Recombinant phosphoglycerate kinase from the hyperthermophilic bacterium Thermotoga maritima: catalytic, spectral and thermodynamic properties

Overview of strategies for developing high thermostability industrial enzymes: Discovery, mechanism, modification and challenges

Biocatalysts such as enzymes are environmentally friendly and have substrate specificity, which are preferred in the production of various industrial products. However, the strict reaction conditions in industry including high temperature, organic solvents, strong acids and bases and other harsh environments often destabilize enzymes, and thus substantially compromise their catalytic functions, and greatly restrict their applications in food, pharmaceutical, textile, bio-refining and feed industries. Therefore, developing industrial enzymes with high thermostability becomes very important in industry as thermozymes have more advantages under high temperature. Discovering new thermostable enzymes using genome sequencing, metagenomics and sample isolation from extreme environments, or performing molecular modification of the existing enzymes with poor thermostability using emerging protein engineering technology have become an effective means of obtaining thermozymes. Based on the thermozymes as biocatalytic chips in industry, this review systematically analyzes the ways to discover thermostable enzymes from extreme environment, clarifies various interaction forces that will affect thermal stability of enzymes, and proposes different strategies to improve enzymes' thermostability. Furthermore, latest development in the thermal stability modification of industrial enzymes through rational design strategies is comprehensively introduced from structure-activity relationship point of view. Challenges and future research perspectives are put forward as well.

Evolution of Protein Structure and Stability in Global Warming

This review focuses on the molecular signatures of protein structures in relation to evolution and survival in global warming. It is based on the premise that the power of evolutionary selection may lead to thermotolerant organisms that will repopulate the planet and continue life in general, but perhaps with different kinds of flora and fauna. Our focus is on molecular mechanisms, whereby known examples of thermoresistance and their physicochemical characteristics were noted. A comparison of interactions of diverse residues in proteins from thermophilic and mesophilic organisms, as well as reverse genetic studies, revealed a set of imprecise molecular signatures that pointed to major roles of hydrophobicity, solvent accessibility, disulfide bonds, hydrogen bonds, ionic and π-electron interactions, and an overall condensed packing of the higher-order structure, especially in the hydrophobic regions. Regardless of mutations, specialized protein chaperones may play a cardinal role. In evolutionary terms, thermoresistance to global warming will likely occur in stepwise mutational changes, conforming to the molecular signatures, such that each "intermediate" fits a temporary niche through punctuated equilibrium, while maintaining protein functionality. Finally, the population response of different species to global warming may vary substantially, and, as such, some may evolve while others will undergo catastrophic mass extinction.

Phosphoglycerate kinase 1 (PGK1) in cancer: A promising target for diagnosis and therapy

Phosphoglycerate kinase 1 (PGK1) is the first critical enzyme to produce ATP in the glycolytic pathway. PGK1 is not only a metabolic enzyme but also a protein kinase, which mediates the tumor growth, migration and invasion through phosphorylation some important substrates. Moreover, PGK1 is associated with poor treatment and prognosis of cancers. This manuscript reviews the structure, functions, post-translational modifications (PTMs) of PGK1 and its relationship with tumors, which demonstrates that PGK1 has indispensable value in the tumor progression. The current review highlights the important role of PGK1 in anticancer treatments.

Optimization of expression and properties of the recombinant acetohydroxyacid synthase of Thermotoga maritima

The data provide additional support of the characterization of the biophysical and biochemical properties of the enzyme acetohydroxyacid synthase from the hyperthermophilic bacterium Thermotoga maritima (Eram et al., 2015) [1]. The genes encoding the enzyme subunits have been cloned and expressed in the mesophilic host Escherichia coli. Detailed data include information about the optimization of the expression conditions, biophysical properties of the enzyme and reconstitution of the holoenzyme from individually expressed and purified subunits.

Protein Stability, Folding and Misfolding in Human PGK1 Deficiency

Conformational diseases are often caused by mutations, altering protein folding and stability in vivo. We review here our recent work on the effects of mutations on the human phosphoglycerate kinase 1 (hPGK1), with a particular focus on thermodynamics and kinetics of protein folding and misfolding. Expression analyses and in vitro biophysical studies indicate that disease-causing mutations enhance protein aggregation propensity. We found a strong correlation among protein aggregation propensity, thermodynamic stability, cooperativity and dynamics. Comparison of folding and unfolding properties with previous reports in PGKs from other species suggests that hPGK1 is very sensitive to mutations leading to enhance protein aggregation through changes in protein folding cooperativity and the structure of the relevant denaturation transition state for aggregation. Overall, we provide a mechanistic framework for protein misfolding of hPGK1, which is insightful to develop new therapeutic strategies aimed to target native state stability and foldability in hPGK1 deficient patients.

