Psychrophily and catalysis

Cold-active microbial enzymes and their biotechnological applications

Microorganisms known as psychrophiles/psychrotrophs, which survive in cold climates, constitute majority of the biosphere on Earth. Their capability to produce cold-active enzymes along with other distinguishing characteristics allows them to survive in the cold environments. Due to the relative ease of large-scale production compared to enzymes from plants and animals, commercial uses of microbial enzyme are alluring. The ocean depths, polar, and alpine regions, which make up over 85% of the planet, are inhabited to cold ecosystems. Microbes living in these regions are important for their metabolic contribution to the ecosphere as well as for their enzymes, which may have potential industrial applications. Cold-adapted microorganisms are a possible source of cold-active enzymes that have high catalytic efficacy at low and moderate temperatures at which homologous mesophilic enzymes are not active. Cold-active enzymes can be used in a variety of biotechnological processes, including food processing, additives in the detergent and food industries, textile industry, waste-water treatment, biopulping, environmental bioremediation in cold climates, biotransformation, and molecular biology applications with great potential for energy savings. Genetically manipulated strains that are suitable for producing a particular cold-active enzyme would be crucial in a variety of industrial and biotechnological applications. The potential advantage of cold-adapted enzymes will probably lead to a greater annual market than for thermo-stable enzymes in the near future. This review includes latest updates on various microbial source of cold-active enzymes and their biotechnological applications.

Biochemical and in silico structural characterization of a cold-active arginase from the psychrophilic yeast, Glaciozyma antarctica PI12

Glaciozyma antarctica PI12 is a psychrophilic yeast isolated from Antarctica. In this work, we describe the heterologous production, biochemical properties and in silico structure analysis of an arginase from this yeast (GaArg). GaArg is a metalloenzyme that catalyses the hydrolysis of L-arginine to L-ornithine and urea. The cDNA of GaArg was reversed transcribed, cloned, expressed and purified as a recombinant protein in Escherichia coli. The purified protein was active against L-arginine as its substrate in a reaction at 20 °C, pH 9. At 10-35 °C and pH 7-9, the catalytic activity of the protein was still present around 50%. Mn2+, Ni2+, Co2+ and K+ were able to enhance the enzyme activity more than two-fold, while GaArg is most sensitive to SDS, EDTA and DTT. The predicted structure model of GaArg showed a very similar overall fold with other known arginases. GaArg possesses predominantly smaller and uncharged amino acids, fewer salt bridges, hydrogen bonds and hydrophobic interactions compared to the other counterparts. GaArg is the first reported arginase that is cold-active, facilitated by unique structural characteristics for its adaptation of catalytic functions at low-temperature environments. The structure and function of cold-active GaArg provide insights into the potentiality of new applications in various biotechnology and pharmaceutical industries.

Metagenomic views on taxonomic and functional profiles of the Himalayan Tsomgo cold lake and unveiling its deterzome potential

Cold habitat is considered a potential source for detergent industry enzymes. This study aims at the metagenomic investigation of Tsomgo lake for taxonomic and functional annotation, unveiling the deterzome potential of the residing microbiota at this site. The present investigation revealed molecular profiling of microbial community structure and functional potential of the high-altitude Tsomgo lake samples of two different temperatures, harvested during March and August. Bacteria were found to be the most dominant phyla, with traces of genomic pieces of evidence belonging to archaea, viruses, and eukaryotes. Proteobacteria and Actinobacteria were noted to be the most abundant bacterial phyla in the cold lake. In-depth metagenomic investigation of the cold aquatic habitat revealed novel genes encoding detergent enzymes, amylase, protease, and lipase. Further, metagenome-assembled genomes (MAGs) belonging to the psychrophilic bacterium, Arthrobacter alpinus, were constructed from the metagenomic data. The annotation depicted the presence of detergent enzymes and genes for low-temperature adaptation in Arthrobacter alpinus. Psychrophilic microbial isolates were screened for lipase, protease, and amylase activities to further strengthen the metagenomic findings. A novel strain of Acinetobacter sp. was identified with the dual enzymatic activity of protease and amylase. The bacterial isolates exhibited hydrolyzing activity at low temperatures. This metagenomic study divulged novel genomic resources for detergent industry enzymes, and the bacterial isolates secreting cold-active amylase, lipase, and protease enzymes. The findings manifest that Tsomgo lake is a potential bioresource of cold-active enzymes, vital for various industrial applications.

