Screening and Characterization of Novel Polyesterases from Environmental Metagenomes with High Hydrolytic Activity against Synthetic Polyesters.

The continuous growth of global plastics production, including polyesters, has resulted in increasing plastic pollution and subsequent negative environmental impacts. Therefore, enzyme-catalyzed depolymerization of synthetic polyesters as a plastics recycling approach has become a focus of research. In this study, we screened over 200 purified uncharacterized hydrolases from environmental metagenomes and sequenced microbial genomes and identified at least 10 proteins with high hydrolytic activity against synthetic polyesters. These include the metagenomic esterases MGS0156 and GEN0105, which hydrolyzed polylactic acid (PLA), polycaprolactone, as well as bis(benzoyloxyethyl)-terephthalate. With solid PLA as a substrate, both enzymes produced a mixture of lactic acid monomers, dimers, and higher oligomers as products. The crystal structure of MGS0156 was determined at 1.95 Å resolution and revealed a modified α/ß hydrolase fold, with a lid domain and highly hydrophobic active site. Mutational studies of MGS0156 identified the residues critical for hydrolytic activity against both polyester and monoester substrates, with two-times higher polyesterase activity in the MGS0156 L169A mutant protein. Thus, our work identified novel, highly active polyesterases in environmental metagenomes and provided molecular insights into their activity, thereby augmenting our understanding of enzymatic polyester hydrolysis.

Exploring Bacterial Carboxylate Reductases for the Reduction of Bifunctional Carboxylic Acids.

Carboxylic acid reductases (CARs) selectively reduce carboxylic acids to aldehydes using ATP and NADPH as cofactors under mild conditions. Although CARs attracts significant interest, only a few enzymes have been characterized to date, whereas the vast majority of CARs have yet to be examined. Herein the authors report that 12 bacterial CARs reduces a broad range of bifunctional carboxylic acids containing oxo-, hydroxy-, amino-, or second carboxyl groups with several enzymes showing activity toward 4-hydroxybutanoic (4-HB) and adipic acids. These CARs exhibits significant reductase activity against substrates whose second functional group is separated from the carboxylate by at least three carbons with both carboxylate groups being reduced in dicarboxylic acids. Purified CARs supplemented with cofactor regenerating systems (for ATP and NADPH), an inorganic pyrophosphatase, and an aldo-keto reductase catalyzes a high conversion (50-76%) of 4-HB to 1,4-butanediol (1,4-BDO) and adipic acid to 1,6-hexanediol (1,6-HDO). Likewise, Escherichia coli strains expressing eight different CARs efficiently reduces 4-HB to 1,4-BDO with 50-95% conversion, whereas adipic acid is reduced to a mixture of 6-hydroxyhexanoic acid (6-HHA) and 1,6-HDO. Thus, our results illustrate the broad biochemical diversity of bacterial CARs and their compatibility with other enzymes for applications in biocatalysis.

Biochemical and Structural Insights into Enzymatic Depolymerization of Polylactic Acid and Other Polyesters by Microbial Carboxylesterases.

Polylactic acid (PLA) is a biodegradable polyester derived from renewable resources, which is a leading candidate for the replacement of traditional petroleum-based polymers. Since the global production of PLA is quickly growing, there is an urgent need for the development of efficient recycling technologies, which will produce lactic acid instead of CO2 as the final product. After screening 90 purified microbial α/ß-hydrolases, we identified hydrolytic activity against emulsified PLA in two uncharacterized proteins, ABO2449 from Alcanivorax borkumensis and RPA1511 from Rhodopseudomonas palustris. Both enzymes were also active against emulsified polycaprolactone and other polyesters as well as against soluble α-naphthyl and p-nitrophenyl monoesters. In addition, both ABO2449 and RPA1511 catalyzed complete or extensive hydrolysis of solid PLA with the production of lactic acid monomers, dimers, and larger oligomers as products. The crystal structure of RPA1511 was determined at 2.2 Å resolution and revealed a classical α/ß-hydrolase fold with a wide-open active site containing a molecule of polyethylene glycol bound near the catalytic triad Ser114-His270-Asp242. Site-directed mutagenesis of both proteins demonstrated that the catalytic triad residues are important for the hydrolysis of both monoester and polyester substrates. We also identified several residues in RPA1511 (Gln172, Leu212, Met215, Trp218, and Leu220) and ABO2449 (Phe38 and Leu152), which were not essential for activity against soluble monoesters but were found to be critical for the hydrolysis of PLA. Our results indicate that microbial carboxyl esterases can efficiently hydrolyze various polyesters making them attractive biocatalysts for plastics depolymerization and recycling.