Stabilizing salt-bridge enhances protein thermostability by reducing the heat capacity change of unfolding

Most thermophilic proteins tend to have more salt bridges, and achieve higher thermostability by up-shifting and broadening their protein stability curves. While the stabilizing effect of salt-bridge has been extensively studied, experimental data on how salt-bridge influences protein stability curves are scarce. Here, we used double mutant cycles to determine the temperature-dependency of the pair-wise interaction energy and the contribution of salt-bridges to ΔC_p in a thermophilic ribosomal protein L30e. Our results showed that the pair-wise interaction energies for the salt-bridges E6/R92 and E62/K46 were stabilizing and insensitive to temperature changes from 298 to 348 K. On the other hand, the pair-wise interaction energies between the control long-range ion-pair of E90/R92 were negligible. The ΔC_p of all single and double mutants were determined by Gibbs-Helmholtz and Kirchhoff analyses. We showed that the two stabilizing salt-bridges contributed to a reduction of ΔC_p by 0.8–1.0 kJ mol⁻¹ K⁻¹. Taken together, our results suggest that the extra salt-bridges found in thermophilic proteins enhance the thermostability of proteins by reducing ΔC_p, leading to the up-shifting and broadening of the protein stability curves.

Evolution and thermodynamics of the slow unfolding of hyperstable monomeric proteins

BACKGROUND

The unfolding speed of some hyperthermophilic proteins is dramatically lower than that of their mesostable homologs. Ribonuclease HII from the hyperthermophilic archaeon Thermococcus kodakaraensis (Tk-RNase HII) is stabilized by its remarkably slow unfolding rate, whereas RNase HI from the thermophilic bacterium Thermus thermophilus (Tt-RNase HI) unfolds rapidly, comparable with to that of RNase HI from Escherichia coli (Ec-RNase HI).

RESULTS

To clarify whether the difference in the unfolding rate is due to differences in the types of RNase H or differences in proteins from archaea and bacteria, we examined the equilibrium stability and unfolding reaction of RNases HII from the hyperthermophilic bacteria Thermotoga maritima (Tm-RNase HII) and Aquifex aeolicus (Aa-RNase HII) and RNase HI from the hyperthermophilic archaeon Sulfolobus tokodaii (Sto-RNase HI). These proteins from hyperthermophiles are more stable than Ec-RNase HI over all the temperature ranges examined. The observed unfolding speeds of all hyperstable proteins at the different denaturant concentrations studied are much lower than those of Ec-RNase HI, which is in accordance with the familiar slow unfolding of hyperstable proteins. However, the unfolding rate constants of these RNases H in water are dispersed, and the unfolding rate constant of thermophilic archaeal proteins is lower than that of thermophilic bacterial proteins.

CONCLUSIONS

These results suggest that the nature of slow unfolding of thermophilic proteins is determined by the evolutionary history of the organisms involved. The unfolding rate constants in water are related to the amount of buried hydrophobic residues in the tertiary structure.

Unfolding of hydrophobic polymers in guanidinium chloride solutions

Guanidinium chloride (GdmCl) is a widely used chemical denaturant that unfolds proteins. Its effects on hydrophobic interactions are, however, not fully understood. We quantify the effects of GdmCl on various manifestations of hydrophobicity--from solvation and interactions of small solutes to folding-unfolding of hydrophobic polymers--in water and in concentrated GdmCl solutions. For comparison, we also perform similar calculations in solutions of NaCl and CsCl in water. Like NaCl and CsCl, GdmCl increases the surface tension of water, decreases the solubility of small hydrophobic solutes, and enhances the strength of hydrophobic interactions at the pair level. However, unlike NaCl and CsCl, GdmCl destabilizes folded states of hydrophobic polymers. We show that Gdm(+) ions preferentially coat the hydrophobic polymer, and it is the direct van der Waals interaction between Gdm(+) ions and the polymer that contributes to the destabilization of folded states. Interestingly, the temperature dependence of the free energy of unfolding of the hydrophobic polymer in water is protein-like, with signatures of both heat and cold denaturation. Addition of GdmCl shifts the cold denaturation temperature higher, into the experimentally accessible region. Finally, translational as well as conformational dynamics of the polymer are slower in GdmCl and correlate with dynamics of water molecules in solution.

Slow unfolding of monomeric proteins from hyperthermophiles with reversible unfolding

Based on the differences in their optimal growth temperatures microorganisms can be classified into psychrophiles, mesophiles, thermophiles, and hyperthermophiles. Proteins from hyperthermophiles generally exhibit greater stability than those from other organisms. In this review, we collect data about the stability and folding of monomeric proteins from hyperthermophilies with reversible unfolding, from the equilibrium and kinetic aspects. The results indicate that slow unfolding is a general strategy by which proteins from hyperthermophiles adapt to higher temperatures. Hydrophobic interaction is one of the factors in the molecular mechanism of the slow unfolding of proteins from hyperthermophiles.

A computational approach for the rational design of stable proteins and enzymes: optimization of surface charge-charge interactions

The design of stable proteins and enzymes is not only of particular biotechnological importance, but also addresses some important fundamental questions. While there are a number of different options available for designing or engineering stable proteins, the field of computational design provides fast and universal methods for stabilizing proteins of interest. One of the successful computational design strategies focuses on stabilizing proteins through the optimization of charge-charge interactions on the protein surface. By optimizing surface interactions, it is possible to alleviate some of the challenges that accompany efforts to redesign the protein core. The rational design of surface charge-charge interactions also allows one to optimize only the interactions that are distant from binding sites or active sites, making it possible to increase stability without adversely affecting activity. The optimization of surface charge-charge interactions is discussed in detail along with the experimental evidence to demonstrate that this is a robust and universal approach to designing proteins with enhanced stability.