Production of cold-active pectinases by three novel Cladosporium species isolated from Egypt and application of the most active enzyme

Cladosporium parasphaerospermum, Cladosporium chlamydosporigenum, and Cladosporium compactisporum have all been discovered and characterized as new Cladosporium species. The three new species seemed to generate cold-active pectinases with high activity at pH 6.0 and 10 °C, pH 6.0 and 15 °C, and pH 5.0 and 15 °C, respectively, with the most active being C. parasphaerospermum pectinase. In submerged fermentation (SmF), C. parasphaerospermum produced the most cold-active pectinase with the highest activity and specific activity (28.84 U/mL and 3797 U/mg) after 8 days. C. parasphaerospermum cold-active pectinase was isolated using DEAE-Cellulose anion exchange resin and a Sephadex G 100 gel filtration column. The enzyme was purified 214.4-fold and 406.4-fold greater than the fermentation medium using DEAE-cellulose and Sephadex G 100, respectively. At pH 7.0 and 10 °C, pure pectinase had the highest activity (6684 U/mg), with Km and Vmax determined to be 26.625 mg/mL and 312.5 U/min, respectively. At 5 mM/mL, EDTA, MgCl2, and SDS inhibited the activity of pure pectinase by 99.21, 96.03, and 94.45%, respectively. The addition of 10 U/mL pure pectinase enhanced the yield of apple, orange, apricot, and peach juice by 17, 20, 13, and 24%, respectively, and improved the clarity and colour of orange juice by 194 and 339%, respectively. We can now add cold-active pectinase production to the long list of Cladosporium species that have been identified. We also report three new species that can be used in biotechnological solutions as active microbial pectinase producers. Although further research is needed, these distinct species might be used to decompose difficult and resistant pertinacious wastes as well as clear fruit juices.

The Activation Parameters of a Cold-Adapted Short Chain Dehydrogenase Are Insensitive to Enzyme Oligomerization

The structural principles of enzyme cold adaptation are of fundamental interest both for understanding protein evolution and for biotechnological applications. It has become clear in recent years that structural flexibility plays a major role in tuning enzyme activity at low temperatures, which is reflected by characteristic changes in the thermodynamic activation parameters for psychrophilic enzymes, compared to those of mesophilic and thermophilic ones. Hence, increased flexibility of the enzyme surface has been shown to lead to a lower enthalpy and a more negative entropy of activation, which leads to higher activity in the cold. This immediately raises the question of how enzyme oligomerization affects the temperature dependence of catalysis. Here, we address this issue by computer simulations of the catalytic reaction of a cold-adapted bacterial short chain dehydrogenase in different oligomeric states. Reaction free energy profiles are calculated at different temperatures for the tetrameric, dimeric, and monomeric states of the enzyme, and activation parameters are obtained from the corresponding computational Arrhenius plots. The results show that the activation free energy, enthalpy, and entropy are remarkably insensitive to the oligomeric state, leading to the conclusion that assembly of the subunit interfaces does not compromise cold adaptation, even though the mobilities of interfacial residues are indeed affected.

Double mutations far from the active site affect cold activity in an Antarctic halophilic β-galactosidase

The Antarctic haloarchaeon, Halorubrum lacusprofundi, contains a polyextremophilic family 42 β-galactosidase, which we are using as a model for cold-active enzymes. Divergent amino acid residues in this 78 kDa protein were identified through comparative genomics and hypothesized to be important for cold activity. Six amino acid residues were previously mutated and five were shown by steady-state kinetic analysis to have altered temperature-dependent catalytic activity profiles via effects on K_m and/or k_cat compared to the wild-type enzyme. In this follow-up study, double-mutated enzymes were constructed and tested for temperature effects, including two new tandem residue pairs (N180T/A181T and T383A/S384A), and pairwise combination of the single residue mutations (N251D, F387L, I476V, and V482L). All double-mutated enzymes were found to be more catalytically active at moderate and/or less active at colder temperatures than wild-type, with both K_m and k_cat effects observed for the two tandem mutations. For pairwise combinations, a K_m effect was seen when the surface exposed F387L mutation located in a domain A TIM barrel α helix 19 Å from the active site was combined with two internal residues, N251D or V482L. When another surface exposed mutation I476V located in a coiled region of domain B 25 Å from the active site was paired with N251D or V482L, a k_cat effect was observed. These results indicate that temperature-dependent kinetic effects may be complex and subtle and are mediated by a combination of a small number of residues distant from the active site via changes to the hydration shell and/or perturbation of internal packing.

