Relationship of sequence and structure to specificity in the alpha-amylase family of enzymes

Crystal structure of barley limit dextrinase-limit dextrinase inhibitor (LD-LDI) complex reveals insights into mechanism and diversity of cereal type inhibitors

Molecular details underlying regulation of starch mobilization in cereal seed endosperm remain unknown despite the paramount role of this process in plant growth. The structure of the complex between the starch debranching enzyme barley limit dextrinase (LD), hydrolyzing α-1,6-glucosidic linkages, and its endogenous inhibitor (LDI) was solved at 2.7 Å. The structure reveals an entirely new and unexpected binding mode of LDI as compared with previously solved complex structures of related cereal type family inhibitors (CTIs) bound to glycoside hydrolases but is structurally analogous to binding of dual specificity CTIs to proteases. Site-directed mutagenesis establishes that a hydrophobic cluster flanked by ionic interactions in the protein-protein interface is vital for the picomolar affinity of LDI to LD as assessed by analysis of binding by using surface plasmon resonance and also supported by LDI inhibition of the enzyme activity. A phylogenetic analysis identified four LDI-like proteins in cereals among the 45 sequences from monocot databases that could be classified as unique CTI sequences. The unprecedented binding mechanism shown here for LDI has likely evolved in cereals from a need for effective inhibition of debranching enzymes having characteristic open active site architecture. The findings give a mechanistic rationale for the potency of LD activity regulation and provide a molecular understanding of the debranching events associated with optimal starch mobilization and utilization during germination. This study unveils a hitherto not recognized structural basis for the features endowing diversity to CTIs.

A review of starch-branching enzymes and their role in amylopectin biosynthesis

Starch-branching enzymes (SBEs) are one of the four major enzyme classes involved in starch biosynthesis in plants and algae, and their activities play a crucial role in determining the structure and physical properties of starch granules. SBEs generate α-1,6-branch linkages in α-glucans through cleavage of internal α-1,4 bonds and transfer of the released reducing ends to C-6 hydroxyls. Starch biosynthesis in plants and algae requires multiple isoforms of SBEs and is distinct from glycogen biosynthesis in both prokaryotes and eukaryotes which uses a single branching enzyme (BE) isoform. One of the unique characteristics of starch structure is the grouping of α-1,6-branch points in clusters within amylopectin. This is a feature of SBEs and their interplay with other starch biosynthetic enzymes, thus facilitating formation of the compact water-insoluble semicrystalline starch granule. In this respect, the activity of SBE isoforms is pivotal in starch granule assembly. SBEs are structurally related to the α-amylase superfamily of enzymes, sharing three domains of secondary structure with prokaryotic Bes: the central (β/α)8 -barrel catalytic domain, an NH2 -terminal domain involved in determining the size of α-glucan chain transferred, and the C-terminal domain responsible for catalytic capacity and substrate preference. In addition, SBEs have conserved plant-specific domains, including phosphorylation sites which are thought to be involved in regulating starch metabolism. SBEs form heteromeric protein complexes with other SBE isoforms as well as other enzymes involved in starch synthesis, and assembly of these protein complexes is regulated by protein phosphorylation. Phosphorylated SBEIIb is found in multienzyme complexes with isoforms of glucan-elongating starch synthases, and these protein complexes are implicated in amylopectin cluster formation. This review presents a comparative overview of plant SBEs and includes a review of their properties, structural and functional characteristics, and recent developments on their post-translational regulation.