Proteomic analysis of soybean nodule cytosol

An isolation procedure for soybean (Glycine max L. cv Williams 82) nodule cytosol proteins was developed which greatly improved protein resolution by two-dimensional polyacrylamide gel electrophoresis. The most abundant proteins were selected and analyzed by mass spectrometry. The identified proteins were categorized by function (% of total proteins analyzed): carbon metabolism (28%), nitrogen metabolism (12%), reactive oxygen metabolism (12%) and vesicular trafficking (11%). The first three categories were expected based on the known physiological functions of the symbiotic nitrogen fixation process. The number of proteins involved in vesicular trafficking suggests a very active exchange of macromolecules and membrane components. Among the 69 identified proteins were the enzymes of the three carbon portion of glycolysis, which were further characterized to support their roles in the sucrose synthase pathway to provide malate for the bacteroids. Proteomic analysis provides a functional tool by which to understand and further investigate nodule function.

Thermostable variants of the recombinant xylanase a from Bacillus subtilis produced by directed evolution show reduced heat capacity changes

Directed evolution techniques have been used to improve the thermal stability of the xylanase A from Bacillus subtilis (XylA). Two generations of random mutant libraries generated by error prone PCR coupled with a single generation of DNA shuffling produced a series of mutant proteins with increasing thermostability. The most Thermostable XylA variant from the third generation contained four mutations Q7H, G13R, S22P, and S179C that showed an increase in melting temperature of 20 degrees C. The thermodynamic properties of a representative subset of nine XylA variants showing a range of thermostabilities were measured by thermal denaturation as monitored by the change in the far ultraviolet circular dichroism signal. Analysis of the data from these thermostable variants demonstrated a correlation between the decrease in the heat capacity change (deltaC(p)) with an increase in the midpoint of the transition temperature (T(m)) on transition from the native to the unfolded state. This result could not be interpreted within the context of the changes in accessible surface area of the protein on transition from the native to unfolded states. Since all the mutations are located at the surface of the protein, these results suggest that an explanation of the decrease in deltaC(p) should include effects arising from the protein/solvent interface.

Structural, thermodynamic, and mutational analyses of a psychrotrophic RNase HI

Ribonuclease (RNase) HI from the psychrotrophic bacterium Shewanella oneidensis MR-1 was overproduced in Escherichia coli, purified, and structurally and biochemically characterized. The amino acid sequence of MR-1 RNase HI is 67% identical to that of E. coli RNase HI. The crystal structure of MR-1 RNase HI determined at 2.0 A resolution was highly similar to that of E. coli RNase HI, except that the number of intramolecular ion pairs and the fraction of polar surface area of MR-1 RNase HI were reduced compared to those of E. coli RNase HI. The enzymatic properties of MR-1 RNase HI were similar to those of E. coli RNase HI. However, MR-1 RNase HI was much less stable than E. coli RNase HI. The stability of MR-1 RNase HI against heat inactivation was lower than that of E. coli RNase HI by 19 degrees C. The conformational stability of MR-1 RNase HI was thermodynamically analyzed by monitoring the CD values at 220 nm. MR-1 RNase HI was less stable than E. coli RNase HI by 22.4 degrees C in Tm and 12.5 kJ/mol in DeltaG(H2O). The thermodynamic stability curve of MR-1 RNase HI was characterized by a downward shift and increased curvature, which results in an increased DeltaCp value, compared to that of E. coli RNase HI. Site-directed mutagenesis studies suggest that the difference in the number of intramolecular ion pairs partly accounts for the difference in stability between MR-1 and E. coli RNases HI.

Folding and assembly pathways of co-chaperonin proteins 10: Origin of bacterial thermostability

To compare folding/assembly processes of heptameric co-chaperonin proteins 10 (cpn10) from different species and search for the origin of thermostability in hyper-thermostable Aquifex aeolicus cpn10 (Aacpn10), we have studied two bacterial variants-Aacpn10 and Escherichia coli cpn10 (GroES)-and compared the results to data on Homo sapiens cpn10 (hmcpn10). Equilibrium denaturation of GroES by urea, guanidine hydrochloride (GuHCl) and temperature results in coupled heptamer-to-monomer transitions in all cases. This is similar to the behavior of Aacpn10 but differs from hmcpn10 denaturation in urea. Time-resolved experiments reveal that GroES unfolds before heptamer dissociation, whereas refolding/reassembly begins with folding of individual monomers; these assemble in a slower step. The sequential folding/assembly mechanism for GroES is rather similar to that observed for Aacpn10 but contradicts the parallel paths of hmcpn10. We reveal that Aacpn10's stability profile is shifted upwards, broadened, and also moved horizontally to higher temperatures, as compared to that of GroES.