Biotechnological potential of psychrophilic microorganisms as the source of cold-active enzymes in food processing applications

Microorganisms striving in extreme environments and exhibiting optimal growth and reproduction at low temperatures, otherwise known as psychrophilic microorganisms, are potential sources of cold-active enzymes. Owing to higher stability and cold activity, these enzymes are gaining enormous attention in numerous industrial bioprocesses. Applications of several cold-active enzymes have been established in the food industry, e.g., β-galactosidase, pectinase, proteases, amylases, xylanases, pullulanases, lipases, and β-mannanases. The enzyme engineering approaches and the accumulating knowledge of protein structure and function have made it possible to improve the catalytic properties of interest and express the candidate enzyme in a heterologous host for a higher level of enzyme production. This review compiles the relevant and recent information on the potential uses of different cold-active enzymes in the food industry.

Isolation of cellulase-producing Microbulbifer sp. from marine teleost blackfish (Girella melanichthys) intestine and the enzyme characterization

Most animals cannot digest cellulose but have symbiotic microbes that degrade the matrix polysaccharides of plant matter. Herbivorous and omnivorous marine fish are similarly expected to rely on symbiotic microbes, but reports to date on cellulase-producing bacteria in fish intestines are limited. Here, we report the isolation of new cellulase-producing bacteria from the marine omnivorous teleost, blackfish (Girella melanichthys), and the characterization of cellulase activity. Three strains of cellulase-producing bacteria sp. were isolated from the hindgut of wild G. melanichthys. The strains of cellulase-producing bacteria grew in medium with artificial seawater but not in NaCl alone. Growth was optimum at 20-35°C, but there was no growth at 40°C, suggesting adaptation in a marine environment at a low temperature. Isolates were identified to Microbulbifer sp., among which GL-2 strain produced a high enzyme activity. The GL-2 strain was further used for enzyme characterization with carboxymethyl cellulose (CMC) as the substrate. Maximum activity of the cellulase was observed at 60°C, and activity was more than 30% at 20°C, while commercial cellulase Enthiron showed an optimum activity at 50°C and 17% activity at 20°C. Hydrolytic products by GL-2 cellulase were cellobiose but not glucose, suggesting a deficiency of β-glucosidase activity. Active gel electrophoresis containing CMC showed five bands, suggesting several cellulolytic enzymes. The GL-2 strain and its enzyme are potential probiotics for aquaculture fish and the industrial production of cellobiose.

Diversity, Ecological Role and Biotechnological Potential of Antarctic Marine Fungi

The Antarctic Ocean is one of the most remote and inaccessible environments on our planet and hosts potentially high biodiversity, being largely unexplored and undescribed. Fungi have key functions and unique physiological and morphological adaptations even in extreme conditions, from shallow habitats to deep-sea sediments. Here, we summarized information on diversity, the ecological role, and biotechnological potential of marine fungi in the coldest biome on Earth. This review also discloses the importance of boosting research on Antarctic fungi as hidden treasures of biodiversity and bioactive molecules to better understand their role in marine ecosystem functioning and their applications in different biotechnological fields.

Hidden Conformational States and Strange Temperature Optima in Enzyme Catalysis

The existence of temperature optima in enzyme catalysis that occur before protein melting sets in can be described by different types of kinetic models. Such optima cause distinctly curved Arrhenius plots and have, for example, been observed in several cold-adapted enzymes from psychrophilic species. The two main explanations proposed for this behavior either invoke conformational equilibria with inactive substrate-bound states or postulate differences in heat capacity between the reactant and transition states. Herein, we analyze the implications of the different types of kinetic models in terms of apparent activation enthalpies, entropies, and heat capacities, using the catalytic reaction of a cold-adapted α-amylase as a prototypic example. We show that the behavior of these thermodynamic activation parameters is fundamentally different between equilibrium and heat capacity models, and in the α-amylase case, computer simulations have shown the former model to be correct. A few other enzyme-catalyzed reactions are also discussed in this context.

