Cold-adapted enzymes: from fundamentals to biotechnology

Psychrophilic enzymes produced by cold-adapted microorganisms display a high catalytic efficiency and are most often, if not always, associated with high thermosensitivity. Using X-ray crystallography, these properties are beginning to become understood, and the rules governing their adaptation to cold appear to be relatively diverse. The application of these enzymes offers considerable potential to the biotechnology industry, for example, in the detergent and food industries, for the production of fine chemicals and in bioremediation processes.

Protein adaptation to low temperatures: a comparative study of alpha-tubulin sequences in mesophilic and psychrophilic algae

The alpha-tubulin genes from two psychrophilic algae belonging to the genus Chloromonas (here named ANT1 and ANT3) have been isolated and sequenced. The genes ant1 and ant3 contain 4 and 2 introns, respectively. The coding DNA sequences are 90% identical but the degree of isology is very high at the polypeptide level (more than 97% strict identities). The ANT1 and ANT3 alpha-tubulin amino acid sequences were compared to the corresponding sequence of the mesophilic alga Chlamydomonas reinhardtii. Of the 15 substitutions detected in ANT1 and/or ANT3, 5 are common to both psychrophilic algae. The recorded substitutions have been analyzed in terms of cold adaptation on the basis of the available three-dimensional structure of the alpha,beta-tubulin heterodimer from pig brain. Most of these are subtle changes, but two substitutions, M268V and A295V occurring in the region of interdimer contacts, could be of great significance for the cold stability of Antarctic algae microtubules due to the fact that the entropic control of microtubule assembly is particularly high in cold adaptes species.

Purification and characterization of the heat-labile alpha-amylase secreted by the psychrophilic bacterium TAC 240B

A total of 59 bacteria samples from Antarctic sea water were collected and screened for their ability to produce alpha-amylase. The highest activity was recorded from an isolate identified as an Alteromonas species. The purified alpha-amylase shows a molecular mass of about 50,000 Da and a pI of 5.2. The enzyme is stable from pH 7.5 to 9 and has a maximal activity at pH 7.5. Compared with other alpha-amylases from mesophiles and thermophiles, the "cold enzyme" displays a higher activity at low temperature and a lower stability at high temperature. The psychrophilic alpha-amylase requires both Cl- and Ca2+ for its amylolytic activity. Br- is also quite efficient as an allosteric effector. The comparison of the amino acid composition with those of other alpha-amylases from various organisms shows that the cold alpha-amylase has the lowest content in Arg and Pro residues. This could be involved in the principle used by the psychrophilic enzyme to adapt its molecular structure to the low temperature of the environment.

Thermodynamic stability of a cold-active alpha-amylase from the Antarctic bacterium Alteromonas haloplanctis

The thermal stability of the cold-active alpha-amylase (AHA) secreted by the Antarctic bacterium Alteromonas haloplanctis has been investigated by intrinsic fluorescence, circular dichroism, and differential scanning calorimetry. It was found that this heat-labile enzyme is the largest known multidomain protein exhibiting a reversible two-state unfolding, as demonstrated by the recovery of DeltaHcal values after consecutive calorimetric transitions, a DeltaHcal/DeltaHeff ratio close to unity, and the independence of unfolding thermodynamic parameters of scan rates. By contrast, the mesophilic alpha-amylases investigated here (from porcine pancreas, human salivary glands, yellow meal beetle, Bacillus amyloliquefaciens, and Bacillus licheniformis) unfold irreversibly according to a non-two-state mechanism. Unlike mesophilic alpha-amylases, the melting point of AHA is independent of calcium and chloride binding while the allosteric and structural functions of these ions are conserved. The thermostability of AHA at optimal conditions is characterized by a Tm of 43.7 degrees C, a DeltaHcal of 238 kcal mol-1, and a DeltaCp of 8.47 kcal mol-1 K-1. These values were used to calculate the Gibbs free energy of unfolding over a wide range of temperatures. This stability curve shows that (a) the specific DeltaGmax of AHA [22 cal (mol of residue)-1] is 4 times lower than that of mesophilic alpha-amylases, (b) group hydration plays a crucial role in the enzyme flexibility at low temperatures, (c) the temperature of cold unfolding closely corresponds to the lower limit of bacterial growth, and (d) the recombinant heat-labile enzyme can be expressed in mesophilic hosts at moderate temperatures. It is also argued that the cold-active alpha-amylase has evolved toward the lowest possible conformational stability of its native state.