Lessons in stability from thermophilic proteins

Studies that compare proteins from thermophilic and mesophilic organisms can provide insights into ability of thermophiles to function at their high habitat temperatures and may provide clues that enable us to better define the forces that stabilize all proteins. Most of the comparative studies have focused on thermal stability and show, as expected, that thermophilic proteins have higher Tm values than their mesophilic counterparts. Although these comparisons are useful, more detailed thermodynamic analyses are required to reach a more complete understanding of the mechanisms thermophilic protein employ to remain folded over a wider range of temperatures. This complete thermodynamic description allows one to generate a stability curve for a protein that defines how the conformational stability (DeltaG) varies with temperature. Here we compare stability curves for many pairs of homologous proteins from thermophilic and mesophilc organisms. Of the basic methods that can be employed to achieve enhanced thermostability, we find that most thermophilic proteins use the simple method that raises the DeltaG at all temperatures as the principal way to increase their Tm. We discuss and compare this thermodynamic method with the possible alternatives. In addition we propose ways that structural alterations and changes to the amino acid sequences might give rise to varied methods used to obtain thermostability.

Correlation between conformational stability of the ternary enzyme-substrate complex and domain closure of 3-phosphoglycerate kinase

3-phosphoglycerate kinase (PGK) is a typical two-domain hinge-bending enzyme with a well-structured interdomain region. The mechanism of domain-domain interaction and its regulation by substrate binding is not yet fully understood. Here the existence of strong cooperativity between the two domains was demonstrated by following heat transitions of pig muscle and yeast PGKs using differential scanning microcalorimetry and fluorimetry. Two mutants of yeast PGK containing a single tryptophan fluorophore either in the N- or in the C-terminal domain were also studied. The coincidence of the calorimetric and fluorimetric heat transitions in all cases indicated simultaneous, highly cooperative unfolding of the two domains. This cooperativity is preserved in the presence of substrates: 3-phosphoglycerate bound to the N domain or the nucleotide (MgADP, MgATP) bound to the C domain increased the structural stability of the whole molecule. A structural explanation of domain-domain interaction is suggested by analysis of the atomic contacts in 12 different PGK crystal structures. Well-defined backbone and side-chain H bonds, and hydrophobic and electrostatic interactions between side chains of conserved residues are proposed to be responsible for domain-domain communication. Upon binding of each substrate newly formed molecular contacts are identified that firstly explain the order of the increased heat stability in the various binary complexes, and secondly describe the possible route of transmission of the substrate-induced conformational effects from one domain to the other. The largest stability is characteristic of the native ternary complex and is abolished in the case of a chemically modified inactive form of PGK, the domain closure of which was previously shown to be prevented [Sinev MA, Razgulyaev OI, Vas M, Timchenko AA & Ptitsyn OB (1989) Eur J Biochem180, 61-66]. Thus, conformational stability correlates with domain closure that requires simultaneous binding of both substrates.

Electrostatic interactions contribute to reduced heat capacity change of unfolding in a thermophilic ribosomal protein l30e

The origin of reduced heat capacity change of unfolding (DeltaC(p)) commonly observed in thermophilic proteins is controversial. The established theory that DeltaC(p) is correlated with change of solvent-accessible surface area cannot account for the large differences in DeltaC(p) observed for thermophilic and mesophilic homologous proteins, which are very similar in structures. We have determined the protein stability curves, which describe the temperature dependency of the free energy change of unfolding, for a thermophilic ribosomal protein L30e from Thermococcus celer, and its mesophilic homologue from yeast. Values of DeltaC(p), obtained by fitting the free energy change of unfolding to the Gibbs-Helmholtz equation, were 5.3 kJ mol(-1) K(-1) and 10.5 kJ mol(-1) K(-1) for T.celer and yeast L30e, respectively. We have created six charge-to-neutral mutants of T.celer L30e. Removal of charges at Glu6, Lys9, and Arg92 decreased the melting temperatures of T.celer L30e by approximately 3-9 degrees C, and the differences in melting temperatures were smaller with increasing concentration of salt. These results suggest that these mutations destabilize T.celer L30e by disrupting favorable electrostatic interactions. To determine whether electrostatic interactions contribute to the reduced DeltaC(p) of the thermophilic protein, we have determined DeltaC(p) for wild-type and mutant T.celer L30e by Gibbs-Helmholtz and by van't Hoff analyses. A concomitant increase in DeltaC(p) was observed for those charge-to-neutral mutants that destabilize T.celer L30e by removing favorable electrostatic interactions. The crystal structures of K9A, E90A, and R92A, were determined, and no structural change was observed. Taken together, our results support the conclusion that electrostatic interactions contribute to the reduced DeltaC(p) of T.celer L30e.