Is the Arrhenius-correction of biodegradation rates, as recommended through REACH guidance, fit for environmentally relevant conditions? An example from petroleum biodegradation in environmental systems

Biodegradation is a major determinant of chemical persistence in the environment and an important consideration for PBT and environmental risk assessments. It is influenced by several environmental factors including temperature and microbial community structure. According to REACH guidance, a temperature correction based on the Arrhenius equation is recommended for chemical persistence data not performed at the recommended EU mean surface water temperature. Such corrections, however, can lead to overly conservative P/vP assessments. In this paper, the relevance of this temperature correction is assessed for petroleum hydrocarbons, using measured surface water (marine and freshwater) degradation half-time (DT50) and degradation half-life (HL) data compiled from relevant literature. Stringent screening criteria were used to specifically select data from biodegradation tests containing indigenous microbes and conducted at temperatures close to their ambient sampling temperature. As a result, ten independent studies were identified, with 993 data points covering 326 hydrocarbon constituents. These data were derived from tests conducted with natural seawater, or freshwater, at temperatures ranging from 5 to 21 °C. Regressions were performed on the full hydrocarbon dataset and on several individual hydrocarbons. The results were compared to the trend as predicted by the Arrhenius equation and using the activation energy (E_a) as recommend in the REACH Guidance. The comparison shows that the correction recommended in REACH Guidance over predicts the effect of temperature on hydrocarbon biodegradation. These results contrast with temperature manipulated inocula where the test temperature is different from the ambient sampling temperature. In these manipulated systems, the effect of temperature follows the Arrhenius equation more closely. In addition, a more striking effect of temperature on the lag phase was observed with longer lag phases more apparent at lower temperatures. This indicates that the effect of temperature may indeed be even lower when considering hydrocarbon biodegradation without the initial lag phase.

Mapping the Transglycosylation Relevant Sites of Cold-Adapted β-d-Galactosidase from Arthrobacter sp. 32cB

β-Galactosidase from Arthrobacter sp. 32cB (ArthβDG) is a cold-adapted enzyme able to catalyze hydrolysis of β-d-galactosides and transglycosylation reaction, where galactosyl moiety is being transferred onto an acceptor larger than a water molecule. Mutants of ArthβDG: D207A and E517Q were designed to determine the significance of specific residues and to enable formation of complexes with lactulose and sucrose and to shed light onto the structural basis of the transglycosylation reaction. The catalytic assays proved loss of function mutation E517 into glutamine and a significant drop of activity for mutation of D207 into alanine. Solving crystal structures of two new mutants, and new complex structures of previously presented mutant E441Q enables description of introduced changes within active site of enzyme and determining the importance of mutated residues for active site size and character. Furthermore, usage of mutants with diminished and abolished enzymatic activity enabled solving six complex structures with galactose, lactulose or sucrose bounds. As a result, not only the galactose binding sites were mapped on the enzyme's surface but also the mode of lactulose, product of transglycosylation reaction, and binding within the enzyme's active site were determined and the glucopyranose binding site in the distal of active site was discovered. The latter two especially show structural details of transglycosylation, providing valuable information that may be used for engineering of ArthβDG or other analogous galactosidases belonging to GH2 family.

Current prospective in using cold-active enzymes as eco-friendly detergent additive

Advanced developments in the field of enzyme technology have increased the use of enzymes in industrial applications, especially in detergents. Enzymes as detergent additives have been extensively studied and the demand is considerably increasing due to its distinct properties and potential applications. Enzymes from microorganisms colonized at various geographical locations ranging from extreme hot to cold are explored for compatibility studies as detergent additives. Especially psychrophiles growing at cold conditions have cold-active enzymes with high catalytic activity and their stability under extreme conditions makes it as an appropriate eco-friendly and cost-effective additive in detergents. Adequate number of reports are available on cold-active enzymes such as proteases, lipases, amylases, and cellulases with high efficiency and exceptional features. These enzymes with increased thermostability and alkaline stability have become the premier choice as detergent additives. Modern approaches in genomics and proteomics paved the way to understand the compatibility of cold-active enzymes as detergent additives in broader dimensions. The molecular techniques such as gene coding, amino acid sequencing, and protein engineering studies helped to solve the mysteries related to alkaline stability of these enzymes and their chemical compatibility with oxidizing agents. The present review provides an overview of cold-active enzymes used as detergent additives and molecular approaches that resulted in development of these enzymes as commercial hit in detergent industries. The scope and challenges in using cold-active enzymes as eco-friendly and sustainable detergent additive are also discussed.

