using machine learning and synthetic genes

Background

Altering a protein's function by changing its sequence allows natural proteins to be converted into useful molecular tools. Current protein engineering methods are limited by a lack of high throughput physical or computational tests that can accurately predict protein activity under conditions relevant to its final application. Here we describe a new synthetic biology approach to protein engineering that avoids these limitations by combining high throughput gene synthesis with machine learning-based design algorithms.

Results

We selected 24 amino acid substitutions to make in proteinase K from alignments of homologous sequences. We then designed and synthesized 59 specific proteinase K variants containing different combinations of the selected substitutions. The 59 variants were tested for their ability to hydrolyze a tetrapeptide substrate after the enzyme was first heated to 68°C for 5 minutes. Sequence and activity data was analyzed using machine learning algorithms. This analysis was used to design a new set of variants predicted to have increased activity over the training set, that were then synthesized and tested. By performing two cycles of machine learning analysis and variant design we obtained 20-fold improved proteinase K variants while only testing a total of 95 variant enzymes.

Conclusion

The number of protein variants that must be tested to obtain significant functional improvements determines the type of tests that can be performed. Protein engineers wishing to modify the property of a protein to shrink tumours or catalyze chemical reactions under industrial conditions have until now been forced to accept high throughput surrogate screens to measure protein properties that they hope will correlate with the functionalities that they intend to modify. By reducing the number of variants that must be tested to fewer than 100, machine learning algorithms make it possible to use more complex and expensive tests so that only protein properties that are directly relevant to the desired application need to be measured. Protein design algorithms that only require the testing of a small number of variants represent a significant step towards a generic, resource-optimized protein engineering process.

Directed evolution of enzyme stability

Modern enzyme development relies to an increasing extent on strategies based on diversity generation followed by screening for variants with optimised properties. In principle, these directed evolution strategies might be used for optimising any enzyme property, which can be screened for in an economically feasible way, even if the molecular basis of that property is not known. Stability is an interesting property of enzymes because (1) it is of great industrial importance, (2) it is relatively easy to screen for, and (3) the molecular basis of stability relates closely to contemporary issues in protein science such as the protein folding problem and protein folding diseases. Thus, engineering enzyme stability is of both commercial and scientific interest. Here, we review how directed evolution has contributed to the development of stable enzymes and to new insight into the principles of protein stability. Several recent examples are described. These examples show that directed evolution is an effective strategy to obtain stable enzymes, especially when used in combination with rational or semi-rational engineering strategies. With respect to the principles of protein stability, some important lessons to learn from recent efforts in directed evolution are (1) that there are many structural ways to stabilize a protein, which are not always easy to rationalize, (2) that proteins may very well be stabilized by optimizing their surfaces, and (3) that high thermal stability may be obtained without forfeiture of catalytic performance at low temperatures.

An engineered disulfide bridge mimics the effect of calcium to protect neutral protease against local unfolding

The extreme thermal stabilization achieved by the introduction of a disulfide bond (G8C/N60C) into the cysteine-free wild-type-like mutant (pWT) of the neutral protease from Bacillus stearothermophilus[Mansfeld J, Vriend G, Dijkstra BW, Veltman OR, Van den Burg B, Venema G, Ulbrich-Hofmann R & Eijsink VG (1997) J Biol Chem272, 11152-11156] was attributed to the fixation of the loop region 56-69. In this study, the role of calcium ions in the guanidine hydrochloride (GdnHCl)-induced unfolding and autoproteolysis kinetics of pWT and G8C/N60C was analyzed by fluorescence spectroscopy, far-UV CD spectroscopy and SDS/PAGE. First-order rate constants (kobs) were evaluated by chevron plots (ln kobs vs. GdnHCl concentration). The kobs of unfolding showed a difference of nearly six orders of magnitude (DeltaDeltaG# = 33.5 kJ.mol(-1) at 25 degrees C) between calcium saturation (at 100 mM CaCl2) and complete removal of calcium ions (in the presence of 100 mM EDTA). Analysis of the protease variant W55F indicated that calcium binding-site III, situated in the critical region 56-69, determines the stability at calcium ion concentrations between 5 and 50 mM. In the chevron plots the disulfide bridge in G8C/N60C shows a similar effect compared with pWT as the addition of calcium ions, suggesting that the introduced disulfide bridge fixes the region (near calcium binding-site III) that is responsible for unfolding and subsequent autoproteolysis. Owing to the presence of the disulfide bridge, the DeltaDeltaG# is 13.2 kJ.mol(-1) at 25 degrees C and 5 mM CaCl2. Non-linear chevron plots reveal an intermediate in unfolding probably caused by local unfolding of the loop 56-69. The occurrence of this intermediate is prevented by calcium concentrations of > 5 mM, or the introduction of the disulfide bridge G8C/N60C.

