Inhibition of enzyme activity by high concentrations of substrate and/or cofactor is a general phenomenon demonstrated in many enzymes, including aldehyde dehydrogenases. Here we show that the uncharacterized protein BetB (SA2613) from Staphylococcus aureus is a highly specific betaine aldehyde dehydrogenase, which exhibits substrate inhibition at concentrations of betaine aldehyde as low as 0.15 mM. In contrast, the aldehyde dehydrogenase YdcW from Escherichia coli, which is also active against betaine aldehyde, shows no inhibition by this substrate. Using the crystal structures of BetB and YdcW, we performed a structure-based mutational analysis of BetB and introduced the YdcW residues into the BetB active site. From a total of 32 mutations, those in five residues located in the substrate binding pocket (Val288, Ser290, His448, Tyr450, and Trp456) greatly reduced the substrate inhibition of BetB, whereas the double mutant protein H448F/Y450L demonstrated a complete loss of substrate inhibition. Substrate inhibition was also reduced by mutations of the semiconserved Gly234 (to Ser, Thr, or Ala) located in the BetB NAD+ binding site, suggesting some cooperativity between the cofactor and substrate binding sites. Substrate docking analysis of the BetB and YdcW active sites revealed that the wild-type BetB can bind betaine aldehyde in both productive and nonproductive conformations, whereas only the productive binding mode can be modeled in the active sites of YdcW and the BetB mutant proteins with reduced substrate inhibition. Thus, our results suggest that the molecular mechanism of substrate inhibition of BetB is associated with the nonproductive binding of betaine aldehyde.
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,β-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.
dITP; dUTP; dUTP pyrophosphatase (dUTPase); enzyme co-operativity; mutagenesis; protein structure
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.
A metagenomic fosmid expression library established from environmental DNA (eDNA) from the shallow hot vent sediment sample collected from the Levante Bay, Vulcano Island (Aeolian archipelago) was established in Escherichia coli. Using activity-based screening assays, we have assessed 9600 fosmid clones corresponding to approximately 350 Mbp of the cloned eDNA, for the lipases/esterases/lactamases, haloalkane and haloacid dehalogenases, and glycoside hydrolases. Thirty-four positive fosmid clones were selected from the total of 120 positive hits and sequenced to yield ca. 1360 kbp of high-quality assemblies. Fosmid inserts were attributed to the members of ten bacterial phyla, including Proteobacteria, Bacteroidetes, Acidobateria, Firmicutes, Verrucomicrobia, Chloroflexi, Spirochaetes, Thermotogae, Armatimonadetes, and Planctomycetes. Of ca. 200 proteins with high biotechnological potential identified therein, we have characterized in detail three distinct α/β-hydrolases (LIPESV12_9, LIPESV12_24, LIPESV12_26) and one new α-arabinopyranosidase (GLV12_5). All LIPESV12 enzymes revealed distinct substrate specificities tested against 43 structurally diverse esters and 4 p-nitrophenol carboxyl esters. Of 16 different glycosides tested, the GLV12_5 hydrolysed only p-nitrophenol-α-(l)-arabinopyranose with a high specific activity of about 2.7 kU/mg protein. Most of the α/β-hydrolases were thermophilic and revealed a high tolerance to, and high activities in the presence of, numerous heavy metal ions. Among them, the LIPESV12_24 was the best temperature-adapted, retaining its activity after 40 min of incubation at 90 °C. Furthermore, enzymes were active in organic solvents (e.g., >30 % methanol). Both LIPESV12_24 and LIPESV12_26 had the GXSXG pentapeptides and the catalytic triads Ser-Asp-His typical to the representatives of carboxylesterases of EC 18.104.22.168.
Electronic supplementary material
The online version of this article (doi:10.1007/s00253-015-6873-x) contains supplementary material, which is available to authorized users.
Vulcano Island; Fosmids; Metagenomic library; Screening; Hydrolase; Lipase; Esterase; Arabinopyranosidase
CRISPR-Cas are small RNA-based adaptive prokaryotic immunity systems protecting cells from foreign DNA or RNA. Type I CRISPR-Cas systems are composed of a multiprotein complex (Cascade) that, when bound to CRISPR RNA (crRNA), can recognize double-stranded DNA targets and recruit the Cas3 nuclease to destroy target-containing DNA. In the Escherichia coli type I-E CRISPR-Cas system, crRNAs are generated upon transcription of CRISPR arrays consisting of multiple palindromic repeats and intervening spacers through the function of Cas6e endoribonuclease, which cleaves at specific positions of repeat sequences of the CRISPR array transcript. Cas6e is also a component of Cascade. Here, we show that when mature unit-sized crRNAs are provided in a Cas6e-independent manner by transcription termination, the CRISPR-Cas system can function without Cas6e. The results should allow facile interrogation of various targets by type I-E CRISPR-Cas system in E. coli using unit-sized crRNAs generated by transcription.