Extremophile - An Adaptive Strategy for Extreme Conditions and Applications

The concurrence of microorganisms in niches that are hostile like extremes of temperature, pH, salt concentration and high pressure depends upon novel molecular mechanisms to enhance the stability of their proteins, nucleic acids, lipids and cell membranes. The structural, physiological and genomic features of extremophiles that make them capable of withstanding extremely selective environmental conditions are particularly fascinating. Highly stable enzymes exhibiting several industrial and biotechnological properties are being isolated and purified from these extremophiles. Successful gene cloning of the purified extremozymes in the mesophilic hosts has already been done. Various extremozymes such as amylase, lipase, xylanase, cellulase and protease from thermophiles, halothermophiles and psychrophiles are of industrial interests due to their enhanced stability at forbidding conditions. In this review, we made an attempt to point out the unique features of extremophiles, particularly thermophiles and psychrophiles, at the structural, genomic and proteomic levels, which allow for functionality at harsh conditions focusing on the temperature tolerance by them.

Structural features of cold-adapted dimeric GH2 β-D-galactosidase from Arthrobacter sp. 32cB

Crystal structures of cold-adapted β-d-galactosidase (EC 3.2.1.23) from the Antarctic bacterium Arthrobacter sp. 32cB (ArthβDG) have been determined in an unliganded form resulting from diffraction experiments conducted at 100 K (at resolution 1.8 Å) and at room temperature (at resolution 3.0 Å). A detailed comparison of those two structures of the same enzyme was performed in order to estimate differences in their molecular flexibility and rigidity and to study structural rationalization for the cold-adaptation of the investigated enzyme. Furthermore, a comparative analysis with structures of homologous enzymes from psychrophilic, mesophilic, and thermophilic sources has been discussed to elucidate the relationship between structure and cold-adaptation in a wider context. The performed studies confirm that the structure of cold-adapted ArthβDG maintains balance between molecular stability and structural flexibility, which can be observed independently on the temperature of conducted X-ray diffraction experiments. Obtained information about proper protein function under given conditions provide a guideline for rational engineering of proteins in terms of their temperature optimum and thermal stability.

Structural and mutational analyses of psychrophilic and mesophilic adenylate kinases highlight the role of hydrophobic interactions in protein thermal stability

Protein thermal stability is an important field since thermally stable proteins are desirable in many academic and industrial settings. Information on protein thermal stabilization can be obtained by comparing homologous proteins from organisms living at distinct temperatures. Here, we report structural and mutational analyses of adenylate kinases (AKs) from psychrophilic Bacillus globisporus (AKp) and mesophilic Bacillus subtilis (AKm). Sequence and structural comparison showed suboptimal hydrophobic packing around Thr26 in the CORE domain of AKp, which was replaced with an Ile residue in AKm. Mutations that improved hydrophobicity of the Thr residue increased the thermal stability of the psychrophilic AKp, and the largest stabilization was observed for a Thr-to-Ile substitution. Furthermore, a reverse Ile-to-Thr mutation in the mesophilic AKm significantly decreased thermal stability. We determined the crystal structures of mutant AKs to confirm the impact of the residue substitutions on the overall stability. Taken together, our results provide a structural basis for the stability difference between psychrophilic and mesophilic AK homologues and highlight the role of hydrophobic interactions in protein thermal stability.

Psychrophilic lifestyles: mechanisms of adaptation and biotechnological tools

Cold-adapted microorganisms inhabiting permanently low-temperature environments were initially just a biological curiosity but have emerged as rich sources of numerous valuable tools for application in a broad spectrum of innovative technologies. To overcome the multiple challenges inherent to life in their cold habitats, these microorganisms have developed a diverse array of highly sophisticated synergistic adaptations at all levels within their cells: from cell envelope and enzyme adaptation, to cryoprotectant and chaperone production, and novel metabolic capabilities. Basic research has provided valuable insights into how these microorganisms can thrive in their challenging habitat conditions and into the mechanisms of action of the various adaptive features employed, and such insights have served as a foundation for the knowledge-based development of numerous novel biotechnological tools. In this review, we describe the current knowledge of the adaptation strategies of cold-adapted microorganisms and the biotechnological perspectives and commercial tools emerging from this knowledge. Adaptive features and, where possible, applications, in relation to membrane fatty acids, membrane pigments, the cell wall peptidoglycan layer, the lipopolysaccharide component of the outer cell membrane, compatible solutes, antifreeze and ice-nucleating proteins, extracellular polymeric substances, biosurfactants, chaperones, storage materials such as polyhydroxyalkanoates and cyanophycins and metabolic adjustments are presented and discussed.