Protein engineering of selected residues from conserved sequence regions of a novel Anoxybacillus α-amylase

The α-amylases from Anoxybacillus species (ASKA and ADTA), Bacillus aquimaris (BaqA) and Geobacillus thermoleovorans (GTA, Pizzo and GtamyII) were proposed as a novel group of the α-amylase family GH13. An ASKA yielding a high percentage of maltose upon its reaction on starch was chosen as a model to study the residues responsible for the biochemical properties. Four residues from conserved sequence regions (CSRs) were thus selected, and the mutants F113V (CSR-I), Y187F and L189I (CSR-II) and A161D (CSR-V) were characterised. Few changes in the optimum reaction temperature and pH were observed for all mutants. Whereas the Y187F (t1/2 43 h) and L189I (t1/2 36 h) mutants had a lower thermostability at 65°C than the native ASKA (t1/2 48 h), the mutants F113V and A161D exhibited an improved t1/2 of 51 h and 53 h, respectively. Among the mutants, only the A161D had a specific activity, k(cat) and k(cat)/K(m) higher (1.23-, 1.17- and 2.88-times, respectively) than the values determined for the ASKA. The replacement of the Ala-161 in the CSR-V with an aspartic acid also caused a significant reduction in the ratio of maltose formed. This finding suggests the Ala-161 may contribute to the high maltose production of the ASKA.

Comprehensive enzymatic analysis of the amylolytic system in the digestive fluid of the sea hare, Aplysia kurodai: Unique properties of two α-amylases and two α-glucosidases

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The amylolytic system of the digestive fluid of sea hare (Aplysia kurodai) was studied.

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Two α-amylases and two α-glucosidases were purified from the digestive fluid.

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Sea hare efficiently digests sea lettuce to glucose by a combination of these enzymes.

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Starch in sea lettuce is a predominant glucose source for sea hare.

Sea lettuce (Ulva pertusa) is a nuisance species of green algae that is found all over the world. East-Asian species of the marine gastropod, the sea hare Aplysia kurodai, shows a clear feeding preference for sea lettuce. Compared with cellulose, sea lettuce contains a higher amount of starch as a storage polysaccharide. However, the entire amylolytic system in the digestive fluid of A. kurodai has not been studied in detail. We purified α-amylases and α-glucosidases from the digestive fluid of A. kurodai and investigated the synergistic action of these enzymes on sea lettuce. A. kurodai contain two α-amylases (59 and 80 kDa) and two α-glucosidases (74 and 86 kDa). The 59-kDa α-amylase, but not the 80-kDa α-amylase, was markedly activated by Ca²⁺ or Cl⁻. Both α-amylases degraded starch and maltoheptaose, producing maltotriose, maltose, and glucose. Glucose production from starch was higher with 80-kDa α-amylase than with 59-kDa α-amylase. Kinetic analysis indicated that 74-kDa α-glucosidase prefers short α-1,4-linked oligosaccharide, whereas 86-kDa α-glucosidase prefers large α-1,6 and α-1,4-linked polysaccharides such as glycogen. When sea lettuce was used as a substrate, a 2-fold greater amount of glucose was released by treatment with 59-kDa α-amylase and 74-kDa α-glucosidase than by treatment with 45-kDa cellulase and 210-kDa β-glucosidase of A. kurodai. Unlike mammals, sea hares efficiently digest sea lettuce to glucose by a combination of two α-amylases and two α-glucosidases in the digestive fluids without membrane-bound maltase–glucoamylase and sucrase–isomaltase complexes.

Structural features underlying the selective cleavage of a novel exo-type maltose-forming amylase fromPyrococcussp. ST04

A novel maltose-forming α-amylase (PSMA) was recently found in the hyperthermophilic archaeonPyrococcussp. ST04. This enzyme shows <13% amino-acid sequence identity to other known α-amylases and displays a unique enzymatic property in that it hydrolyzes both α-1,4-glucosidic and α-1,6-glucosidic linkages of substrates, recognizing only maltose units, in an exo-type manner. Here, the crystal structure of PSMA at a resolution of 1.8 Å is reported, showing a tight ring-shaped tetramer with monomers composed of two domains: an N-domain (amino acids 1–341) with a typical GH57 family (β/α)7-barrel fold and a C-domain (amino acids 342–597) composed of α-helical bundles. A small closed cavity observed in proximity to the catalytic residues Glu153 and Asp253 at the domain interface has the appropriate volume and geometry to bind a maltose unit, accounting for the selective exo-type maltose hydrolysis of the enzyme. A narrow gate at the putative subsite +1 formed by residue Phe218 and Phe452 is essential for specific cleavage of glucosidic bonds. The closed cavity at the active site is connected to a short substrate-binding channel that extends to the central hole of the tetramer, exhibiting a geometry that is significantly different from classical maltogenic amylases or β-amylases. The structural features of this novel exo-type maltose-forming α-amylase provide a molecular basis for its unique enzymatic characteristics and for its potential use in industrial applications and protein engineering.