Modification of the Thermoresistance to Spray-Drying of a Cold-Adapted Subtilisin by Genetic Engineering

The thermoresistance of a cold-adapted subtilisin dried by spray-drying was studied. Proteolytic activity of this enzyme was measured before and after spray-drying. Without chemical additives, spray-drying yields ranged from 2-13%. The use of arabic gum and lactose in the composition of the enzyme solutions allowed the strengthening of the enzyme structures and increased water mobility in the product. Increase of water mobility led to a shorter residence time of the product in the spray-drier and a net yield increase was obtained (yield higher than 50%). The effect of two selective mutations on the thermoresistance to spray-drying of the cold-adapted subtilisin was also investigated. Mutation T85D (introduction of an additional link with an ion Ca2+ necessary for enzyme activity, by substitution of Asp for Thr 85) had no effect on the thermoresistance of the subtilisin to spray-drying. Mutation H121W (introduction of an additional aromatic link by substitution of Trp for His 121) reduced the drying yield from 66% (not modified subtilisin) to 52%. This higher thermosensitivity could be explained by an increase of the hygroscopic character of the modified subtilisin (mutation H121W).

Structures of the psychrophilic Alteromonas haloplanctis alpha-amylase give insights into cold adaptation at a molecular level

BACKGROUND

. Enzymes from psychrophilic (cold-adapted) microorganisms operate at temperatures close to 0 degreesC, where the activity of their mesophilic and thermophilic counterparts is drastically reduced. It has generally been assumed that thermophily is associated with rigid proteins, whereas psychrophilic enzymes have a tendency to be more flexible.

RESULTS

. Insights into the cold adaptation of proteins are gained on the basis of a psychrophilic protein's molecular structure. To this end, we have determined the structure of the recombinant form of a psychrophilic alpha-amylase from Alteromonas haloplanctis at 2.4 A resolution. We have compared this with the structure of the wild-type enzyme, recently solved at 2.0 A resolution, and with available structures of their mesophilic counterparts. These comparative studies have enabled us to identify possible determinants of cold adaptation.

CONCLUSIONS

. We propose that an increased resilience of the molecular surface and a less rigid protein core, with less interdomain interactions, are determining factors of the conformational flexibility that allows efficient enzyme catalysis in cold environments.

Purification and characterization of a 315 kDa keratinolytic subtilisin-like serine protease from Microsporum canis and evidence of its secretion in naturally infected cats

A keratinolytic protease, secreted as the major component by a feline clinical isolate of Microsporum canis cultivated in a minimal medium containing cat keratin, was purified by affinity chromatography on bacitracin agarose and gel filtration. The apparent molecular mass of the enzyme was 31.5 kDa and the pI was 11.8. The enzyme was not glycosylated and its first 15 N-terminal amino acids showed numerous similarities with other fungal subtilisins. The optimum pH was around 9 while inactivation of the enzyme was reversible at pH 4, but not at pH 11. The enzyme was stable at 37 degrees C with an apparent optimum temperature around 55 degrees C. PMSF, soybean trypsin inhibitor (SBTI) and chymostatin strongly inhibited the proteinase. The highest affinity (Km of 0.37 mM) and physiological efficiency (k(cat)/Km) were obtained for the synthetic substrate N-Suc-Ala-Ala-Pro-Phe-p-nitroanilide. These results indicate that the keratinase belongs to the subtilisin-like serine protease family. Purified rabbit immunoglobulins G prepared against the keratinase and used in an immunohistochemical test allowed the detection of the keratinase produced by the fungus invading hair structures in naturally infected cats. The in vitro keratinolytic activity of the enzyme and its production in vivo suggest that it may contribute to pathogenicity.