Roles of Active-Site Aromatic Residues in Cold Adaptation of Sphingomonas glacialis Esterase EstSP1

The aromatic amino acids, Tyr or Trp, which line the active-site walls of esterases, stabilize the catalytic His loop via hydrogen bonding. A Tyr residue is preferred in extremophilic esterases (psychrophilic or hyperthermophilic esterases), whereas a Trp residue is preferred in moderate-temperature esterases. Here, we provide evidence that Tyr and Trp play distinct roles in cold adaptation of the psychrophilic esterase EstSP1 isolated from an Arctic bacterium Sphingomonas glacialis PAMC 26605. Stern-Volmer plots showed that the mutation of Tyr191 to Ala, Phe, Trp, and His resulted in reduced conformational flexibility of the overall protein structure. Interestingly, the Y191W and Y191H mutants showed increased thermal stability at moderate temperatures. All Tyr191 mutants showed reduced catalytic activity relative to wild-type EstSP1. Our results indicate that Tyr with its phenyl hydroxyl group is favored for increased conformational flexibility and high catalytic activity of EstSP1 at low temperatures at the expense of thermal stability. The results of this study suggest that, in the permanently cold Arctic zone, enzyme activity has been selected for psychrophilic enzymes over thermal stability. The results presented herein provide novel insight into the roles of Tyr and Trp residues for temperature adaptation of enzymes that function at low, moderate, and high temperatures.

Structural analyses of adenylate kinases from Antarctic and tropical fishes for understanding cold adaptation of enzymes

Psychrophiles are extremophilic organisms capable of thriving in cold environments. Proteins from these cold-adapted organisms can remain physiologically functional at low temperatures, but are structurally unstable even at moderate temperatures. Here, we report the crystal structure of adenylate kinase (AK) from the Antarctic fish Notothenia coriiceps, and identify the structural basis of cold adaptation by comparison with homologues from tropical fishes including Danio rerio. The structure of N. coriiceps AK (AKNc) revealed suboptimal hydrophobic packing around three Val residues in its central CORE domain, which are replaced with Ile residues in D. rerio AK (AKDr). The Val-to-Ile mutations that improve hydrophobic CORE packing in AKNc increased stability at high temperatures but decreased activity at low temperatures, suggesting that the suboptimal hydrophobic CORE packing is important for cold adaptation. Such linkage between stability and activity was also observed in AKDr. Ile-to-Val mutations that destabilized the tropical AK resulted in increased activity at low temperatures. Our results provide the structural basis of cold adaptation of a psychrophilic enzyme from a multicellular, eukaryotic organism, and highlight the similarities and differences in the structural adjustment of vertebrate and bacterial psychrophilic AKs during cold adaptation.

A comparative study of cold- and warm-adapted Endonucleases A using sequence analyses and molecular dynamics simulations

The psychrophilic and mesophilic endonucleases A (EndA) from Aliivibrio salmonicida (VsEndA) and Vibrio cholera (VcEndA) have been studied experimentally in terms of the biophysical properties related to thermal adaptation. The analyses of their static X-ray structures was no sufficient to rationalize the determinants of their adaptive traits at the molecular level. Thus, we used Molecular Dynamics (MD) simulations to compare the two proteins and unveil their structural and dynamical differences. Our simulations did not show a substantial increase in flexibility in the cold-adapted variant on the nanosecond time scale. The only exception is a more rigid C-terminal region in VcEndA, which is ascribable to a cluster of electrostatic interactions and hydrogen bonds, as also supported by MD simulations of the VsEndA mutant variant where the cluster of interactions was introduced. Moreover, we identified three additional amino acidic substitutions through multiple sequence alignment and the analyses of MD-based protein structure networks. In particular, T120V occurs in the proximity of the catalytic residue H80 and alters the interaction with the residue Y43, which belongs to the second coordination sphere of the Mg2+ ion. This makes T120V an amenable candidate for future experimental mutagenesis.