Modeling of a new tubercular maltosyl transferase, GlgE, study of its binding sites and virtual screening

Recently maltosyl transferase of Mycobacterium tuberculosis (mtb GlgE) belonging to α-amylase family has been identified as a potential drug target. Despite its importance, its three dimensional (3D) structure is unavailable. In this study we have modeled its 3D homo-dimeric structure using its homologue in Streptomyces ceolicolor (stp GlgE) as the template. Its monomer consists of five domains and four inserts, out of which two inserts are unique to mtb GlgE. It also has three binding cavities. One primary (pbs) and two secondary (sbs1 and sbs2), with one unique insert appearing within sbs2. Investigation of its homo-dimeric model revealed the presence of a disulphide bridge between Cys-29 of both the chains which is absent in stp GlgE. Virtual screening with known substrates and substrate analogues of α-amylase family proteins indicated better binding of maltose to sbs1 than pbs. Among all computationally screened substrates 3-O-Alpha-D-Glucopyranosyl-D-Fructose (OTU) docked with best binding affinity to pbs. Interaction of known inhibitors of α-amylase family proteins from CHEMBL is also studied. This reveals for the first time the unique 3D structure of mtb GlgE and provides insights into its active sites and substrate binding affinities. This may help in developing new anti-tubercular drugs and its analogues.

Structural insight into how Streptomyces coelicolor maltosyl transferase GlgE binds α-maltose 1-phosphate and forms a maltosyl-enzyme intermediate

GlgE (EC 2.4.99.16) is an α-maltose 1-phosphate:(1→4)-α-d-glucan 4-α-d-maltosyltransferase of the CAZy glycoside hydrolase 13_3 family. It is the defining enzyme of a bacterial α-glucan biosynthetic pathway and is a genetically validated anti-tuberculosis target. It catalyzes the α-retaining transfer of maltosyl units from α-maltose 1-phosphate to maltooligosaccharides and is predicted to use a double-displacement mechanism. Evidence of this mechanism was obtained using a combination of site-directed mutagenesis of Streptomyces coelicolor GlgE isoform I, substrate analogues, protein crystallography, and mass spectrometry. The X-ray structures of α-maltose 1-phosphate bound to a D394A mutein and a β-2-deoxy-2-fluoromaltosyl-enzyme intermediate with a E423A mutein were determined. There are few examples of CAZy glycoside hydrolase family 13 members that have had their glycosyl-enzyme intermediate structures determined, and none before now have been obtained with a 2-deoxy-2-fluoro substrate analogue. The covalent modification of Asp394 was confirmed using mass spectrometry. A similar modification of wild-type GlgE proteins from S. coelicolor and Mycobacterium tuberculosis was also observed. Small-angle X-ray scattering of the M. tuberculosis enzyme revealed a homodimeric assembly similar to that of the S. coelicolor enzyme but with slightly differently oriented monomers. The deeper understanding of the structure–function relationships of S. coelicolor GlgE will aid the development of inhibitors of the M. tuberculosis enzyme.