Opposite effects of cooling on twitch contractions of skeletal muscle isolated from tropical toads (Leptodactylidae) and northern frogs (Ranidae)

Cooling increases the twitch force of frog skeletal muscle (Rana temporaria; Rana pipiens), but decreases the twitch force of tropical toad muscle (Leptodactylus insularis). Action potentials and intramembranous charge movement in frog and toad fibers were slowed identically by cooling. Cooling increased the integral of twitch Ca2+ detected by aequorin in frog fibers (1.4-fold), while also decreasing the peak and slowing the rate of decay. Conversely, cooling decreased the integral (0.6-fold) and the peak of twitch Ca2+ in toad fibers, without affecting the rate of decay. The difference in entire Ca2+ transients may account for cold-induced twitch potentiation in frogs and twitch paralysis in toads. In sustained contractions of toad fibers, cooling markedly decreased maximum force caused by: (i) tetanic stimulation, (ii) two-microelectrode voltage clamp steps, (iii) high [K+], or (iv) caffeine. Maximum force in sustained contractions was decreased moderately by cooling frog fibers. Rapid rewarming and simultaneous removal of high [K+] or caffeine during a sustained contraction, caused toad muscle force to rise towards the value corresponding to the warm temperature. This did not occur after removing high [K+] or caffeine from toad fibers kept in the cold. Transmission electron micrographs showed no relevant structural differences. Parvalbumins are thought to promote relaxation of frog muscle in the cold. The unique parvalbumin isoforms in toad muscle apparently lack this property.

Characterization of the C-terminal propeptide involved in bacterial wall spanning of alpha-amylase from the psychrophile Alteromonas haloplanctis

The antarctic psychrophile Alteromonas haloplanctis secretes a Ca2+- and Cl--dependent alpha-amylase. The nucleotide sequence of the amy gene and the amino acid sequences of the gene products indicate that the alpha-amylase precursor is a preproenzyme composed by the signal peptide (24 residues), the mature alpha-amylase (453 residues, 49 kDa), and a long C-terminal propeptide or secretion helper (192 residues, 21 kDa). In cultures of the wild-type strain, the 70-kDa precursor is secreted at the mid-exponential phase and is cleaved by a nonspecific protease into the mature enzyme and the propeptide. The purified C-terminal propeptide displays several features common to beta-pleated transmembrane proteins. It has no intramolecular chaperone function because active alpha-amylase is expressed by Escherichia coli in the absence of the propeptide coding region. In E. coli, the 70-kDa precursor is directed toward the supernatant. When the alpha-amylase coding region is excised from the gene, the secretion helper can still promote its own membrane spanning. It can also accept a foreign passenger, as shown by the extracellular routing of a beta-lactamase-propeptide fusion protein.

Expression of psychrophilic genes in mesophilic hosts: assessment of the folding state of a recombinant alpha-amylase

Alpha-Amylase from the antarctic psychrophile Altermonas haloplanktis is synthesized at 0 +/- 2 degrees C by the wild strain. This heat-labile alpha-amylase folds correctly when overexpressed in Escherichia coli, providing the culture temperature is sufficiently low to avoid irreversible denaturation. In the described expression system, a compromise between enzyme stability and E. coli growth rate is reached at 18 degrees C.

Crystal structures of the psychrophilic alpha-amylase from Alteromonas haloplanctis in its native form and complexed with an inhibitor

Alteromonas haloplanctis is a bacterium that flourishes in Antarctic sea-water and it is considered as an extreme psychrophile. We have determined the crystal structures of the alpha-amylase (AHA) secreted by this bacterium, in its native state to 2.0 angstroms resolution as well as in complex with Tris to 1.85 angstroms resolution. The structure of AHA, which is the first experimentally determined three-dimensional structure of a psychrophilic enzyme, resembles those of other known alpha-amylases of various origins with a surprisingly greatest similarity to mammalian alpha-amylases. AHA contains a chloride ion which activates the hydrolytic cleavage of substrate alpha-1,4-glycosidic bonds. The chloride binding site is situated approximately 5 angstroms from the active site which is characterized by a triad of acid residues (Asp 174, Glu 200, Asp 264). These are all involved in firm binding of the Tris moiety. A reaction mechanism for substrate hydrolysis is proposed on the basis of the Tris inhibitor binding and the chloride activation. A trio of residues (Ser 303, His 337, Glu 19) having a striking spatial resemblance with serine-protease like catalytic triads was found approximately 22 angstroms from the active site. We found that this triad is equally present in other chloride dependent alpha-amylases, and suggest that it could be responsible for autoproteolytic events observed in solution for this cold adapted alpha-amylase.