Distribution and phylogenies of enzymes of the Embden-Meyerhof-Parnas pathway from archaea and hyperthermophilic bacteria support a gluconeogenic origin of metabolism

Enzymes of the gluconeogenic/glycolytic pathway (the Embden-Meyerhof-Parnas (EMP) pathway), the reductive tricarboxylic acid cycle, the reductive pentose phosphate cycle and the Entner-Doudoroff pathway are widely distributed and are often considered to be central to the origins of metabolism. In particular, several enzymes of the lower portion of the EMP pathway (the so-called trunk pathway), including triosephosphate isomerase (TPI; EC 5.3.1.1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12/13), phosphoglycerate kinase (PGK; EC 2.7.2.3) and enolase (EC 4.2.1.11), are extremely well conserved and universally distributed among the three domains of life. In this paper, the distribution of enzymes of gluconeogenesis/glycolysis in hyperthermophiles--microorganisms that many believe represent the least evolved organisms on the planet--is reviewed. In addition, the phylogenies of the trunk pathway enzymes (TPIs, GAPDHs, PGKs and enolases) are examined. The enzymes catalyzing each of the six-carbon transformations in the upper portion of the EMP pathway, with the possible exception of aldolase, are all derived from multiple gene sequence families. In contrast, single sequence families can account for the archaeal and hyperthermophilic bacterial enzyme activities of the lower portion of the EMP pathway. The universal distribution of the trunk pathway enzymes, in combination with their phylogenies, supports the notion that the EMP pathway evolved in the direction of gluconeogenesis, i.e., from the bottom up.

An electrostatic basis for the stability of thermophilic proteins

Two factors provide key contributions to the stability of thermophilic proteins relative to their mesophilic homologues: electrostatic interactions of charged residues in the folded state and the dielectric response of the folded protein. The dielectric response for proteins in a "thermophilic series" globally modulates the thermal stability of its members, with the calculated dielectric constant for the protein increasing from mesophiles to hyperthermophiles. This variability results from differences in the distribution of charged residues on the surface of the protein, in agreement with structural and genetic observations. Furthermore, the contribution of electrostatic interactions to the stability of the folded state is more favorable for thermophilic proteins than for their mesophilic homologues. This leads to the conclusion that electrostatic interactions play an important role in determining the stability of proteins at high temperatures. The interplay between electrostatic interactions and dielectric response also provides further rationalization for the enhanced stability of thermophilic proteins with respect to cold-denaturation. Taken together, the distribution of charged residues and their fluctuations have been shown to be factors in modulating protein stability over the entire range of biologically relevant temperatures.

Mechanism of thermostabilization in a designed cold shock protein with optimized surface electrostatic interactions

Using computational and sequence analysis of bacterial cold shock proteins, we designed a protein (CspB-TB) that has the core residues of mesophilic protein from Bacillus subtilis(CspB-Bs) and altered distribution of surface charged residues. This designed protein was characterized by circular dichroism spectroscopy, and found to have secondary and tertiary structure similar to that of CspB-Bs. The activity of the CspB-TB protein as measured by the affinity to a single-stranded DNA (ssDNA) template at 25 degrees C is somewhat higher than that of CspB-Bs. Furthermore, the decrease in the apparent binding constant to ssDNA upon increase in temperature is much more pronounced for CspB-Bs than for CspB-TB. Temperature-induced unfolding (as monitored by differential scanning calorimetry and circular dichroism spectroscopy) and urea-induced unfolding experiments were used to compare the stabilities of CspB-Bs and CspB-TB. It was found that CspB-TB is approximately 20 degrees C more thermostable than CspB-Bs. The thermostabilization of CspB-TB relative to CspB-Bs is achieved by decrease in the enthalpy and entropy of unfolding without affecting their temperature dependencies, i.e. these proteins have similar heat capacity changes upon unfolding. These changes in the thermodynamic parameters result in the global stability function, i.e. Gibbs energy, deltaG(T), that is shifted to higher temperatures with only small changes in the maximum stability. Such a mechanism of thermostabilization, although predicted from the basic thermodynamic considerations, has never been identified experimentally.

Folding and association of an extremely stable dimeric protein from Sulfolobus islandicus

ORF56 is a plasmid-encoded protein from Sulfolobus islandicus, which probably controls the copy number of the pRN1 plasmid by binding to its own promotor. The protein showed an extremely high stability in denaturant, heat, and pH-induced unfolding transitions, which can be well described by a two-state reaction between native dimers and unfolded monomers. The homodimeric character of native ORF56 was confirmed by analytical ultracentrifugation. Far-UV circular dichroism and fluorescence spectroscopy gave superimposable denaturant-induced unfolding transitions and the midpoints of both heat as well as denaturant-induced unfolding depend on the protein concentration supporting the two-state model. This model was confirmed by GdmSCN-induced unfolding monitored by heteronuclear 2D NMR spectroscopy. Chemical denaturation was accomplished by GdmCl and GdmSCN, revealing a Gibbs free energy of stabilization of -85.1 kJ/mol at 25 degrees C. Thermal unfolding was possible only above 1 M GdmCl, which shifted the melting temperature (t(m)) below the boiling point of water. Linear extrapolation of t(m) to 0 M GdmCl yielded a t(m) of 107.5 degrees C (5 microM monomer concentration). Additionally, ORF56 remains natively structured over a remarkable pH range from pH 2 to pH 12. Folding kinetics were followed by far-UV CD and fluorescence after either stopped-flow or manual mixing. All kinetic traces showed only a single phase and the two probes revealed coincident folding rates (k(f), k(u)), indicating the absence of intermediates. Apparent first-order refolding rates depend linearly on the protein concentration, whereas the unfolding rates do not. Both lnk(f) and lnk(u) depend linearly on the GdmCl concentration. Together, folding and association of homodimeric ORF56 are concurrent events. In the absence of denaturant ORF56 refolds fast (7.0 x 10(7)M(-1)s(-1)) and unfolds extremely slowly (5.7 year(-1)). Therefore, high stability is coupled to a slow unfolding rate, which is often observed for proteins of extremophilic organisms.