Experimental validation of in silico model-predicted isocitrate dehydrogenase and phosphomannose isomerase from Dehalococcoides mccartyi.

Gene sequences annotated as proteins of unknown or non-specific function and hypothetical proteins account for a large fraction of most genomes. In the strictly anaerobic and organohalide respiring Dehalococcoides mccartyi, this lack of annotation plagues almost half the genome. Using a combination of bioinformatics analyses and genome-wide metabolic modelling, new or more specific annotations were proposed for about 80 of these poorly annotated genes in previous investigations of D. mccartyi metabolism. Herein, we report the experimental validation of the proposed reannotations for two such genes (KB1_0495 and KB1_0553) from D. mccartyi strains in the KB-1 community. KB1_0495 or DmIDH was originally annotated as an NAD(+)-dependent isocitrate dehydrogenase, but biochemical assays revealed its activity primarily with NADP(+) as a cofactor. KB1_0553, also denoted as DmPMI, was originally annotated as a hypothetical protein/sugar isomerase domain protein. We previously proposed that it was a bifunctional phosphoglucose isomerase/phosphomannose isomerase, but only phosphomannose isomerase activity was identified and confirmed experimentally. Further bioinformatics analyses of these two protein sequences suggest their affiliation to potentially novel enzyme families within their respective larger enzyme super families.

Identification and characterization of carboxyl esterases of gill chamber-associated microbiota in the deep-sea shrimp Rimicaris exoculata by using functional metagenomics.

The shrimp Rimicaris exoculata dominates the fauna in deep-sea hydrothermal vent sites along the Mid-Atlantic Ridge (depth, 2,320 m). Here, we identified and biochemically characterized three carboxyl esterases from microbial communities inhabiting the R. exoculata gill that were isolated by naive screens of a gill chamber metagenomic library. These proteins exhibit low to moderate identity to known esterase sequences (≤52%) and to each other (11.9 to 63.7%) and appear to have originated from unknown species or from genera of Proteobacteria related to Thiothrix/Leucothrix (MGS-RG1/RG2) and to the Rhodobacteraceae group (MGS-RG3). A library of 131 esters and 31 additional esterase/lipase preparations was used to evaluate the activity profiles of these enzymes. All 3 of these enzymes had greater esterase than lipase activity and exhibited specific activities with ester substrates (≤356 U mg(-1)) in the range of similar enzymes. MGS-RG3 was inhibited by salts and pressure and had a low optimal temperature (30°C), and its substrate profile clustered within a group of low-activity and substrate-restricted marine enzymes. In contrast, MGS-RG1 and MGS-RG2 were most active at 45 to 50°C and were salt activated and barotolerant. They also exhibited wider substrate profiles that were close to those of highly active promiscuous enzymes from a marine hydrothermal vent (MGS-RG2) and from a cold brackish lake (MGS-RG1). The data presented are discussed in the context of promoting the examination of enzyme activities of taxa found in habitats that have been neglected for enzyme prospecting; the enzymes found in these taxa may reflect distinct habitat-specific adaptations and may constitute new sources of rare reaction specificities.

The environment shapes microbial enzymes: five cold-active and salt-resistant carboxylesterases from marine metagenomes.