Subtilisin from psychrophilic antarctic bacteria: characterization and site-directed mutagenesis of residues possibly involved in the adaptation to cold

A subtilisin excreted by the Antarctic Bacillus TA39 has been purified to homogeneity and characterised. Two independent genes subt1 and subt2 are present but only subt1 is expressed significantly in the culture medium. The enzyme displays the usual characteristics of cold enzymes i.e. a high catalytic efficiency at low and moderate temperatures and an increased thermosensitivity originating from a 3D structure probably more flexible than its mesophilic counterpart. This is corroborated by the analysis of the computerized structure which shows a significant decrease in the number and strength of intramolecular weak bonds such as salt bridges and aromatic interactions. The affinity for calcium is also almost three orders of magnitude lower than that of mesophilic subtilisin and the interactions with the solvent are significantly higher thanks to a large increase in the number of Asp residues in the loops connecting secondary structures. The relation between flexibility and activity was investigated by site-directed mutagenesis tending mainly to increase the rigidity of the molecular edifice through the incorporation of additional salt bridge, disulfide bridge, aromatic interaction and by increasing the affinity of the enzyme for calcium. An important stabilization of the molecular structure was achieved through a modification of a calcium ligand T85D. The thermostability of the mutated product expressed in a mesophilic Bacillus reaches that of mesophilic subtilisin. Most important is the fact that this mutation further enhances the specific activity by a factor close to 2 when compared to the wild type enzyme so that the overall activity of the mutated cold enzyme is about 20 times higher than that of mesophilic subtilisin, illustrating the fact that thermostability is not systematically inversely related to specific activity. This opens new perspectives in the use of cold enzymes in biotechnology.

Preliminary crystal structure determination of the alkaline protease from the Antarctic psychrophile Pseudomonas aeruginosa

A cold alkaline protease, isolated from an Antarctic Pseudomonas aeruginosa strain, has been purified and crystallized. Large crystals were obtained in the presence of PEG 6000 at pH 7 and pH 8. They belong to the space group P2(1)2(1)2(1). A complete data set to 2.1 A resolution has been measured. The structure has been determined by the molecular replacement method using the coordinates of the mesophilic alkaline protease as a model. The molecular replacement solution displays a correlation coefficient of 0.39 and an R-factor of 0.48. Subsequent inspection of the electron density map in the active site region has confirmed the correctness of the solution. Model building and structure refinement will be initiated when the protease sequence becomes fully available. This is the second report, following one on an alpha-amylase, of the preliminary crystallographic characterization of a psychrophilic enzyme.

Psychrophilic enzymes: a thermodynamic challenge

Psychrophilic microorganisms, hosts of permanently cold habitats, produce enzymes which are adapted to work at low temperatures. When compared to their mesophilic counterparts, these enzymes display a higher catalytic efficiency over a temperature range of roughly 0-30 degrees C and a high thermosensitivity. The molecular characteristics of cold enzymes originating from Antarctic bacteria have been approached through protein modelling and X-ray crystallography. The deduced three-dimensional structures of cold alpha-amylase, beta-lactamase, lipase and subtilisin have been compared to their mesophilic homologs. It appears that the molecular adaptation resides in a weakening of the intramolecular interactions, and in some cases in an increase of the interaction with the solvent, leading to more flexible molecular edifices capable of performing catalysis at a lower energy cost.

Psychrophilic enzymes: molecular basis of cold adaptation

Psychrophilic organisms have successfully colonized polar and alpine regions and are able to grow efficiently at sub-zero temperatures. At the enzymatic level, such organisms have to cope with the reduction of chemical reaction rates induced by low temperatures in order to maintain adequate metabolic fluxes. Thermal compensation in cold-adapted enzymes is reached through improved turnover number and catalytic efficiency. This optimization of the catalytic parameters can originate from a highly flexible structure which provides enhanced abilities to undergo conformational changes during catalysis. Thermal instability of cold-adapted enzymes is therefore regarded as a consequence of their conformational flexibility. A survey of the psychrophilic enzymes studied so far reveals only minor alterations of the primary structure when compared to mesophilic or thermophilic homologues. However, all known structural factors and weak interactions involved in protein stability are either reduced in number or modified in order to increase their flexibility.