Directed Coevolution of Stability and Catalytic Activity in Calcium-free Subtilisin

We have coevolved high activity and hyperstability in subtilisin by sequentially randomizing 12 amino acid positions in calcium-free subtilisin. The optimal amino acid for each randomized site was chosen based on stability and catalytic properties and became the parent clone for the next round of mutagenesis. Together, the 12 selected mutations increased the half-life of calcium-free subtilisin at elevated temperature by 15,000-fold. The catalytic properties of the mutants were examined against a range of substrates. In general, only mutations occurring at or near the substrate-binding surface have measurable effects on catalytic constants. No direct influence of stability on catalytic properties was observed. A high-stability mutant, Sbt140, was a more efficient enzyme in terms of k(cat)/K(m) than a commercial version of subtilisin across a range of substrates but had a lower k(cat) against tight-binding substrates. The reason for this behavior was discerned by examining microscopic rate constants for the hydrolysis of a tight-binding peptide substrate. Burst kinetics were observed for this substrate, indicating that acylation is not rate-limiting. Although acylation occurs at the rate of substrate binding, k(cat) is attenuated by the slow release of the N-terminal product. Natural evolution appears to have optimized catalytic activity against a range of sequences by achieving a balance between substrate binding and the rate of release of the N-terminal product.

Trapping a 96 degrees domain rotation in two distinct conformations by engineered disulfide bridges

Engineering disulfide bridges is a common technique to lock a protein movement in a defined conformational state. We have designed two double mutants of Escherichia coli 5'-nucleotidase to trap the enzyme in both an open (S228C, P513C) and a closed (P90C, L424C) conformation by the formation of disulfide bridges. The mutant proteins have been expressed, purified, and crystallized, to structurally characterize the designed variants. The S228C, P513C is a double mutant crystallized in two different crystal forms with three independent conformers, which differ from each other by a rotation of up to 12 degrees of the C-terminal domain with respect to the N-terminal domain. This finding, as well as an analysis of the domain motion in the crystal, indicates that the enzyme still exhibits considerable residual domain flexibility. In the double mutant that was designed to trap the enzyme in the closed conformation, the structure analysis reveals an unexpected intermediate conformation along the 96 degrees rotation trajectory between the open and closed enzyme forms. A comparison of the five independent conformers analyzed in this study shows that the domain movement of the variant enzymes is characterized by a sliding movement of the residues of the domain interface along the interface, which is in contrast to a classical closure motion where the residues of the domain interface move perpendicular to the interface.

Structural basis of selection and thermostability of laboratory evolved Bacillus subtilis lipase

Variation in gene sequences generated by directed evolution approaches often does not assure a minimalist design for obtaining a desired property in proteins. While screening for enhanced thermostability, structural information was utilized in selecting mutations that are generated by error-prone PCR. By this approach we have increased the half-life of denaturation by 300-fold compared to the wild-type Bacillus subtilis lipase through three point mutations generated by only two cycles of error-prone PCR. At lower temperatures the activity parameters of the thermostable mutants are unaltered. High-resolution crystal structures of the mutants show subtle changes, which include stacking of tyrosine residues, peptide plane flipping and a better anchoring of the terminus, that challenge rational design and explain the structural basis for enhanced thermostability. The approach may offer an efficient and minimalist solution for the enhancement of a desired property of a protein.