Most of the Earth's biosphere is cold and is populated by cold-adapted microorganisms. To explore the natural enzyme diversity of these environments and identify new carboxylesterases, we have screened three marine metagenome gene libraries for esterase activity. The screens identified 23 unique active clones, from which five highly active esterases were selected for biochemical characterization. The purified metagenomic esterases exhibited high activity against α-naphthyl and p-nitrophenyl esters with different chain lengths. All five esterases retained high activity at 5 °C indicating that they are cold-adapted enzymes. The activity of MGS0010 increased more than two times in the presence of up to 3.5 M NaCl or KCl, whereas the other four metagenomic esterases were inhibited to various degrees by these salts. The purified enzymes showed different sensitivities to inhibition by solvents and detergents, and the activities of MGS0010, MGS0105 and MGS0109 were stimulated three to five times by the addition of glycerol. Screening of purified esterases against 89 monoester substrates revealed broad substrate profiles with a preference for different esters. The metagenomic esterases also hydrolyzed several polyester substrates including polylactic acid suggesting that they can be used for polyester depolymerization. Thus, esterases from marine metagenomes are cold-adapted enzymes exhibiting broad biochemical diversity reflecting the environmental conditions where they evolved.

Pressure adaptation is linked to thermal adaptation in salt-saturated marine habitats.

The present study provides a deeper view of protein functionality as a function of temperature, salt and pressure in deep-sea habitats. A set of eight different enzymes from five distinct deep-sea (3040-4908 m depth), moderately warm (14.0-16.5°C) biotopes, characterized by a wide range of salinities (39-348 practical salinity units), were investigated for this purpose. An enzyme from a 'superficial' marine hydrothermal habitat (65°C) was isolated and characterized for comparative purposes. We report here the first experimental evidence suggesting that in salt-saturated deep-sea habitats, the adaptation to high pressure is linked to high thermal resistance (P value = 0.0036). Salinity might therefore increase the temperature window for enzyme activity, and possibly microbial growth, in deep-sea habitats. As an example, Lake Medee, the largest hypersaline deep-sea anoxic lake of the Eastern Mediterranean Sea, where the water temperature is never higher than 16°C, was shown to contain halopiezophilic-like enzymes that are most active at 70°C and with denaturing temperatures of 71.4°C. The determination of the crystal structures of five proteins revealed unknown molecular mechanisms involved in protein adaptation to poly-extremes as well as distinct active site architectures and substrate preferences relative to other structurally characterized enzymes.

Biochemical and structural studies of conserved Maf proteins revealed nucleotide pyrophosphatases with a preference for modified nucleotides.

Maf (for multicopy associated filamentation) proteins represent a large family of conserved proteins implicated in cell division arrest but whose biochemical activity remains unknown. Here, we show that the prokaryotic and eukaryotic Maf proteins exhibit nucleotide pyrophosphatase activity against 5-methyl-UTP, pseudo-UTP, 5-methyl-CTP, and 7-methyl-GTP, which represent the most abundant modified bases in all organisms, as well as against canonical nucleotides dTTP, UTP, and CTP. Overexpression of the Maf protein YhdE in E. coli cells increased intracellular levels of dTMP and UMP, confirming that dTTP and UTP are the in vivo substrates of this protein. Crystal structures and site-directed mutagenesis of Maf proteins revealed the determinants of their activity and substrate specificity. Thus, pyrophosphatase activity of Maf proteins toward canonical and modified nucleotides might provide the molecular mechanism for a dual role of these proteins in cell division arrest and house cleaning.

Genome sequence and functional genomic analysis of the oil-degrading bacterium Oleispira antarctica.

Ubiquitous bacteria from the genus Oleispira drive oil degradation in the largest environment on Earth, the cold and deep sea. Here we report the genome sequence of Oleispira antarctica and show that compared with Alcanivorax borkumensis--the paradigm of mesophilic hydrocarbonoclastic bacteria--O. antarctica has a larger genome that has witnessed massive gene-transfer events. We identify an array of alkane monooxygenases, osmoprotectants, siderophores and micronutrient-scavenging pathways. We also show that at low temperatures, the main protein-folding machine Cpn60 functions as a single heptameric barrel that uses larger proteins as substrates compared with the classical double-barrel structure observed at higher temperatures. With 11 protein crystal structures, we further report the largest set of structures from one psychrotolerant organism. The most common structural feature is an increased content of surface-exposed negatively charged residues compared to their mesophilic counterparts. Our findings are relevant in the context of microbial cold-adaptation mechanisms and the development of strategies for oil-spill mitigation in cold environments.