Elucidation of factors responsible for enhanced thermal stability of proteins: a structural genomics based study

Understanding the molecular basis for the enhanced stability of proteins from thermophiles has been hindered by a lack of structural data for homologous pairs of proteins from thermophiles and mesophiles. To overcome this difficulty, complete genome sequences from 9 thermophilic and 21 mesophilic bacterial genomes were aligned with protein sequences with known structures from the protein data bank. Sequences with high homology to proteins with known structures were chosen for further analysis. High quality models of these chosen sequences were obtained using homology modeling. The current study is based on a data set of models of 900 mesophilic and 300 thermophilic protein single chains and also includes 178 templates of known structure. Structural comparisons of models of homologous proteins allowed several factors responsible for enhanced thermostability to be identified. Several statistically significant, specific amino acid substitutions that occur going from mesophiles to thermophiles are identified. Most of these are at solvent-exposed sites. Salt bridges occur significantly more often in thermophiles. The additional salt bridges in thermophiles are almost exclusively in solvent-exposed regions, and 35% are in the same element of secondary structure. Helices in thermophiles are stabilized by intrahelical salt bridges and by an increase in negative charge at the N-terminus. There is an approximate decrease of 1% in the overall loop content and a corresponding increase in helical content in thermophiles. Previously overlooked cation-pi interactions, estimated to be twice as strong as ion-pairs, are significantly enriched in thermophiles. At buried sites, statistically significant hydrophobic amino acid substitutions are typically consistent with decreased side chain conformational entropy.

High thermal stability of 3-isopropylmalate dehydrogenase from Thermus thermophilus resulting from low DeltaC(p) of unfolding

To characterize the thermal stability of 3-isopropylmalate dehydrogenase (IPMDH) from an extreme thermophile, Thermus thermophilus, urea-induced unfolding of the enzyme and of its mesophilic counterpart from Escherichia coli was investigated at various temperatures. The unfolding curves were analyzed with a three-state model for E.coli IPMDH and with a two-state model for T.thermophilus IPMDH, to obtain the free energy change DeltaG degrees of each unfolding process. Other thermodynamic parameters, enthalpy change DeltaH, entropy change DeltaS and heat capacity change DeltaC(p), were derived from the temperature dependence of DeltaG degrees. The main feature of the thermophilic enzyme was its lower dependence of DeltaG degrees on temperature resulting from a low DeltaC(p). The thermophilic IPMDH had a significantly lower DeltaC(p), 1.73 kcal/mol.K, than that of E.coli IPMDH (20.7 kcal/mol.K). The low DeltaC(p) of T.thermophilus IPMDH could not be predicted from its change in solvent-accessible surface area DeltaASA. The results suggested that there is a large structural difference between the unfolded state of T.thermophilus and that of E.coli IPMDH. Another responsible factor for the higher thermal stability of T.thermophilus IPMDH was the increase in the most stable temperature T(s). The DeltaG degrees maximum of T.thermophilus IPMDH was much smaller than that of E.coli IPMDH. The present results clearly demonstrated that a higher melting temperature T(m) is not necessarily accompanied by a higher DeltaG degrees maximum.

Some thermodynamic implications for the thermostability of proteins

An analysis of the thermodynamics of protein stability reveals a general tendency for proteins that denature at higher temperatures to have greater free energies of maximal stability. To a reasonable approximation, the temperature of maximal stability for the set of globular, water-soluble proteins surveyed by Robertson and Murphy occurs at T* approximately 283K, independent of the heat denaturation temperature, T(m). This observation indicates, at least for these proteins, that thermostability tends to be achieved through elevation of the stability curve rather than by broadening or through a horizontal shift to higher temperatures. The relationship between the free energy of maximal stability and the temperature of heat denaturation is such that an increase in maximal stability of approximately 0.008 kJ/mole/residue is, on average, associated with a 1 degrees C increase in T(m). An estimate of the energetic consequences of thermal expansion suggests that these effects may contribute significantly to the destabilization of the native state of proteins with increasing temperature.

