The difficult road from sequence to function

Ethylphenol Formation by Lactobacillus plantarum: Identification of the Enzyme Involved in the Reduction of Vinylphenols

Ethylphenols are strong odorants produced by microbial activity that are described as off flavors in several foods. Lactobacillus plantarum is a lactic acid bacterial species able to produce ethylphenols by the reduction of vinylphenols during the metabolism of hydroxycinnamic acids. However, the reductase involved has not been yet uncovered. In this study, the involvement in vinylphenol reduction of a gene encoding a putative reductase (lp_3125) was confirmed by the absence of reduction activity in the Δlp_3125 knockout mutant. The protein encoded by lp_3125, VprA, was recombinantly produced in Escherichia coli VprA was assayed against vinylphenols (4-vinylphenol, 4-vinylcatechol, and 4-vinylguaiacol), and all were reduced to their corresponding ethylphenols (4-ethylphenol, 4-ethylcatechol, and 4-ethylguaiacol). PCR and high-performance liquid chromatography (HPLC) detection methods revealed that the VprA reductase is not widely distributed among the lactic acid bacteria studied and that only the bacteria possessing the vprA gene were able to produce ethylphenol from vinylphenol. However, all the species belonging to the L. plantarum group were ethylphenol producers. The identification of the L. plantarum VprA protein involved in hydroxycinnamate degradation completes the route of degradation of these compounds in lactic acid bacteria.IMPORTANCE The presence of volatile phenols is considered a major organoleptic defect of several fermented alcoholic beverages. The biosynthesis of these compounds has been mainly associated with Brettanomyces/Dekkera yeasts. However, the potential importance of lactic acid bacteria in volatile phenol spoilage is emphasized by reports describing a faster ethylphenol production by these bacteria than by yeasts. The genetic identification of the bacterial vinylphenol reductase involved in volatile phenol production provides new insights into the role of lactic acid bacteria in the production of these off flavors. The development of a molecular method for the detection of ethylphenol-producing bacteria could be helpful to design strategies to reduce the bacterial production of vinylphenols in fermented foods.

Protein classification with Extended-Sequence Coding by sliding window

A large number of unclassified sequences is still found in public databases, which suggests that there is still need for new investigations in the area. In this contribution, we present a methodology based on Artificial Neural Networks for protein functional classification. A new protein coding scheme, called here Extended-Sequence Coding by Sliding Windows, is presented with the goal of overcoming some of the difficulties of the well method Sequence Coding by Sliding Window. The new protein coding scheme uses more than one sliding window length with a weight factor that is proportional to the window length, avoiding the ambiguity problem without ignoring the identity of small subsequences Accuracy for Sequence Coding by Sliding Windows ranged from 60.1 to 77.7 percent for the first bacterium protein set and from 61.9 to 76.7 percent for the second one, whereas the accuracy for the proposed Extended-Sequence Coding by Sliding Windows scheme ranged from 70.7 to 97.1 percent for the first bacterium protein set and from 61.1 to 93.3 percent for the second one. Additionally, protein sequences classified inconsistently by the Artificial Neural Networks were analyzed by CD-Search revealing that there are some disagreement in public repositories, calling the attention for the relevant issue of error propagation in annotated databases due the incorrect transferred annotations.

A new class of adenylate kinase in methanogens is related to uridylate kinase

The protein derived from the Methanocaldococcus jannaschii MJ0458 gene is annotated as a δ-1-pyrroline 5-carboxylate synthetase and is predicted to be related to aspartokinase and uridylate kinase. Analysis of the predicted protein sequence indicated that it is a unique kinase with few similarities to either uridylate or adenylate kinase. Here, we report that the MJ0458 gene product is a second type of archaeal adenylate kinase, AdkB. This enzyme is different from the established archaeal-specific adenylate kinase in both sequence and predicted tertiary structure.

Substrate specificity of the Bacillus licheniformis lyxose isomerase YdaE and its application in in vitro catalysis for bioproduction of lyxose and glucose by two-step isomerization