Biochemical diversity of carboxyl esterases and lipases from Lake Arreo (Spain): a metagenomic approach.

The esterases and lipases from the α/ß hydrolase superfamily exhibit an enormous sequence diversity, fold plasticity, and activities. Here, we present the comprehensive sequence and biochemical analyses of seven distinct esterases and lipases from the metagenome of Lake Arreo, an evaporite karstic lake in Spain (42°46'N, 2°59'W; altitude, 655 m). Together with oligonucleotide usage patterns and BLASTP analysis, our study of esterases/lipases mined from Lake Arreo suggests that its sediment contains moderately halophilic and cold-adapted proteobacteria containing DNA fragments of distantly related plasmids or chromosomal genomic islands of plasmid and phage origins. This metagenome encodes esterases/lipases with broad substrate profiles (tested over a set of 101 structurally diverse esters) and habitat-specific characteristics, as they exhibit maximal activity at alkaline pH (8.0 to 8.5) and temperature of 16 to 40°C, and they are stimulated (1.5 to 2.2 times) by chloride ions (0.1 to 1.2 M), reflecting an adaptation to environmental conditions. Our work provides further insights into the potential significance of the Lake Arreo esterases/lipases for biotechnology processes (i.e., production of enantiomers and sugar esters), because these enzymes are salt tolerant and are active at low temperatures and against a broad range of substrates. As an example, the ability of a single protein to hydrolyze triacylglycerols, (non)halogenated alkyl and aryl esters, cinnamoyl and carbohydrate esters, lactones, and chiral epoxides to a similar extent was demonstrated.

Nuclease activity of the human SAMHD1 protein implicated in the Aicardi-Goutieres syndrome and HIV-1 restriction.

The human HD domain protein SAMHD1 is implicated in the Aicardi-Goutières autoimmune syndrome and in the restriction of HIV-1 replication in myeloid cells. Recently, this protein has been shown to possess dNTP triphosphatase activity, which is proposed to inhibit HIV-1 replication and the autoimmune response by hydrolyzing cellular dNTPs. Here, we show that the purified full-length human SAMHD1 protein also possesses metal-dependent 3'→5' exonuclease activity against single-stranded DNAs and RNAs in vitro. In double-stranded substrates, this protein preferentially cleaved 3'-overhangs and RNA in blunt-ended DNA/RNA duplexes. Full-length SAMHD1 also exhibited strong DNA and RNA binding to substrates with complex secondary structures. Both nuclease and dNTP triphosphatase activities of SAMHD1 are associated with its HD domain, but the SAM domain is required for maximal activity and nucleic acid binding. The nuclease activity of SAMHD1 could represent an additional mechanism contributing to HIV-1 restriction and suppression of the autoimmune response through direct cleavage of viral and endogenous nucleic acids. In addition, we demonstrated the presence of dGTP triphosphohydrolase and nuclease activities in several microbial HD domain proteins, suggesting that these proteins might contribute to antiviral defense in prokaryotes.

Structure and activity of the NAD(P)+-dependent succinate semialdehyde dehydrogenase YneI from Salmonella typhimurium.

Aldehyde dehydrogenases are found in all organisms and play an important role in the metabolic conversion and detoxification of endogenous and exogenous aldehydes. Genomes of many organisms including Escherichia coli and Salmonella typhimurium encode two succinate semialdehyde dehydrogenases with low sequence similarity and different cofactor preference (YneI and GabD). Here, we present the crystal structure and biochemical characterization of the NAD(P)(+)-dependent succinate semialdehyde dehydrogenase YneI from S. typhimurium. This enzyme shows high activity and affinity toward succinate semialdehyde and exhibits substrate inhibition at concentrations of SSA higher than 0.1 mM. YneI can use both NAD(+) and NADP(+) as cofactors, although affinity to NAD(+) is 10 times higher. High resolution crystal structures of YneI were solved in a free state (1.85 Å) and in complex with NAD(+) (1.90 Å) revealing a two domain protein with the active site located in the interdomain interface. The NAD(+) molecule is bound in the long channel with its nicotinamide ring positioned close to the side chain of the catalytic Cys268. Site-directed mutagenesis demonstrated that this residue, as well as the conserved Trp136, Glu365, and Asp426 are important for activity of YneI, and that the conserved Lys160 contributes to the enzyme preference to NAD(+) . Our work has provided further insight into the molecular mechanisms of substrate selectivity and activity of succinate semialdehyde dehydrogenases.