Characterization of the single tyrosine containing troponin C from lungfish white muscle. Comparison with several fast skeletal muscle troponin C's from fish species

Troponin C molecules from fast skeletal muscle of the following fish species (trout, whiting, lungfish, tilapia, and cod) have been purified to homogeneity. Upon binding of Ca2+ or Mg2+, lungfish troponin C is the only troponin C from fish white muscle to show the typical increase of tyrosine fluorescence emission quantum yield reported for rabbit fast skeletal muscle troponin C. The increase of tyrosine fluorescence signal occurring upon Ca2+ and Mg2+ titration of lungfish troponin C has been used to determine the corresponding affinity constants. With K(Ca) = 7.0 10(7) M-1 and K(Mg) = 3.6 10(3) M-1, the sites probed by the tyrosine residue of lungfish troponin C are typical of the COOH-terminal domain of fast skeletal troponin C's. The amino acid sequencing of the tyrosine containing tryptic peptides has allowed us to position the single tyrosine residue at position 7 in the Ca2+ binding loop of the third site, in identical position to Tyr109 of troponin C from rabbit fast skeletal muscle. Metal ion binding studies followed by intrinsic fluorescence or Tb3+ luminescence indicate that the conformation of the structural domain of lungfish troponin C with one metal ion bound is close to the physiological conformation of this domain.

Ca(2+)-induced conformational shift of the COOH-domain of eel skeletal muscle troponin C in the presence of physiological concentrations of Mg2+

The spectroscopic properties of Trp152 of eel skeletal muscle troponin C have been studied under conditions in which the COOH-domain is depleted of metal ions, titrated with Mg2+ and subsequently with Ca2+. This spectroscopic study clearly shows that the Mg2+ or Ca(2+)-bound states substitution lead to distinct conformations of the COOH-domain. The analysis of eel troponin C absorption and Trp152 fluorescence emission spectra indicates a more polar environment in the Mg(2+)-bound state of the protein. Steady state tryptophan polarization and lifetime distribution data indicate that the motion of the indole moiety is more restricted in the Mg2+ state of the protein than in the Ca(2+)-bound state. However, fluorescence quenching data using I- and Cs+ show that Trp152 is more accessible to the solvent in the Mg(2+)-bound state of eel troponin C. This spectroscopic analysis of the distinct Ca2+ and Mg(2+)-bound states of eel troponin C is consistent with the description of the three-dimensional structure of the corresponding states of pike (pI = 4.10) parvalbumin which is structurally homologous to the COOH-domain of troponin C. Since it appears that during muscular contraction, magnesium ions, which occupy the binding sites of the COOH-domain of troponin C in the resting cell are replaced by calcium ions, the structural shift occurring upon Mg2+/Ca2+ substitution, must have a physiological significance. The role of this domain is probably not limited to a structural role.

Cold adaptation parameters derived from cDNA sequencing and molecular modelling of elastase from Antarctic fish Notothenia neglecta

The primary structure of an elastase from the Antarctic fish Notothenia neglecta (NE) was elucidated by molecular cloning and cDNA sequence analysis. The cDNA of interest was isolated from a cDNA library obtained from Notothenia's pyloric caeca. The amino acid sequence identity with mammalian elastases ranges between 53 and 64%, but interestingly reaches 79% with one isoform (CEB) of two recently isolated cod elastases. The most interesting changes distinguishing the model of NE, predicted from the three dimentional structure of the native porcine elastase (PE), concern the catalytic crevice located in the inter-domains region. These features might be involved in the adaptation to cold of the Antarctic elastase.