Hyperthermophilic enzymes: sources, uses, and molecular mechanisms for thermostability

Enzymes synthesized by hyperthermophiles (bacteria and archaea with optimal growth temperatures of > 80 degrees C), also called hyperthermophilic enzymes, are typically thermostable (i.e., resistant to irreversible inactivation at high temperatures) and are optimally active at high temperatures. These enzymes share the same catalytic mechanisms with their mesophilic counterparts. When cloned and expressed in mesophilic hosts, hyperthermophilic enzymes usually retain their thermal properties, indicating that these properties are genetically encoded. Sequence alignments, amino acid content comparisons, crystal structure comparisons, and mutagenesis experiments indicate that hyperthermophilic enzymes are, indeed, very similar to their mesophilic homologues. No single mechanism is responsible for the remarkable stability of hyperthermophilic enzymes. Increased thermostability must be found, instead, in a small number of highly specific alterations that often do not obey any obvious traffic rules. After briefly discussing the diversity of hyperthermophilic organisms, this review concentrates on the remarkable thermostability of their enzymes. The biochemical and molecular properties of hyperthermophilic enzymes are described. Mechanisms responsible for protein inactivation are reviewed. The molecular mechanisms involved in protein thermostabilization are discussed, including ion pairs, hydrogen bonds, hydrophobic interactions, disulfide bridges, packing, decrease of the entropy of unfolding, and intersubunit interactions. Finally, current uses and potential applications of thermophilic and hyperthermophilic enzymes as research reagents and as catalysts for industrial processes are described.

Functionally accepted insertions of proteins within protein domains

Experiments were designed to explore the tolerance of protein structure and folding to very large insertions of folded protein within a structural domain. Dihydrofolate reductase and beta-lactamase have been inserted in four different positions of phosphoglycerate kinase. The resultant chimeric proteins are all overexpressed, and the host as well as the inserted partners are functional. Although not explicitly designed, functional coupling between the two fused partners was observed in some of the chimeras. These results show that the tolerance of protein structures to very large structured insertions is more general than previously expected and supports the idea that the natural sequence continuity of a structural domain is not required for the folding process. These results directly suggest a new experimental approach to screen, for example, for folded protein in randomized polypeptide sequences.

Maltose-binding protein from the hyperthermophilic bacterium Thermotoga maritima: stability and binding properties

Recombinant maltose-binding protein from Thermotoga maritima (TmMBP) was expressed in Escherichia coli and purified to homogeneity, applying heat incubation of the crude extract at 75 degrees C. As taken from the spectral, physicochemical and binding properties, the recombinant protein is indistinguishable from the natural protein isolated from the periplasm of Thermotoga maritima. At neutral pH, TmMBP exhibits extremely high intrinsic stability with a thermal transition >105 degrees C. Guanidinium chloride-induced equilibrium unfolding transitions at varying temperatures result in a stability maximum at approximately 40 degrees C. At room temperature, the thermodynamic analysis of the highly cooperative unfolding equilibrium transition yields DeltaG(N-->U)=100(+/-5) kJ mol(-1 )for the free energy of stabilization. Compared to mesophilic MBP from E. coli as a reference, this value is increased by about 60 kJ mol(-1). At temperatures around the optimal growth temperature of T. maritima (t(opt) approximately 80 degrees C), the yield of refolding does not exceed 80 %; the residual 20 % are misfolded, as indicated by a decrease in stability as well as loss of the maltose-binding capacity. TmMBP is able to bind maltose, maltotriose and trehalose with dissociation constants in the nanomolar to micromolar range, combining the substrate specificities of the homologs from the mesophilic bacterium E. coli and the hyperthermophilic archaeon Thermococcus litoralis. Fluorescence quench experiments allowed the dissociation constants of ligand binding to be quantified. Binding of maltose was found to be endothermic and entropy-driven, with DeltaH(b)=+47 kJ mol(-1) and DeltaS(b)=+257 J mol(-1) K(-1). Extrapolation of the linear vant'Hoff plot to t(opt) resulted in K(d) approximately 0.3 microM. This result is in agreement with data reported for the MBPs from E. coli and T. litoralis at their respective optimum growth temperatures, corroborating the general observation that proteins under their specific physiological conditions are in corresponding states.

Lys13 plays a crucial role in the functional adaptation of the thermophilic triose-phosphate isomerase from Bacillus stearothermophilus to high temperatures

The thermophilic triose-phosphate isomerases (TIMs) of Bacillus stearothermophilus (bTIM) and Thermotoga maritima (tTIM) have been found to possess a His12-Lys13 pair instead of the Asn12-Gly13 pair normally present in mesophilic TIMs. His12 in bTIM was proposed to prevent deamidation at high temperature, while the precise role of Lys13 is unknown. To investigate the role of the His12 and Lys13 pair in the enzyme's thermoadaptation, we reintroduced the "mesophilic residues" Asn and Gly into both thermophilic TIMs. Neither double mutant displayed diminished structural stability, but the bTIM double mutant showed drastically reduced catalytic activity. No similar behavior was observed with the tTIM double mutant, suggesting that the presence of the His12 and Lys13 cannot be systematically correlated to thermoadaptation in TIMs. We determined the crystal structure of the bTIM double mutant complexed with 2-phosphoglycolate to 2.4-A resolution. A molecular dynamics simulation showed that upon substitution of Lys13 to Gly an increase of the flexibility of loop 1 is observed, causing an incorrect orientation of the catalytic Lys10. This suggests that Lys13 in bTIM plays a crucial role in the functional adaptation of this enzyme to high temperature. Analysis of bTIM single mutants supports this assumption.