Structure and activity of the cold-active and anion-activated carboxyl esterase OLEI01171 from the oil-degrading marine bacterium Oleispira antarctica.

The uncharacterized α/ß-hydrolase protein OLEI01171 from the psychrophilic marine bacterium Oleispira antarctica belongs to the PF00756 family of putative esterases, which also includes human esterase D. In the present paper we show that purified recombinant OLEI01171 exhibits high esterase activity against the model esterase substrate α-naphthyl acetate at 5-30°C with maximal activity at 15-20°C. The esterase activity of OLEI01171 was stimulated 3-8-fold by the addition of chloride or several other anions (0.1-1.0 M). Compared with mesophilic PF00756 esterases, OLEI01171 exhibited a lower overall protein thermostability. Two crystal structures of OLEI01171 were solved at 1.75 and 2.1 Å resolution and revealed a classical serine hydrolase catalytic triad and the presence of a chloride or bromide ion bound in the active site close to the catalytic Ser148. Both anions were found to co-ordinate a potential catalytic water molecule located in the vicinity of the catalytic triad His257. The results of the present study suggest that the bound anion perhaps contributes to the polarization of the catalytic water molecule and increases the rate of the hydrolysis of an acyl-enzyme intermediate. Alanine replacement mutagenesis of OLEI01171 identified ten amino acid residues important for esterase activity. The replacement of Asn225 by lysine had no significant effect on the activity or thermostability of OLEI01171, but resulted in a detectable increase of activity at 35-45°C. The present study has provided insight into the molecular mechanisms of activity of a cold-active and anion-activated carboxyl esterase.

Structure and activity of the Pseudomonas aeruginosa hotdog-fold thioesterases PA5202 and PA2801.

The hotdog fold is one of the basic protein folds widely present in bacteria, archaea and eukaryotes. Many of these proteins exhibit thioesterase activity against fatty acyl-CoAs and play important roles in lipid metabolism, cellular signalling and degradation of xenobiotics. The genome of the opportunistic pathogen Pseudomonas aeruginosa contains over 20 genes encoding predicted hotdog-fold proteins, none of which have been experimentally characterized. We have found that two P. aeruginosa hotdog proteins display high thioesterase activity against 3-hydroxy-3-methylglutaryl-CoA and glutaryl-CoA (PA5202), and octanoyl-CoA (PA2801). Crystal structures of these proteins were solved (at 1.70 and 1.75 Å for PA5202 and PA2801 respectively) and revealed a hotdog fold with a potential catalytic carboxylate residue located on the long α-helix (Asp(57) in PA5202 and Glu(35) in PA2801). Alanine residue replacement mutagenesis of PA5202 identified four residues (Asn(42), Arg(43), Asp(57) and Thr(76)) that are critical for its activity and are located in the active site. A P. aeruginosa PA5202 deletion strain showed an increased secretion of the antimicrobial pigment pyocyanine and an increased expression of genes involved in pyocyanin biosynthesis, suggesting a functional link between PA5202 activity and pyocyanin production. Thus the P. aeruginosa hotdog thioesterases PA5202 and PA2801 have similar structures, but exhibit different substrate preferences and functions.

Biochemical and structural studies of uncharacterized protein PA0743 from Pseudomonas aeruginosa revealed NAD+-dependent L-serine dehydrogenase.