The crystal structure of the protein product of the gene locus At1g05000, a hypothetical protein from A. thaliana, was determined by the multiple-wavelength anomalous diffraction method and was refined to an R factor of 20.4% (Rfree = 24.9%) at 3.3 Å. The protein adopts the α/β fold found in cysteine phosphatases, a superfamily of phosphatases that possess a catalytic cysteine and form a covalent thiol-phosphate intermediate during the catalytic cycle. In At1g05000, the analogous cysteine (Cys150) is located at the bottom of a positively-charged pocket formed by residues that include the conserved arginine (Arg156) of the signature active site motif, HCxxGxxRT. Of 74 model phosphatase substrates tested, purified recombinant At1g05000 showed highest activity toward polyphosphate (poly-P12–13) and deoxyribo- and ribonucleoside triphosphates, and less activity toward phosphoenolpyruvate, phosphotyrosine, phosphotyrosine-containing peptides, and phosphatidyl inositols. Divalent metal cations were not required for activity and had little effect on the reaction.
phosphatase; Arabidopsis; polyphosphate; At1g05000; structure; crystallography
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) and the associated proteins (Cas) comprise a system of adaptive immunity against viruses and plasmids in prokaryotes. Cas1 is a CRISPR-associated protein that is common to all CRISPR-containing prokaryotes but its function remains obscure. Here we show that the purified Cas1 protein of Escherichia coli (YgbT) exhibits nuclease activity against single-stranded and branched DNAs including Holliday junctions, replication forks, and 5′-flaps. The crystal structure of YgbT and site-directed mutagenesis have revealed the potential active site. Genome-wide screens show that YgbT physically and genetically interacts with key components of DNA repair systems, including recB, recC and ruvB. Consistent with these findings, the ygbT deletion strain showed increased sensitivity to DNA damage and impaired chromosomal segregation. Similar phenotypes were observed in strains with deletion of CRISPR clusters, suggesting that the function of YgbT in repair involves interaction with the CRISPRs. These results show that YgbT belongs to a novel, structurally distinct family of nucleases acting on branched DNAs and suggest that, in addition to antiviral immunity, at least some components of the CRISPR-Cas system have a function in DNA repair.
Cas1; CRISPR; DNA recombination; DNA repair; nuclease; YgbT
The purified putative betaine aldehyde dehydrogenase SACOL2628 from the early methicillin-resistant S. aureus COL has betaine aldehyde dehydrogenase activity and is structurally similar to aldehyde dehydrogenases.
When exposed to high osmolarity, methicillin-resistant Staphylococcus aureus (MRSA) restores its growth and establishes a new steady state by accumulating the osmoprotectant metabolite betaine. Effective osmoregulation has also been implicated in the acquirement of a profound antibiotic resistance by MRSA. Betaine can be obtained from the bacterial habitat or produced intracellularly from choline via the toxic betaine aldehyde (BA) employing the choline dehydrogenase and betaine aldehyde dehydrogenase (BADH) enzymes. Here, it is shown that the putative betaine aldehyde dehydrogenase SACOL2628 from the early MRSA isolate COL (SaBADH) utilizes betaine aldehyde as the primary substrate and nicotinamide adenine dinucleotide (NAD+) as the cofactor. Surface plasmon resonance experiments revealed that the affinity of NAD+, NADH and BA for SaBADH is affected by temperature, pH and buffer composition. Five crystal structures of the wild type and three structures of the Gly234Ser mutant of SaBADH in the apo and holo forms provide details of the molecular mechanisms of activity and substrate specificity/inhibition of this enzyme.