The structural basis of the thermostability of SP1, a novel plant (Populus tremula) boiling stable protein

We previously reported on a new boiling stable protein isolated from aspen plants (Populus tremula), which we named SP1. SP1 is a stress-related protein with no significant sequence homology to other stress-related proteins. It is a 108-amino-acid hydrophilic polypeptide with a molecular mass of 12.4 kDa (Wang, W. X., Pelah, D., Alergand, T., Shoseyov, O., and Altman, A. (2002) Plant Physiol. 130, 865-875) and is found in an oligomeric form. Preliminary electron microscopy studies and matrix-assisted laser desorption ionization time-of-flight mass spectrometry experiments showed that SP1 is a dodecamer composed of two stacking hexamers. We performed a SDS-PAGE analysis, a differential scanning calorimetric study, and crystal structure determination to further characterize SP1. SDS-PAGE indicated a spontaneous assembly of SP1 to one stable oligomeric form, a dodecamer. Differential scanning calorimetric showed that SP1 has high thermostability i.e. Tm of 107 degrees C (at pH 7.8). The crystal structure of SP1 was initially determined to 2.4 A resolution by multi-wavelength anomalous dispersion method from a crystal belonging to the space group I422. The phases were extended to 1.8 A resolution using data from a different crystal form (P21). The final refined molecule includes 106 of the 108 residues and 132 water molecules (on average for each chain). The R-free is 20.1%. The crystal structure indicated that the SP1 molecule has a ferredoxin-like fold. Strong interactions between each two molecules create a stable dimer. Six dimers associate to form a ring-like-shaped dodecamer strongly resembling the particle visualized in the electron microscopy studies. No structural similarity was found between the crystal structure of SP1 and the crystal structure of other stress-related proteins such as small heat shock proteins, whose structure has been already determined. This structural study further supports our previous report that SP1 may represent a new family of stress-related proteins with high thermostability and oligomerization.

The 0.93Å Crystal Structure of Sphericase: A Calcium-loaded Serine Protease from Bacillus sphaericus

We have previously isolated sphericase (Sph), an extracellular mesophilic serine protease produced by Bacillus sphaericus. The Sph amino acid sequence is highly homologous to two cold-adapted subtilisins from Antarctic bacilli S39 and S41 (76% and 74% identity, respectively). Sph is calcium-dependent, 310 amino acid residues long and has optimal activity at pH 10.0. S41 and S39 have not as yet been structurally analysed. In the present work, we determined the crystal structure of Sph by the Eu/multiwavelength anomalous diffraction method. The structure was extended to 0.93A resolution and refined to a crystallographic R-factor of 9.7%. The final model included all 310 amino acid residues, one disulfide bond, 679 water molecules and five calcium ions. Although Sph is a mesophilic subtilisin, its amino acid sequence is similar to that of the psychrophilic subtilisins, which suggests that the crystal structure of these subtilisins is very similar. The presence of five calcium ions bound to a subtilisin molecule, as found here for Sph, has not been reported for the subtilisin superfamily. None of these calcium-binding sites correlates with the well-known high-affinity calcium-binding site (site I or site A), and only one site has been described previously. This calcium-binding pattern suggests that a reduction in the flexibility of the surface loops of Sph by calcium binding may be responsible for its adaptation to mesophilic organisms.

A simple electrostatic criterion for predicting the thermal stability of proteins

The enhancement of protein thermostability is an important issue for both basic science and biotechnology purposes. We have developed a thermostability criterion for a protein in terms of a quasi-electric dipole moment (contributed by its charged residues) defined for an electric charge distribution whose total charge is not zero. It was found that minimization of the modulus of this dipole moment increased its thermal stability, as demonstrated by surveying these values in pairs of mesostable-thermostable homologous proteins and in mutations described in the literature. The analysis of these observations provides criteria for thermostabilization of a protein, by computing its dipole profile. This profile is obtained by direct substitution of each amino acid of the sequence by either a positive, negative or neutral amino acid, followed by a recalculation of the dipole moment. As an experimental example, these criteria were applied to a beta-glucanase to enhance its thermal stability.