Enzymatic processes are useful for industrially important sugar production, and in vitro two-step isomerization has proven to be an efficient process in utilizing readily available sugar sources. A hypothetical uncharacterized protein encoded by ydaE of Bacillus licheniformis was found to have broad substrate specificities and has shown high catalytic efficiency on D-lyxose, suggesting that the enzyme is D-lyxose isomerase. Escherichia coli BL21 expressing the recombinant protein, of 19.5 kDa, showed higher activity at 40 to 45°C and pH 7.5 to 8.0 in the presence of 1.0 mM Mn²+. The apparent K(m) values for D-lyxose and D-mannose were 30.4 ± 0.7 mM and 26 ± 0.8 mM, respectively. The catalytic efficiency (k(cat)/K(m)) for lyxose (3.2 ± 0.1 mM⁻¹ s⁻¹) was higher than that for D-mannose (1.6 mM⁻¹ s⁻¹). The purified protein was applied to the bioproduction of D-lyxose and D-glucose from d-xylose and D-mannose, respectively, along with the thermostable xylose isomerase of Thermus thermophilus HB08. From an initial concentration of 10 mM D-lyxose and D-mannose, 3.7 mM and 3.8 mM D-lyxose and D-glucose, respectively, were produced by two-step isomerization. This two-step isomerization is an easy method for in vitro catalysis and can be applied to industrial production.

Application of NMR and molecular docking in structure-based drug discovery

Drug discovery is a complex and costly endeavor, where few drugs that reach the clinical testing phase make it to market. High-throughput screening (HTS) is the primary method used by the pharmaceutical industry to identify initial lead compounds. Unfortunately, HTS has a high failure rate and is not particularly efficient at identifying viable drug leads. These shortcomings have encouraged the development of alternative methods to drive the drug discovery process. Specifically, nuclear magnetic resonance (NMR) spectroscopy and molecular docking are routinely being employed as important components of drug discovery research. Molecular docking provides an extremely rapid way to evaluate likely binders from a large chemical library with minimal cost. NMR ligand-affinity screens can directly detect a protein-ligand interaction, can measure a corresponding dissociation constant, and can reliably identify the ligand binding site and generate a co-structure. Furthermore, NMR ligand affinity screens and molecular docking are perfectly complementary techniques, where the combination of the two has the potential to improve the efficiency and success rate of drug discovery. This review will highlight the use of NMR ligand affinity screens and molecular docking in drug discovery and describe recent examples where the two techniques were combined to identify new and effective therapeutic drugs.

Nature versus nurture in two highly enantioselective esterases from Bacillus cereus and Thermoanaerobacter tengcongensis

There is an increasing need for the use of biocatalysis to obtain enantiopure compounds as chiral building blocks for drug synthesis such as antibiotics. The principal findings of this study are: (i) the complete sequenced genomes of Bacillus cereus ATCC 14579 and Thermoanaerobacter tengcongensis MB4 contain a hitherto undescribed enantioselective and alkaliphilic esterase (BcEST and TtEST respectively) that is specific for the production of (R)‐2‐benzyloxy‐propionic acid ethyl ester, a key intermediate in the synthesis of levofloxacin, a potent antibiotic; and (ii) directed evolution targeted for increased thermostability of BcEST produced two improved variants, but in either case the 3–5°C increase in the apparent melting temperature (T_m) of the mutants over the native BcEST that has a T_m of 50°C was outperformed by TtEST, a naturally occurring homologue with a T_m of 65°C. Protein modelling of BcEST mapped the S148C and K272R mutations at protein surface and the I88T and Q110L mutations at more buried locations. This work expands the repertoire of characterized members of the α/β‐fold hydrolase superfamily. Further, it shows that genome mining is an economical option for new biocatalyst discovery and we provide a rare example of a naturally occurring thermostable biocatalyst that outperforms experimentally evolved homologues that carry out the same hydrolysis.

Investigative mining of sequence data for novel enzymes: a case study with nitrilases

Mining sequence data is increasingly important for biocatalysis research. However, when relying on sequence data alone, prediction of the reaction catalyzed by a specific protein sequence is often elusive, and substrate specificity is far from trivial. The present study demonstrated an approach of combining sequence data and structures from distant homologs to target identification of new nitrilases that specifically utilize hindered nitrile substrates like mandelonitrile. A total of 212 non-identical target nitrilases were identified from GenBank. Evolutionary trace and sequence clustering methods were used combinatorily to identify a set of nitrilases with presumably distinct substrate specificities. Selected encoding genes were cloned into Escherichia coli. Recombinant E. coli expressing NitA (gi91784632) from Burkholderia xenovorans LB400 was capable of growth on glutaronitrile or adiponitrile as the sole nitrogen source. Purified NitA exhibited highest activity with mandelonitrile, showing a catalytic efficiency (k(cat)/K(m)) of 3.6 x 10(4)M(-1)s(-1). A second nitrilase predicted from our studies from Bradyrhizobium zaponicum USDA 110 (gi27381513) was likewise shown to prefer mandelonitrile [Zhu, D., Mukherjee, C., Biehl, E.R., Hua, L., 2007. Discovery of a mandelonitrile hydrolase from Bradyrhizobium japonicum USDA110 by rational genome mining. J. Biotechnol. 129 (4), 645-650]. Thus, predictions from sequence analysis and distant superfamily structures yielded enzyme activities with high selectivity for mandelonitrile. These data suggest that similar data mining techniques can be used to identify other substrate-specific enzymes from published, unannotated sequences.