betaine aldehyde dehydrogenase; Staphylococcus aureus; structural genomics; high-throughput approach; infectious diseases
Clustered regularly interspaced short palindromic repeats (CRISPRs) and their associated Cas proteins comprise a prokaryotic RNA-guided adaptive immune system that interferes with mobile genetic elements, such as plasmids and phages. The type I-E CRISPR interference complex Cascade from Escherichia coli is composed of five different Cas proteins and a 61-nt-long guide RNA (crRNA). crRNAs contain a unique 32-nt spacer flanked by a repeat-derived 5′ handle (8 nt) and a 3′ handle (21 nt). The spacer part of crRNA directs Cascade to DNA targets. Here, we show that the E. coli Cascade can be expressed and purified from cells lacking crRNAs and loaded in vitro with synthetic crRNAs, which direct it to targets complementary to crRNA spacer. The deletion of even one nucleotide from the crRNA 5′ handle disrupted its binding to Cascade and target DNA recognition. In contrast, crRNA variants with just a single nucleotide downstream of the spacer part bound Cascade and the resulting ribonucleotide complex containing a 41-nt-long crRNA specifically recognized DNA targets. Thus, the E. coli Cascade-crRNA system exhibits significant flexibility suggesting that this complex can be engineered for applications in genome editing and opening the way for incorporation of site-specific labels in crRNA.
Cas4 proteins, a core protein family associated with the microbial system of adaptive immunity CRISPR, are predicted to function in the adaptation step of the CRISPR mechanism. Here we show that the Cas4 protein SSO0001 from the archaeon Sulfolobus solfataricus has metal-dependent endonuclease and 5' to 3' exonuclease activities against single-stranded DNA, as well as ATP-independent DNA unwinding activity toward double-stranded DNA. The crystal structure of SSO0001 revealed a decameric toroid formed by five dimers with each protomer containing one [4Fe-4S] cluster and one Mn2+ ion bound in the active site located inside the internal tunnel. The conserved RecB motif and four Cys residues are important for DNA binding and cleavage activities, whereas DNA unwinding depends on several residues located near the [4Fe-4S]-cluster. Our results suggest that Cas4 proteins might contribute to the addition of novel CRISPR spacers through the formation of 3'-DNA overhangs and to the degradation of foreign DNA.
CRISPR interference; Cas4; exonuclease; RecB motif; [4Fe-4S] cluster
Cas4 nucleases constitute a core family of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) associated proteins, but little is known about their structure and activity. Here we report the crystal structure of the Cas4 protein Pcal_0546 from Pyrobaculum calidifontis, which revealed a monomeric protein with a RecB-like fold and one [2Fe-2S] cluster coordinated by four conserved Cys residues. Pcal_0546 exhibits metal-dependent 5′ to 3′ exonuclease activity against ssDNA substrates, whereas the Cas4 protein SSO1391 from Sulfolobus solfataricus can cleave ssDNA in both the 5′ to 3′ and 3′ to 5′ directions. The active site of Pcal_0546 contains a bound metal ion coordinated by the side chains of Asp123, Glu136, His146, and the main chain carbonyl of Ile137. Site-directed mutagenesis of Pcal_0546 and SSO1391 revealed that the residues of RecB motifs II, III and QhXXY are critical for nuclease activity, whereas mutations of the conserved Cys residues resulted in a loss of the iron-sulfur cluster, but had no effect on DNA cleavage. Our results revealed the biochemical diversity of Cas4 nucleases, which can have different oligomeric states, contain [4Fe-4S] or [2Fe-2S] clusters, and cleave single stranded DNA in different directions producing single-stranded DNA overhangs, which are potential intermediates for the synthesis of new CRISPR spacers.
The uncharacterized α/β-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 ofOLEI01171 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.
anion activation; carboxyl esterase; cold-active enzyme; crystal structure; Oleispira antarctica; protein thermostability
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 signaling, 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 (1.70 and 1.75 Å) and revealed a hotdog fold with a potential catalytic carboxylate residue located on the long alpha helix (Asp57 in PA5202 and Glu35 in PA2801). Alanine replacement mutagenesis of PA5202 identified four residues (Asn42, Arg43, Asp57, and Thr76), which are critical for 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 the PA5202 activity and pyocyanin production. Thus, the P. aeruginosa hotdog thioesterases PA5202 and PA2801 have similar structures, but exhibit different substrate preferences and functions.
hotdog fold; thioesterase; crystal structure; pyocyanin; Pseudomonas aeruginosa
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.
•Maf proteins represent a family of nucleoside triphosphate pyrophosphatases•Maf proteins hydrolyze the canonical nucleotides dTTP, UTP, and CTP•Maf proteins are also active against m5UTP, m5CTP, pseudo-UTP, and m7GTP•Maf structures reveal the molecular mechanisms of their substrate selectivity
Tchigvintsev et al. show that Maf proteins are a family of nucleotide pyrophosphatases active against both canonical and modified nucleotides. This suggests that Mafs might have a dual role in cell division and in the prevention of the incorporation of modified nucleotides into cellular nucleic acids.