α-Amylase: an enzyme specificity found in various families of glycoside hydrolases

α-Amylase (EC 3.2.1.1) represents the best known amylolytic enzyme. It catalyzes the hydrolysis of α-1,4-glucosidic bonds in starch and related α-glucans. In general, the α-amylase is an enzyme with a broad substrate preference and product specificity. In the sequence-based classification system of all carbohydrate-active enzymes, it is one of the most frequently occurring glycoside hydrolases (GH). α-Amylase is the main representative of family GH13, but it is probably also present in the families GH57 and GH119, and possibly even in GH126. Family GH13, known generally as the main α-amylase family, forms clan GH-H together with families GH70 and GH77 that, however, contain no α-amylase. Within the family GH13, the α-amylase specificity is currently present in several subfamilies, such as GH13_1, 5, 6, 7, 15, 24, 27, 28, 36, 37, and, possibly in a few more that are not yet defined. The α-amylases classified in family GH13 employ a reaction mechanism giving retention of configuration, share 4-7 conserved sequence regions (CSRs) and catalytic machinery, and adopt the (β/α)8-barrel catalytic domain. Although the family GH57 α-amylases also employ the retaining reaction mechanism, they possess their own five CSRs and catalytic machinery, and adopt a (β/α)7-barrel fold. These family GH57 attributes are likely to be characteristic of α-amylases from the family GH119, too. With regard to family GH126, confirmation of the unambiguous presence of the α-amylase specificity may need more biochemical investigation because of an obvious, but unexpected, homology with inverting β-glucan-active hydrolases.

The specificity of α-glucosidase from Saccharomyces cerevisiae differs depending on the type of reaction: hydrolysis versus transglucosylation

Our investigation of the catalytic properties of Saccharomyces cerevisiae α-glucosidase (AGL) using hydroxybenzyl alcohol (HBA) isomers as transglucosylation substrates and their glucosides in hydrolytic reactions demonstrated interesting findings pertaining to the aglycon specificity of this important enzyme. AGL specificity increased from the para(p)- to the ortho(o)-HBA isomer in transglucosylation, whereas such AGL aglycon specificity was not seen in hydrolysis, thus indicating that the second step of the reaction (i.e., binding of the glucosyl acceptor) is rate-determining. To study the influence of substitution pattern on AGL kinetics, we compared AGL specificity, inferred from kinetic constants, for HBA isomers and other aglycon substrates. The demonstrated inhibitory effects of HBA isomers and their corresponding glucosides on AGL-catalyzed hydrolysis of p-nitrophenyl α-glucoside (PNPG) suggest that HBA glucosides act as competitive, whereas HBA isomers are noncompetitive, inhibitors. As such, we postulate that aromatic moieties cannot bind to an active site unless an enzyme-glucosyl complex has already formed, but they can interact with other regions of the enzyme molecule resulting in inhibition.

Similarities and differences in the biochemical and enzymological properties of the four isomaltases from Saccharomyces cerevisiae

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Isomaltases (Imap) preferably cleave α-(1,6) bonds, yet show clear substrate ambiguity.

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With only 3 different aa, Ima3p activities and thermostability diverge from Ima2p.

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The most distant protein, Ima5p, is extremely sensitive to temperature.

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Ima5p nevertheless displays most of the same catalytic properties as Ima1p and Ima2p.

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Ima5p challenges previous conclusions about specific aa needs for the active site.

The yeast Saccharomyces cerevisiae IMA multigene family encodes four isomaltases sharing high sequence identity from 65% to 99%. Here, we explore their functional diversity, with exhaustive in-vitro characterization of their enzymological and biochemical properties. The four isoenzymes exhibited a preference for the α-(1,6) disaccharides isomaltose and palatinose, with Michaëlis–Menten kinetics and inhibition at high substrates concentration. They were also able to hydrolyze trisaccharides bearing an α-(1,6) linkage, but also α-(1,2), α-(1,3) and α-(1,5) disaccharides including sucrose, highlighting their substrate ambiguity. While Ima1p and Ima2p presented almost identical characteristics, our results nevertheless showed many singularities within this protein family. In particular, Ima3p presented lower activities and thermostability than Ima2p despite only three different amino acids between the sequences of these two isoforms. The Ima3p_R279Q variant recovered activity levels of Ima2p, while the Leu-to-Pro substitution at position 240 significantly increased the stability of Ima3p and supported the role of prolines in thermostability. The most distant protein, Ima5p, presented the lowest optimal temperature and was also extremely sensitive to temperature. Isomaltose hydrolysis by Ima5p challenged previous conclusions about the requirement of specific amino acids for determining the specificity for α-(1,6) substrates. We finally found a mixed inhibition by maltose for Ima5p while, contrary to a previous work, Ima1p inhibition by maltose was competitive at very low isomaltose concentrations and uncompetitive as the substrate concentration increased. Altogether, this work illustrates that a gene family encoding proteins with strong sequence similarities can lead to enzyme with notable differences in biochemical and enzymological properties.