The ATPase activity of an 'essential' Bacillus subtilis enzyme, YdiB, is required for its cellular function and is modulated by oligomerization

Characterization of 'unknown' proteins is one of the challenges of the post-genomic era. Here, we report a study of Bacillus subtilis YdiB, which belongs to an uncharted class of bacterial P-loop ATPases. Precise deletion of the ydiB gene yielded a mutant with much reduced growth rate compared to the wild-type strain. In vitro, purified YdiB was in equilibrium among different forms, monomers, dimers and oligomers, and this equilibrium was strongly affected by salts; high concentrations of NaCl favoured the monomeric over the oligomeric form of the enzyme. Interestingly, the ATPase activity of the monomer was about three times higher than that of the oligomer, and the monomer showed a K(m) of about 60 microM for ATP and a V(max) of about 10 nmol min(-1) (mg protein)(-1) (k(cat) approximately 10 h(-1)). This low ATPase activity was shown to be specific to YdiB because mutation of an invariant lysine residue in the P-loop motif (K41A) strongly attenuated this rate. This mutant was unable to restore a normal growth phenotype when introduced into a conditional knockout strain for ydiB, showing that the ATPase activity of YdiB is required for the in vivo function of the protein. Oligomerization was also observed with the purified YjeE from Escherichia coli, a YdiB orthologue, suggesting that this property is shared by all members of this family of ATPases. Importantly, dimers of YdiB were also observed in a B. subtilis extract, or when stabilized by formaldehyde cross-linking for YjeE from E. coli, suggesting that oligomerization might regulate the function of this new class of proteins in vivo.

Metabolic capacity of Bacillus cereus strains ATCC 14579 and ATCC 10987 interlinked with comparative genomics

Bacillus cereus is an important food-borne pathogen and spoilage organism. In this study, numerous phenotypes and the genomes of B. cereus strains ATCC 14579 and ATCC 10987 were analysed to compare their metabolic capacity and stress resistance potential. The growth performance of the two strains was assessed for nearly 2000 phenotypes, including use of nutrient sources, performance in acid and basic environments, osmo-tolerance and antibiotic resistance. Several food-relevant phenotypic differences were found between ATCC 14579 and ATCC 10987, such as differences in utilization of carbohydrates, peptides, amino acids and ammonia. Subsequently, the genomes of both strains were analysed with INPARANOID to search for strain-specific open reading frames (ORFs). B. cereus ATCC 14579 and ATCC 10987 were found to harbour 983 and 1360 strain-specific ORFs respectively. The strain-specific phenotypic features were interlinked with corresponding genetic features and for several phenotypic differences a related strain-specific genetic feature could be identified. In conclusion, the combination of phenotypic data with strain-specific genomic differences has led to detailed insight into the performance of the two B. cereus strains, and may supply indicators for the performance of these bacteria in different environments and ecological niches.

Genome scale enzyme-metabolite and drug-target interaction predictions using the signature molecular descriptor

MOTIVATION

Identifying protein enzymatic or pharmacological activities are important areas of research in biology and chemistry. Biological and chemical databases are increasingly being populated with linkages between protein sequences and chemical structures. There is now sufficient information to apply machine-learning techniques to predict interactions between chemicals and proteins at a genome scale. Current machine-learning techniques use as input either protein sequences and structures or chemical information. We propose here a method to infer protein-chemical interactions using heterogeneous input consisting of both protein sequence and chemical information.

RESULTS

Our method relies on expressing proteins and chemicals with a common cheminformatics representation. We demonstrate our approach by predicting whether proteins can catalyze reactions not present in training sets. We also predict whether a given drug can bind a target, in the absence of prior binding information for that drug and target. Such predictions cannot be made with current machine-learning techniques requiring binding information for individual reactions or individual targets.