Multicopper oxidases can act on a broad spectrum of phenolic and non-phenolic compounds. These enzymes include laccases, which are widely distributed in plants and fungi, and were more recently identified in bacteria. Here, we present the results of biochemical and mutational studies of small laccase (SLAC), a multicopper oxidase from Streptomyces coelicolor (SCO6712). In addition to typical laccase substrates, SLAC was tested using phenolic compounds that exhibit antioxidant activity. SLAC showed oxidase activity against 12 of 23 substrates tested, including caffeic acid, ferulic acid, resveratrol, quercetin, morin, kaempferol and myricetin. The kinetic parameters of SLAC were determined for 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid), 2,6-dimethoxyphenol, quercetin, morin and myricetin, and maximum reaction rates were observed with myricetin, where kcat and Km values at 60°C were 8.1 (± 0.8) s−1 and 0.9 (± 0.3) mM respectively. SLAC had a broad pH optimum for activity (between pH 4 and 8) and temperature optimum at 60–70°C. It demonstrated remarkable thermostability with a half-life of over 10 h at 80°C and over 7 h at 90°C. Site-directed mutagenesis revealed 17 amino acid residues important for SLAC activity including the 10 His residues involved in copper coordination. Most notably, the Y229A and Y230A mutant proteins showed over 10-fold increase in activity compared with the wild-type SLAC, which was correlated to higher copper incorporation, while kinetic analyses with S929A predicts localization of this residue near the meta-position of aromatic substrates.
Funding Information Funding for this research was provided by the Government of Ontario for the project ‘FFABnet: Functionalized Fibre and Biochemicals’ (ORF-RE-05-005), and the Natural Sciences and Engineering Research Council of Canada.
The esterases and lipases from the α/β 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.
Metabolomics, genetics and biochemistry were combined to obtain the first complete map of the nucleotide degradation and ribose salvage pathway in yeast. This pathway promotes yeast survival in starvation and oxidative stress.
During carbon starvation, ribose salvage from nucleotides promotes yeast survival.The salvage pathway requires the previously misannotated nucleotidase Phm8.Ribose-derived carbon accumulates as sedoheptulose-7-phosphate.This carbon reserve enables rapid NADPH production in oxidative stress.
Nucleotide degradation is a universal metabolic capability. Here we combine metabolomics, genetics and biochemistry to characterize the yeast pathway. Nutrient starvation, via PKA, AMPK/SNF1, and TOR, triggers autophagic breakdown of ribosomes into nucleotides. A protein not previously associated with nucleotide degradation, Phm8, converts nucleotide monophosphates into nucleosides. Downstream steps, which involve the purine nucleoside phosphorylase, Pnp1, and pyrimidine nucleoside hydrolase, Urh1, funnel ribose into the nonoxidative pentose phosphate pathway. During carbon starvation, the ribose-derived carbon accumulates as sedoheptulose-7-phosphate, whose consumption by transaldolase is impaired due to depletion of transaldolase's other substrate, glyceraldehyde-3-phosphate. Oxidative stress increases glyceraldehyde-3-phosphate, resulting in rapid consumption of sedoheptulose-7-phosphate to make NADPH for antioxidant defense. Ablation of Phm8 or double deletion of Pnp1 and Urh1 prevent effective nucleotide salvage, resulting in metabolite depletion and impaired survival of starving yeast. Thus, ribose salvage provides means of surviving nutrient starvation and oxidative stress.
autophagy; mass spectrometry; metabolism; nutrient starvation; Saccharomyces cerevisiae
The CRISPR–Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated proteins) modules are adaptive immunity systems that are present in many archaea and bacteria. These defence systems are encoded by operons that have an extraordinarily diverse architecture and a high rate of evolution for both the cas genes and the unique spacer content. Here, we provide an updated analysis of the evolutionary relationships between CRISPR–Cas systems and Cas proteins. Three major types of CRISPR–Cas system are delineated, with a further division into several subtypes and a few chimeric variants. Given the complexity of the genomic architectures and the extremely dynamic evolution of the CRISPR–Cas systems, a unified classification of these systems should be based on multiple criteria. Accordingly, we propose a `polythetic' classification that integrates the phylogenies of the most common cas genes, the sequence and organization of the CRISPR repeats and the architecture of the CRISPR–cas loci.