Different transcription regulation routes are exerted by L- and D-amino acid enantiomers of peptide hormones

Conversion of one or more amino acids in eukaryotic peptides to the D-configuration is catalyzed by specific L/D peptide isomerases and it is a poorly investigated post-translational modification. No common modified amino acid and no specific modified position have been recognized and mechanisms underlying changes in the peptide function provided by this conversion were not sufficiently studied. The 72 amino acid crustacean hyperglycemic hormone (CHH) of Astacidea crustaceans exhibits a co-existence of two peptide enantiomers alternately having D- or L-phenylalanine in their third position. It is a pleiotropic hormone regulating several physiological processes in different target tissues and along different time scales. CHH enantiomers differently affect time courses and intensities of examined processes. The short-term effects of the two isomers on gene expression are presented here, examined in the hepatopancreas, gills, hemocytes and muscles of the astacid Pontastacus leptodactylus. Muscles and hemocytes were poorly affected by both isomers. Two CHH modes of action were elucidated in the hepatopancreas and the gills: specific gene induction by D-CHH only, elucidated in both organs and mutual targeted attenuation affected by both enantiomers elucidated in the gills. Consequently a two-receptor system is hypothesized for conveying the effect of the two CHH isomers.

A novel cold-active and salt-tolerant α-amylase from marine bacterium Zunongwangia profunda: molecular cloning, heterologous expression and biochemical characterization

A novel gene (amyZ) encoding a cold-active and salt-tolerant α-amylase (AmyZ) was cloned from marine bacterium Zunongwangia profunda (MCCC 1A01486) and the protein was expressed in Escherichia coli. The gene has a length of 1785 bp and encodes an α-amylase of 594 amino acids with an estimated molecular mass of 66 kDa by SDS-PAGE. The enzyme belongs to glycoside hydrolase family 13 and shows the highest identity (25%) to the characterized α-amylase TVA II from thermoactinomyces vulgaris R-47. The recombinant α-amylase showed the maximum activity at 35 °C and pH 7.0, and retained about 39% activity at 0 °C. AmyZ displayed extreme salt tolerance, with the highest activity at 1.5 M NaCl and 93% activity even at 4 M NaCl. The catalytic efficiency (k cat/K m) of AmyZ increased from 115.51 (with 0 M NaCl) to 143.30 ml mg(-1) s(-1) (with 1.5 M NaCl) at 35 °C and pH 7.0, using soluble starch as substrate. Besides, the thermostability of the enzyme was significantly improved in the presence of 1.5 M NaCl or 1 mM CaCl2. AmyZ is one of the very few α-amylases that tolerate both high salinity and low temperatures, making it a potential candidate for research in basic and applied biology.

Novel maltotriose-hydrolyzing thermoacidophilic type III pullulan hydrolase from Thermococcus kodakarensis

A novel thermoacidophilic pullulan-hydrolyzing enzyme (PUL) from hyperthermophilic archaeon Thermococcus kodakarensis (TK-PUL) that efficiently hydrolyzes starch under industrial conditions in the absence of any additional metal ions was cloned and characterized. TK-PUL possessed both pullulanase and α-amylase activities. The highest activities were observed at 95 to 100°C. Although the enzyme was active over a broad pH range (3.0 to 8.5), the pH optima for both activities were 3.5 in acetate buffer and 4.2 in citrate buffer. TK-PUL was stable for several hours at 90°C. Its half-life at 100°C was 45 min when incubated either at pH 6.5 or 8.5. The Km value toward pullulan was 2 mg ml(-1), with a Vmax of 109 U mg(-1). Metal ions were not required for the activity and stability of recombinant TK-PUL. The enzyme was able to hydrolyze both α-1,6 and α-1,4 glycosidic linkages in pullulan. The most preferred substrate, after pullulan, was γ-cyclodextrin, which is a novel feature for this type of enzyme. Additionally, the enzyme hydrolyzed a variety of polysaccharides, including starch, glycogen, dextrin, amylose, amylopectin, and cyclodextrins (α, β, and γ), mainly into maltose. A unique feature of TK-PUL was the ability to hydrolyze maltotriose into maltose and glucose.