The archaeon Methanosarcina acetivorans contains a protein disulfide reductase with an iron-sulfur cluster

Methanosarcina acetivorans, a strictly anaerobic methane-producing species belonging to the domain Archaea, contains a gene cluster annotated with homologs encoding oxidative stress proteins. One of the genes (MA3736) is annotated as a gene encoding an uncharacterized carboxymuconolactone decarboxylase, an enzyme required for aerobic growth with aromatic compounds by species in the domain Bacteria. Methane-producing species are not known to utilize aromatic compounds, suggesting that MA3736 is incorrectly annotated. The product of MA3736, overproduced in Escherichia coli, had protein disulfide reductase activity dependent on a C(67)XXC(70) motif not found in carboxymuconolactone decarboxylase. We propose that MA3736 be renamed mdrA (methanosarcina disulfide reductase). Further, unlike carboxymuconolactone decarboxylase, MdrA contained an Fe-S cluster. Binding of the Fe-S cluster was dependent on essential cysteines C(67) and C(70), while cysteines C(39) and C(107) were not required. Loss of the Fe-S cluster resulted in conversion of MdrA from an inactive hexamer to a trimer with protein disulfide reductase activity. The data suggest that MdrA is the prototype of a previously unrecognized protein disulfide reductase family which contains an intermolecular Fe-S cluster that controls oligomerization as a mechanism to regulate protein disulfide reductase activity.

Multivariate-activity mining for molecular quasi-species in a glutathione transferase mutant library

A library of recombinant glutathione transferases (GSTs) generated by shuffling of DNA encoding human GST M1-1 and GST M2-2 was screened with eight alternative substrates, and the activities were subjected to multivariate analysis. Assays were made in lysates of bacteria in which the GST variants had been expressed. The primary data showed clustering of the activities in eight-dimensional substrate-activity space. For an incisive analysis, the rows of the data matrix, corresponding to the different enzyme variants, were individually scaled to unit length, thus accounting for different expression levels of the enzymes. The columns representing the activities with alternative substrates were subsequently individually normalized to unit variance and a zero mean. By this standardization, the data were adjusted to comparable orders of magnitude. Three molecular quasi-species were recognized by multivariate K-means and principal component analyses. Two of them encompassed the parental GST M1-1 and GST M2-2. A third one diverged functionally by displaying enhanced activities with some substrates and suppressed activities with signature substrates for GST M1-1 and GST M2-2. A fourth cluster contained mutants with impaired functions and was not regarded as a quasi-species. Sequence analysis of representatives of the mutant clusters demonstrated that the majority of the variants in the diverging novel quasi-species were structurally similar to the M1-like GSTs, but distinguished themselves from GST M1-1 by a Ser to Thr substitution in the active site. The data show that multivariate analysis of functional profiles can identify small structural changes influencing the evolution of enzymes with novel substrate-activity profiles.

Characterization of a novel D-lyxose isomerase from Cohnella laevoribosii RI-39 sp. nov

A newly isolated bacterium, Cohnella laevoribosii RI-39, could grow in a defined medium with L-ribose as the sole carbon source. A 21-kDa protein isomerizing L-ribose to L-ribulose, as well as D-lyxose to D-xylulose, was purified to homogeneity from this bacterium. Based on the N-terminal and internal amino acid sequences of the purified enzyme obtained by N-terminal sequencing and quantitative time of flight mass spectrometry-mass spectrometry analyses, a 549-bp gene (lyxA) encoding D-lyxose (L-ribose) isomerase was cloned and expressed in Escherichia coli. The purified endogenous enzyme and the recombinant enzyme formed homodimers that were activated by Mn(2+). C. laevoribosii D-lyxose (L-ribose) isomerase (CLLI) exhibits maximal activity at pH 6.5 and 70 degrees C in the presence of Mn(2+) for D-lyxose and L-ribose, and its isoelectric point (pI) is 4.2 (calculated pI, 4.9). The enzyme is specific for D-lyxose, L-ribose, and D-mannose, with apparent K(m) values of 22.4 +/- 1.5 mM, 121.7 +/- 10.8 mM, and 34.0 +/- 1.1 mM, respectively. The catalytic efficiencies (k(cat)/K(m)) of CLLI were 84.9 +/- 5.8 mM(-1) s(-1) for D-lyxose (V(max), 5,434.8 U mg(-1)), 0.2 mM(-1) s(-1) for L-ribose (V(max), 75.5 +/- 6.0 U mg(-1)), and 1.4 +/- 0.1 mM(-1) s(-1) for D-mannose (V(max), 131.8 +/- 7.4 U mg(-1)). The ability of lyxA to permit E. coli cells to grow on D-lyxose and L-ribose and homology searches of other sugar-related enzymes, as well as previously described sugar isomerases, suggest that CLLI is a novel type of rare sugar isomerase.