Gluconeogenesis converts three carbon units into glucose. Here we identify an analogous pathway in Saccharomyces cerevisiae for converting three carbon units into ribose, a component of nucleic acids and nucleotides. This riboneogenic pathway involves the enzyme sedoheptulose-1,7-bisphosphatase (SHB17), whose activity was identified based on accumulation of sedoheptulose-1,7-bisphosphate in the corresponding knockout strain. We determined the crystal structure of Shb17 in complex with sedoheptulose-1,7-bisphosphate, and found that the sugar is bound in the closed furan form in the active site. Like fructose-1,6-bisphosphate, sedoheptulose-1,7-bisphosphate is produced by aldolase, in this case from erythrose 4-phosphate and dihydroxyacetone phosphate. Hydrolysis of sedoheptulose-1,7-bisphosphate by SHB17 provides an energetically favorable input to the non-oxidative pentose phosphate pathway to drive ribose production. Flux through SHB17 is enhanced under conditions when ribose demand is high relative to demand for NADPH, including during ribosome biogenesis in metabolically synchronized yeast cells. Thus, riboneogenesis provides a thermodynamically-driven route of ribose production uncoupled from formation of NADPH.
The carbon-fluorine bond is the strongest covalent bond in organic chemistry, yet fluoroacetate dehalogenases can readily hydrolyze this bond under mild physiological conditions. Elucidating the molecular basis of this rare biocatalytic activity will provide the fundamental chemical insights of how this formidable feat is achieved. Here, we present a series of high-resolution (1.15–1.80 Å) crystal structures of a fluoroacetate dehalogenase, capturing snapshots along the defluorination reaction: the free enzyme, enzyme-fluoroacetate Michaelis complex, glycolyl-enzyme covalent intermediate and enzyme-product complex. We demonstrate that enzymatic defluorination requires a halide pocket that not only supplies three hydrogen bonds to stabilize the fluoride ion, but is also finely tailored for the smaller fluorine halogen atom to establish selectivity towards fluorinated substrates. We have further uncovered dynamics near the active site which may play pivotal roles in enzymatic defluorination. These findings may ultimately lead to the development of novel defluorinases that will enable the biotransformation of more complex fluorinated organic compounds, which in turn will assist the synthesis, detoxification, biodegradation, disposal, recycling and regulatory strategies for the growing markets of organofluorines across major industrial sectors.
Cyclic diguanylate (c-di-GMP) is a ubiquitous second messenger regulating 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 TDB1265 EAL domain was determined in a free state (1.8 Å) and in complex with c-di-GMP (2.35 Å) and unveiled the role of the 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 the 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.
EAL domain; cyclic di-GMP; phosphodiesterase; X-ray crystallography; Thiobacillus denitrificans
Type II chromosomal toxin-antitoxin (TA) modules consist of a pair of genes that encode two components: a stable toxin and a labile antitoxin interfering with the lethal action of the toxin through protein complex formation. Bioinformatic analysis of Streptococcus mutans UA159 genome identified a pair of linked genes encoding a MazEF-like TA. Our results show that S. mutans mazEF genes form a bicistronic operon that is cotranscribed from a σ70-like promoter. Overproduction of S. mutans MazF toxin had a toxic effect on S. mutans which can be neutralized by coexpression of its cognate antitoxin, S. mutans MazE. Although mazF expression inhibited cell growth, no cell lysis of S. mutans cultures was observed under the conditions tested. The MazEF TA is also functional in E. coli, where S. mutans MazF did not kill the cells but rather caused reversible cell growth arrest. Recombinant S. mutans MazE and MazF proteins were purified and were shown to interact with each other in vivo, confirming the nature of this TA as a type II addiction system. Our data indicate that MazF is a toxic nuclease arresting cell growth through the mechanism of RNA cleavage and that MazE inhibits the RNase activity of MazF by forming a complex. Our results suggest that the MazEF TA module might represent a cell growth modulator facilitating the persistence of S. mutans under the harsh conditions of the oral cavity.