Thermodynamic study of phosphoglycerate kinase from Thermotoga maritima and its isolated domains: reversible thermal unfolding monitored by differential scanning calorimetry and circular dichroism spectroscopy

The folding of phosphoglycerate kinase (PGK) from the hyperthermophilic bacterium Thermotoga maritima and its isolated N- and C-terminal domains (N1/2 and C1/2) was characterized by differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy. At pH 3.0-4.0, reversible thermal denaturation of TmPGK occurred below 90 degrees C. The corresponding peaks in the partial molar heat capacity function were fitted by a four-state model, describing three well-defined unfolding transitions. Using CD spectroscopy, these are ascribed to the disruption of the domain interactions and subsequent sequential unfolding of the two domains. The isolated N-terminal domain unfolds reversibly between pH 3.0 and pH 4.0 to >90% and at pH 7.0 to about 70%. In contrast, the isolated engineered C-terminal domain only shows reversible thermal denaturation between pH 3.0 and pH 3.5. Neither N1/2 nor C1/2 obeys the simple two-state mechanism of unfolding. Instead, both unfold via a partially structured intermediate. In the case of N1/2, the intermediate exhibits native secondary structure and perturbed tertiary structure, whereas for C1/2 the intermediate could not be defined with certainty.

A thermodynamic comparison of mesophilic and thermophilic ribonucleases H

The mechanisms by which thermophilic proteins attain their increased thermostability remain unclear, as usually the sequence and structure of these proteins are very similar to those of their mesophilic homologues. To gain insight into the basis of thermostability, we have determined protein stability curves describing the temperature dependence of the free energy of unfolding for two ribonucleases H, one from the mesophile Escherichia coli and one from the thermophile Thermus thermophilus. The circular dichroism signal was monitored as a function of temperature and guanidinium chloride concentration, and the resulting free energies of unfolding were fit to the Gibbs-Helmholtz equation to obtain a set of thermodynamic parameters for these proteins. Although the maximal stabilities for these proteins occur at similar temperatures, the heat capacity of unfolding for T. thermophilus RNase H is lower, resulting in a smaller temperature dependence of the free energy of unfolding and therefore a higher thermal melting temperature. In addition, the stabilities of these proteins are similar at the optimal growth temperatures for their respective organisms, suggesting that a balance of thermodynamic stability and flexibility is important for function.

Comparing the thermodynamic stabilities of a related thermophilic and mesophilic enzyme

Several models have been proposed to explain the high temperatures required to denature enzymes from thermophilic organisms; some involve greater maximum thermodynamic stability for the thermophile, and others do not. To test these models, we reversibly melted two analogous protein domains in a two-state manner. E2cd is the isolated catalytic domain of cellulase E2 from the thermophile Thermomonospora fusca. CenAP30 is the analogous domain of the cellulase CenA from the mesophile Cellulomonas fimi. When reversibly denatured in a common buffer, the thermophilic enzyme E2cd had a temperature of melting (Tm) of 72.2 degrees C, a van't Hoff enthalpy of unfolding (DeltaHVH) of 190 kcal/mol, and an entropy of unfolding (DeltaSu) of 0.55 kcal/(mol*K); the mesophilic enzyme CenAP30 had a Tm of 56.4 degrees C, a DeltaHVH of 107 kcal/mol, and a DeltaSu of 0. 32 kcal/(mol*K). The higher DeltaHVH and DeltaSu values for E2cd suggest that its free energy of unfolding (DeltaGu) has a steeper dependence on temperature at the Tm than CenAP30. This result supports models that predict a greater maximum thermodynamic stability for thermophilic enzymes than for their mesophilic counterparts. This was further explored by urea denaturation. Under reducing conditions at 30 degrees C, E2cd had a concentration of melting (Cm) of 5.2 M and a DeltaGu of 11.2 kcal/mol; CenAP30 had a Cm of 2.6 M and a DeltaGu of 4.3 kcal/mol. Under nonreducing conditions, the Cm and DeltaGu of CenAP30 were increased to 4.5 M and 10.8 kcal/mol at 30 degrees C; the Cm for E2cd was increased to at least 7.4 M at 32 degrees C. We were unable to determine a DeltaGu value for E2cd under nonreducing conditions due to problems with reversibility. These data suggest that E2cd attains its greater thermal stability (DeltaTm = 15.8 degrees C) through a greater thermodynamic stability (DeltaDeltaGu = 6.9 kcal/mol) compared to its mesophilic analogue CenAP30.

The stability of proteins in extreme environments

Three complete genome sequences of thermophilic bacteria provide a wealth of information challenging current ideas concerning phylogeny and evolution, as well as the determinants of protein stability. Considering known protein structures from extremophiles, it becomes clear that no general conclusions can be drawn regarding adaptive mechanisms to extremes of physical conditions. Proteins are individuals that accumulate increments of stabilization; in thermophiles these come from charge clusters, networks of hydrogen bonds, optimization of packing and hydrophobic interactions, each in its own way. Recent examples indicate ways for the rational design of ultrastable proteins.