The ß-hydroxyacid dehydrogenases form a large family of ubiquitous enzymes that catalyze oxidation of various ß-hydroxy acid substrates to corresponding semialdehydes. Several known enzymes include ß-hydroxyisobutyrate dehydrogenase, 6-phosphogluconate dehydrogenase, 2-(hydroxymethyl)glutarate dehydrogenase, and phenylserine dehydrogenase, but the vast majority of ß-hydroxyacid dehydrogenases remain uncharacterized. Here, we demonstrate that the predicted ß-hydroxyisobutyrate dehydrogenase PA0743 from Pseudomonas aeruginosa catalyzes an NAD(+)-dependent oxidation of l-serine and methyl-l-serine but exhibits low activity against ß-hydroxyisobutyrate. Two crystal structures of PA0743 were solved at 2.2-2.3-Å resolution and revealed an N-terminal Rossmann fold domain connected by a long α-helix to the C-terminal all-α domain. The PA0743 apostructure showed the presence of additional density modeled as HEPES bound in the interdomain cleft close to the predicted catalytic Lys-171, revealing the molecular details of the PA0743 substrate-binding site. The structure of the PA0743-NAD(+) complex demonstrated that the opposite side of the enzyme active site accommodates the cofactor, which is also bound near Lys-171. Site-directed mutagenesis of PA0743 emphasized the critical role of four amino acid residues in catalysis including the primary catalytic residue Lys-171. Our results provide further insight into the molecular mechanisms of substrate selectivity and activity of ß-hydroxyacid dehydrogenases.

Structure and activity of the Saccharomyces cerevisiae dUTP pyrophosphatase DUT1, an essential housekeeping enzyme.

Genomes of all free-living organisms encode the enzyme dUTPase (dUTP pyrophosphatase), which plays a key role in preventing uracil incorporation into DNA. In the present paper, we describe the biochemical and structural characterization of DUT1 (Saccharomyces cerevisiae dUTPase). The hydrolysis of dUTP by DUT1 was strictly dependent on a bivalent metal cation with significant activity observed in the presence of Mg2+, Co2+, Mn2+, Ni2+ or Zn2+. In addition, DUT1 showed a significant activity against another potentially mutagenic nucleotide: dITP. With both substrates, DUT1 demonstrated a sigmoidal saturation curve, suggesting a positive co-operativity between the subunits. The crystal structure of DUT1 was solved at 2 Å resolution (1 Å=0.1 nm) in an apo state and in complex with the non-hydrolysable substrate α,ß-imido dUTP or dUMP product. Alanine-replacement mutagenesis of the active-site residues revealed seven residues important for activity including the conserved triad Asp87/Arg137/Asp85. The Y88A mutant protein was equally active against both dUTP and UTP, indicating that this conserved tyrosine residue is responsible for discrimination against ribonucleotides. The structure of DUT1 and site-directed mutagenesis support a role of the conserved Phe142 in the interaction with the uracil base. Our work provides further insight into the molecular mechanisms of substrate selectivity and catalysis of dUTPases.

Structural insight into the mechanism of c-di-GMP hydrolysis by EAL domain phosphodiesterases.

Cyclic diguanylate (or bis-(3'-5') cyclic dimeric guanosine monophosphate; c-di-GMP) is a ubiquitous second messenger that regulates diverse cellular functions, including motility, biofilm formation, cell cycle progression, and virulence in bacteria. In the cell, degradation of c-di-GMP is catalyzed by highly specific EAL domain phosphodiesterases whose catalytic mechanism is still unclear. Here, we purified 13 EAL domain proteins from various organisms and demonstrated that their catalytic activity is associated with the presence of 10 conserved EAL domain residues. The crystal structure of the TBD1265 EAL domain was determined in free state (1.8 Å) and in complex with c-di-GMP (2.35 A), and unveiled the role of conserved residues in substrate binding and catalysis. The structure revealed the presence of two metal ions directly coordinated by six conserved residues, two oxygens of c-di-GMP phosphate, and potential catalytic water molecule. Our results support a two-metal-ion catalytic mechanism of c-di-GMP hydrolysis by EAL domain phosphodiesterases.