Kinetics and Structure of a Cold-Adapted Hetero-Octameric ATP Phosphoribosyltransferase

Adenosine 5'-triphosphate phosphoribosyltransferase (ATPPRT) catalyzes the first step in histidine biosynthesis, the condensation of ATP and 5-phospho-α-d-ribosyl-1-pyrophosphate to generate N¹-(5-phospho-β-d-ribosyl)-ATP and inorganic pyrophosphate. The enzyme is allosterically inhibited by histidine. Two forms of ATPPRT, encoded by the hisG gene, exist in nature, depending on the species. The long form, HisG_L, is a single polypeptide chain with catalytic and regulatory domains. The short form, HisG_S, lacks a regulatory domain and cannot bind histidine. HisG_S instead is found in complex with a regulatory protein, HisZ, constituting the ATPPRT holoenzyme. HisZ triggers HisG_S catalytic activity while rendering it sensitive to allosteric inhibition by histidine. Until recently, HisG_S was thought to be catalytically inactive without HisZ. Here, recombinant HisG_S and HisZ from the psychrophilic bacterium Psychrobacter arcticus were independently overexpressed and purified. The crystal structure of P. arcticus ATPPRT was determined at 2.34 Å resolution, revealing an equimolar HisG_S-HisZ hetero-octamer. Steady-state kinetics indicate that both the ATPPRT holoenzyme and HisG_S are catalytically active. Surprisingly, HisZ confers only a modest 2-4-fold increase in k_cat. Reaction profiles for both enzymes cannot be distinguished by ³¹P nuclear magnetic resonance, indicating that the same reaction is catalyzed. The temperature dependence of k_cat shows deviation from Arrhenius behavior at 308 K with the holoenzyme. Interestingly, such deviation is detected only at 313 K with HisG_S. Thermal denaturation by CD spectroscopy resulted in T_m's of 312 and 316 K for HisZ and HisG_S, respectively, suggesting that HisZ renders the ATPPRT complex more thermolabile. This is the first characterization of a psychrophilic ATPPRT.

MutT from the fish pathogen Aliivibrio salmonicida is a cold-active nucleotide-pool sanitization enzyme with unexpectedly high thermostability

Upon infection by pathogenic bacteria, production of reactive oxygen species (ROS) is part of the host organism's first line of defence. ROS damage a number of macromolecules, and in order to withstand such a harsh environment, the bacteria need to have well-functioning ROS scavenging and repair systems. Herein, MutT is an important nucleotide-pool sanitization enzyme, which degrades 8-oxo-dGTP and thus prevents it from being incorporated into DNA. In this context, we have performed a comparative biochemical and structural analysis of MutT from the fish pathogen Aliivibrio salmonicida (AsMutT) and the human pathogen Vibrio cholerae (VcMutT), in order to analyse their function as nucleotide sanitization enzymes and also determine possible cold-adapted properties of AsMutT. The biochemical characterisation revealed that both enzymes possess activity towards the 8-oxo-dGTP substrate, and that AsMutT has a higher catalytic efficiency than VcMutT at all temperatures studied. Calculations based on the biochemical data also revealed a lower activation energy (E a) for AsMutT compared to VcMutT, and differential scanning calorimetry experiments showed that AsMutT displayed an unexpected higher melting temperature (T m) value than VcMutT. A comparative analysis of the crystal structure of VcMutT, determined to 2.42 Å resolution, and homology models of AsMutT indicate that three unique Gly residues in loops of VcMutT, and additional long range ion-pairs in AsMutT could explain the difference in temperature stability of the two enzymes. We conclude that AsMutT is a stable, cold-active enzyme with high catalytic efficiency and reduced E a, compared to the mesophilic VcMutT.

Polar Microalgae: New Approaches towards Understanding Adaptations to an Extreme and Changing Environment

Polar Regions are unique and highly prolific ecosystems characterized by extreme environmental gradients. Photosynthetic autotrophs, the base of the food web, have had to adapt physiological mechanisms to maintain growth, reproduction and metabolic activity despite environmental conditions that would shut-down cellular processes in most organisms. High latitudes are characterized by temperatures below the freezing point, complete darkness in winter and continuous light and high UV in the summer. Additionally, sea-ice, an ecological niche exploited by microbes during the long winter seasons when the ocean and land freezes over, is characterized by large salinity fluctuations, limited gas exchange, and highly oxic conditions. The last decade has been an exciting period of insights into the molecular mechanisms behind adaptation of microalgae to the cryosphere facilitated by the advancement of new scientific tools, particularly "omics" techniques. We review recent insights derived from genomics, transcriptomics, and proteomics studies. Genes, proteins and pathways identified from these highly adaptable polar microbes have far-reaching biotechnological applications. Furthermore, they may provide insights into life outside this planet, as well as glimpses into the past. High latitude regions also have disproportionately large inputs into global biogeochemical cycles and are the region most sensitive to climate change.