Characterization of Treponema denticola pyrF encoding orotidine-5'-monophosphate decarboxylase

The Treponema denticola ATCC 35405 genome annotation contains most of the genes for de novo pyrimidine biosynthesis. To initiate characterization of pyrimidine synthesis in Treponema, we focused on TDE2110 (the putative pyrF, encoding orotidine-5'-monophosphate decarboxlyase). Unlike the parent strain, an isogenic pyrF mutant was resistant to 5-fluoroorotic acid. In complex medium, growth of the pyrF mutant was independent of added uracil, indicating activity of a uracil uptake/salvage pathway. Transcription of pyrF was greatly reduced in T. denticola grown in excess uracil, demonstrating that de novo pyrimidine synthesis is regulated and suggesting a feedback mechanism. Treponema denticola PyrF complemented uracil auxotrophy in an Escherichia coli pyrF mutant. This study provides biochemical confirmation of T. denticola genome predictions of de novo and salvage pyrimidine pathways and provides proof of concept that pyrF has potential as a selectable marker in T. denticola.

High precision multi-genome scale reannotation of enzyme function by EFICAz

BACKGROUND

The functional annotation of most genes in newly sequenced genomes is inferred from similarity to previously characterized sequences, an annotation strategy that often leads to erroneous assignments. We have performed a reannotation of 245 genomes using an updated version of EFICAz, a highly precise method for enzyme function prediction.

RESULTS

Based on our three-field EC number predictions, we have obtained lower-bound estimates for the average enzyme content in Archaea (29%), Bacteria (30%) and Eukarya (18%). Most annotations added in KEGG from 2005 to 2006 agree with EFICAz predictions made in 2005. The coverage of EFICAz predictions is significantly higher than that of KEGG, especially for eukaryotes. Thousands of our novel predictions correspond to hypothetical proteins. We have identified a subset of 64 hypothetical proteins with low sequence identity to EFICAz training enzymes, whose biochemical functions have been recently characterized and find that in 96% (84%) of the cases we correctly identified their three-field (four-field) EC numbers. For two of the 64 hypothetical proteins: PA1167 from Pseudomonas aeruginosa, an alginate lyase (EC 4.2.2.3) and Rv1700 of Mycobacterium tuberculosis H37Rv, an ADP-ribose diphosphatase (EC 3.6.1.13), we have detected annotation lag of more than two years in databases. Two examples are presented where EFICAz predictions act as hypothesis generators for understanding the functional roles of hypothetical proteins: FLJ11151, a human protein overexpressed in cancer that EFICAz identifies as an endopolyphosphatase (EC 3.6.1.10), and MW0119, a protein of Staphylococcus aureus strain MW2 that we propose as candidate virulence factor based on its EFICAz predicted activity, sphingomyelin phosphodiesterase (EC 3.1.4.12).

CONCLUSION

Our results suggest that we have generated enzyme function annotations of high precision and recall. These predictions can be mined and correlated with other information sources to generate biologically significant hypotheses and can be useful for comparative genome analysis and automated metabolic pathway reconstruction.

Biosynthesis of phosphoserine in the Methanococcales

Methanococcus maripaludis and Methanocaldococcus jannaschii produce cysteine for protein synthesis using a tRNA-dependent pathway. These methanogens charge tRNA(Cys) with l-phosphoserine, which is also an intermediate in the predicted pathways for serine and cystathionine biosynthesis. To establish the mode of phosphoserine production in Methanococcales, cell extracts of M. maripaludis were shown to have phosphoglycerate dehydrogenase and phosphoserine aminotransferase activities. The heterologously expressed and purified phosphoglycerate dehydrogenase from M. maripaludis had enzymological properties similar to those of its bacterial homologs but was poorly inhibited by serine. While bacterial enzymes are inhibited by micromolar concentrations of serine bound to an allosteric site, the low sensitivity of the archaeal protein to serine is consistent with phosphoserine's position as a branch point in several pathways. A broad-specificity class V aspartate aminotransferase from M. jannaschii converted the phosphohydroxypyruvate product to phosphoserine. This enzyme catalyzed the transamination of aspartate, glutamate, phosphoserine, alanine, and cysteate. The M. maripaludis homolog complemented a serC mutation in the Escherichia coli phosphoserine aminotransferase. All methanogenic archaea apparently share this pathway, providing sufficient phosphoserine for the tRNA-dependent cysteine biosynthetic pathway.