Molecular evolution and diversity of dimeric alpha-amylase inhibitor gene in Kengyilia species (Triticeae: Poaceae)

Kengyilia Yen et J. L. Yang is a group of allohexaploid species with StYP genomic constitutions in the wheat tribe. To investigate the evolution and diversity of dimeric alpha-amylase inhibitor genes in the Kengyilia, forty-five homoeologous DAAI gene sequences were isolated from sampled Kengyilia species and analyzed together with those of its close relatives. These results suggested that (1) Kengyilia species from Central Asia and the Qinghai-Tibetan Plateau had different origins from those of the geographically differentiated P genome; (2) the St and P genomes of Kengyilia were donated by Pseudoroegneria and Agropyron, respectively, and the Y genome had an independent origin and showed an affinity with the St genome; (3) purifying selection dominated the DAAI gene members and the St-DAAI gene was evolving at faster rate than the P- and Y-DAAI genes in Kengyilia; and (4) natural selection was the main factor on the codon usage pattern of the DAAI gene in Kengyilia.

Probing impact of active site residue mutations on stability and activity of Neisseria polysaccharea amylosucrase

The amylosucrase from Neisseria polysaccharea is a transglucosidase from the GH13 family of glycoside-hydrolases that naturally catalyzes the synthesis of α-glucans from the widely available donor sucrose. Interestingly, natural molecular evolution has modeled a dense hydrogen bond network at subsite -1 responsible for the specific recognition of sucrose and conversely, it has loosened interactions at the subsite +1 creating a highly promiscuous subsite +1. The residues forming these subsites are considered to be likely involved in the activity as well as the overall stability of the enzyme. To assess their role, a structure-based approach was followed to reshape the subsite -1. A strategy based on stability change predictions, using the FoldX algorithm, was considered to identify the best candidates for site-directed mutagenesis and guide the construction of a small targeted library. A miniaturized purification protocol was developed and both mutant stability and substrate promiscuity were explored. A range of 8 °C between extreme melting temperature values was observed and some variants were able to synthesize series of oligosaccharides with distributions differing from that of the parental enzyme. The crucial role of subsite -1 was thus highlighted and the biocatalysts generated can now be considered as starting points for further engineering purposes.

Defining structural and evolutionary modules in proteins: a community detection approach to explore sub-domain architecture

Background

Assessing protein modularity is important to understand protein evolution. Still the question of the existence of a sub-domain modular architecture remains. We propose a graph-theory approach with significance and power testing to identify modules in protein structures. In the first step, clusters are determined by optimizing the partition that maximizes the modularity score. Second, each cluster is tested for significance. Significant clusters are referred to as modules. Evolutionary modules are identified by analyzing homologous structures. Dynamic modules are inferred from sets of snapshots of molecular simulations. We present here a methodology to identify sub-domain architecture robustly, biologically meaningful, and statistically supported.

Results

The robustness of this new method is tested using simulated data with known modularity. Modules are correctly identified even when there is a low correlation between landmarks within a module. We also analyzed the evolutionary modularity of a data set of α-amylase catalytic domain homologs, and the dynamic modularity of the Niemann-Pick C1 (NPC1) protein N-terminal domain.

The α-amylase contains an (α/β)₈ barrel (TIM barrel) with the polysaccharides cleavage site and a calcium-binding domain. In this data set we identified four robust evolutionary modules, one of which forms the minimal functional TIM barrel topology.