One hundred and seventy‐one genes encoding potential esterases from 11 bacterial genomes were cloned and overexpressed in Escherichia coli; 74 of the clones produced soluble proteins. All 74 soluble proteins were purified and screened for esterase activity; 36 proteins showed carboxyl esterase activity on short‐chain esters, 17 demonstrated arylesterase activity, while 38 proteins did not exhibit any activity towards the test substrates. Esterases from Rhodopseudomonas palustris (RpEST‐1, RpEST‐2 and RpEST‐3), Pseudomonas putida (PpEST‐1, PpEST‐2 and PpEST‐3), Pseudomonas aeruginosa (PaEST‐1) and Streptomyces avermitilis (SavEST‐1) were selected for detailed biochemical characterization. All of the enzymes showed optimal activity at neutral or alkaline pH, and the half‐life of each enzyme at 50°C ranged from < 5 min to over 5 h. PpEST‐3, RpEST‐1 and RpEST‐2 demonstrated the highest specific activity with pNP‐esters; these enzymes were also among the most stable at 50°C and in the presence of detergents, polar and non‐polar organic solvents, and imidazolium ionic liquids. Accordingly, these enzymes are particularly interesting targets for subsequent application trials. Finally, biochemical and bioinformatic analyses were compared to reveal sequence features that could be correlated to enzymes with arylesterase activity, facilitating subsequent searches for new esterases in microbial genome sequences.
The genome of the extremely radiation resistant bacterium Deinococcus radiodurans encodes 21 Nudix hydrolases of which only two have been characterized in detail. Here we report the activity and crystal structure for DR_0079, the first Nudix hydrolase observed to have a marked preference for cytosine ribonucleoside 5’-diphosphate (CDP) and cytosine ribonucleoside 5’-triphosphate (CTP). After CDP and CTP the next most preferred substrates for DR_0079, with a relative activity of < 50%, were the corresponding deoxyribose nucleotides, dCDP and dCTP. Hydrolase activity at the site of the phosphodiester bond was corroborated using 31P NMR spectroscopy to follow the phosphorus resonances for three substrates, CDP, IDP, and CTP, and their respective hydrolysis products, CMP + Pi, IMP + Pi, and CMP + PPi. Nucleophilic substitution at the β-phosphorus of CDP and CTP was established, using 31P NMR spectroscopy, by the appearance of an upfield shifted Pi resonance and line-broadened PPi resonance, respectively, when performing the hydrolysis in 40% H218O enriched water. Optimum activity for CDP was at pH 9.0 – 9.5 with the reaction requiring divalent metal cation (Mg2+ > Mn2+ > Co2+). The biochemical data is discussed with reference to the crystal structure for DR_0079 that was determined in the metal-free form at 1.9 Å resolution. The protein contains nine β-strands, three α-helices, and two 310-helices organized into three subdomains; an N-terminal β-sheet, a central Nudix core, and a C-terminal helix-turn-helix motif. As observed for all known structures of Nudix hydrolases, the α-helix of the ‘Nudix box’ is one of two helices that sandwich a ‘four-strand’ mixed β-sheet. To identify residues potentially involved in metal and substrate binding, NMR chemical shift mapping experiments were performed on 15N-labelled DR_0079 with the paramagnetic divalent cation Co2+ and the non-hydrolyzable substrate thymidine-5’-O-(α,β-methylenediphosphate) and the results mapped onto the crystal structure.
chemical shift mapping; phosphorus-31 NMR; function screening; Nudix hydrolase; cytidine 5`diphosphate
Dehalogenases are environmentally important enzymes that detoxify organohalogens by cleaving their carbon‐halogen bonds. Many microbial genomes harbour enzyme families containing dehalogenases, but a sequence‐based identification of genuine dehalogenases with high confidence is challenging because of the low sequence conservation among these enzymes. Furthermore, these protein families harbour a rich diversity of other enzymes including esterases and phosphatases. Reliable sequence determinants are necessary to harness genome sequencing‐efforts for accelerating the discovery of novel dehalogenases with improved or modified activities. In an attempt to extract dehalogenase sequence fingerprints, 103 uncharacterized potential dehalogenase candidates belonging to the α/β hydrolase (ABH) and haloacid dehalogenase‐like hydrolase (HAD) superfamilies were screened for dehalogenase, esterase and phosphatase activity. In this first biochemical screen, 1 haloalkane dehalogenase, 1 fluoroacetate dehalogenase and 5 l‐2‐haloacid dehalogenases were found (success rate 7%), as well as 19 esterases and 31 phosphatases. Using this functional data, we refined the sequence‐based dehalogenase selection criteria and applied them to a second functional screen, which identified novel dehalogenase activity in 13 out of only 24 proteins (54%), increasing the success rate eightfold. Four new l‐2‐haloacid dehalogenases from the HAD superfamily were found to hydrolyse fluoroacetate, an activity never previously ascribed to enzymes in this superfamily.