Enzymes from cold-adapted microorganisms. The class C beta-lactamase from the antarctic psychrophile Psychrobacter immobilis A5

A heat-labile beta-lactamase has been purified from culture supernatants of Psychrobacter immobilis A5 grown at 4 degrees C and the corresponding chromosomal ampC gene has been cloned and sequenced. All structural and kinetic properties clearly relate this enzyme to class C beta-lactamases. The kinetic parameters of P. immobilis beta-lactamase for the hydrolysis of some beta-lactam antibiotics are in the same range as the values recorded for the highly specialized cephalosporinases from pathogenic mesophilic bacteria. By contrast, the enzyme displays a low apparent optimum temperature of activity and a reduced thermal stability. Structural factors responsible for the latter property were analysed from the three-dimensional structure built by homology modelling. The deletion of proline residues in loops, the low number of arginine-mediated H-bonds and aromatic-aromatic interactions, the lower global hydrophobicity and the improved solvent interactions through additional surface acidic residues appear to be the main determinants of the enzyme flexibility.

Trypsin and trypsinogen from an Antarctic fish: molecular basis of cold adaptation

Trypsin from Antarctic fish Paranotothenia magellanica displays molecular and kinetic properties typical of enzymes produced by psychrophilic organisms. The enzyme has a high catalytic efficiency at low and moderate temperatures and is rapidly inactivated at temperatures higher than 30 degrees C. The nucleotide sequence was determined after mRNA extraction and cDNA synthesis. The cDNA encodes a pretrypsinogen which includes a seven residue activation peptide containing only three acidic residues preceeding the 222 amino-acid mature enzyme. A three-dimensional model of the enzyme was built. Structural parameters possibly involved in the adaptation to cold have been derived from comparison with the three-dimensional structure of the bovine enzyme. Among them are the lack of Tyr-151 in the substrate binding pocket, an overall decrease in the number of salt bridges and hydrophobicity and the increase in the surface hydrophilicity.

Crystallization and preliminary X-ray diffraction studies of alpha-amylase from the antarctic psychrophile Alteromonas haloplanctis A23

A cold-active alpha-amylase was purified from culture supernatants of the antarctic psychrophile Alteromonas haloplanctis A23 grown at 4 degrees C. In order to contribute to the understanding of the molecular basis of cold adaptations, crystallographic studies of this cold-adapted enzyme have been initiated because a three-dimensional structure of a mesophilic counterpart, pig pancreatic alpha-amylase, already exists. alpha-Amylase from A. haloplanctis, which shares 53% sequence identity with pig pancreatic alpha-amylase, has been crystallized and data to 1.85 A have been collected. The space group is found to be C222(1) with a = 71.40 A, b = 138.88 A, and c = 115.66 A. Until now, a three-dimensional structure of a psychrophilic enzyme is lacking.

Structural and functional aspects of chloride binding to Alteromonas haloplanctis alpha-amylase

Chloride is the allosteric effector of vertebrate pancreatic and salivary alpha-amylases and of the bacterial alpha-amylase from Alteromonas haloplanctis. Activation experiments of A. haloplanctis alpha-amylase by several monovalent anions show that a negative charge, not restricted to that of Cl-, is essential for the amylolytic reaction. Engineering of the chloride binding site reveals that a basic residue is an essential component of the site. The mutation K337R alters the Cl--binding properties, whereas the mutation K337Q produces an active, chloride-independent enzyme. Comparison of the Kd values for Cl- in three homologous alpha-amylases also indicates that the binding affinity is dependent on the chloride coordination mode by this basic residue. Analysis of substrate and chloride binding according to the allosteric kinetic model shows that the chloride effector is not involved in substrate binding. By contrast, the pH dependence of activity and experiments of chemical modifications and Ca2+ inhibition show that the chloride ion is responsible for the pKa shift of catalytic groups and interacts with active site carboxyl groups.

The beta-lactamase secreted by the antarctic psychrophile Psychrobacter immobilis A8

A class C beta-lactamase has been purified from the culture supernatant of the antarctic psychrophile Psychrobacter immobilis A8. This psychrophilic beta-lactamase displays a low level of thermal stability and a low optimal temperature of activity. In contrast to other cold-adapted enzymes, its level of specific activity is not higher than that of mesophilic class C beta-lactamases.