The NPC1 protein is involved in the intracellular lipid metabolism coordinating sterol trafficking. NPC1 N-terminus is the first luminal domain which binds to cholesterol and its oxygenated derivatives. Our inferred dynamic modules in the protein NPC1 are also shown to match functional components of the protein related to the NPC1 disease.

Conclusions

A domain compartmentalization can be found and described in correlation space. To our knowledge, there is no other method attempting to identify sub-domain architecture from the correlation among residues. Most attempts made focus on sequence motifs of protein-protein interactions, binding sites, or sequence conservancy. We were able to describe functional/structural sub-domain architecture related to key residues for starch cleavage, calcium, and chloride binding sites in the α-amylase, and sterol opening-defining modules and disease-related residues in the NPC1. We also described the evolutionary sub-domain architecture of the α-amylase catalytic domain, identifying the already reported minimum functional TIM barrel.

Two potentially novel amylolytic enzyme specificities in the prokaryotic glycoside hydrolase α-amylase family GH57

Glycoside hydrolase (GH) family 57 consists of more than 900 proteins from Archaea (roughly one-quarter) and Bacteria (roughly three-quarters), mostly from thermophiles. Fewer than 20 GH57 members have already been biochemically characterized as real, (almost exclusively) amylolytic enzymes. In addition to a recently described dual-specificity amylopullulanase-cyclomaltodextrinase, five enzyme specificities have been well established in the family--α-amylase, amylopullulanase, branching enzyme, 4-α-glucanotransferase and α-galactosidase--plus a group of the so-called α-amylase-like homologues probably without the enzyme activity. A (β/α)7-barrel succeeded by a bundle of a few α-helices forming the catalytic domain, and five conserved sequence regions (CSRs), are the main characteristics of family GH57. The main goal of the present bioinformatics study was to describe two novel groups within family GH57 that represent potential non-specified amylases (127 sequences mostly from Bacteria) and maltogenic amylases (12 sequences from Archaea). These were collected from sequence databases based on an indication of their biochemical characterization. Although both the non-specified amylases and the maltogenic amylases share the in silico identified catalytic machinery and predicted fold with the experimentally determined GH57 members, the two novel groups may define new GH57 subfamilies. They are distinguishable from the other, previously recognized, subfamilies by specific sequence features present especially in their CSRs (the so-called sequence fingerprints), also reflecting their own evolutionary histories.

Arabidopsis thaliana AMY3 is a unique redox-regulated chloroplastic α-amylase

α-Amylases are glucan hydrolases that cleave α-1,4-glucosidic bonds in starch. In vascular plants, α-amylases can be classified into three subfamilies. Arabidopsis has one member of each subfamily. Among them, only AtAMY3 is localized in the chloroplast. We expressed and purified AtAMY3 from Escherichia coli and carried out a biochemical characterization of the protein to find factors that regulate its activity. Recombinant AtAMY3 was active toward both insoluble starch granules and soluble substrates, with a strong preference for β-limit dextrin over amylopectin. Activity was shown to be dependent on a conserved aspartic acid residue (Asp(666)), identified as the catalytic nucleophile in other plant α-amylases such as the barley AMY1. AtAMY3 released small linear and branched glucans from Arabidopsis starch granules, and the proportion of branched glucans increased after the predigestion of starch with a β-amylase. Optimal rates of starch digestion in vitro was achieved when both AtAMY3 and β-amylase activities were present, suggesting that the two enzymes work synergistically at the granule surface. We also found that AtAMY3 has unique properties among other characterized plant α-amylases, with a pH optimum of 7.5-8, appropriate for activity in the chloroplast stroma. AtAMY3 is also redox-regulated, and the inactive oxidized form of AtAMY3 could be reactivated by reduced thioredoxins. Site-directed mutagenesis combined with mass spectrometry analysis showed that a disulfide bridge between Cys(499) and Cys(587) is central to this regulation. This work provides new insights into how α-amylase activity may be regulated in the chloroplast.

Coupled high-throughput functional screening and next generation sequencing for identification of plant polymer decomposing enzymes in metagenomic libraries

Recent advances in sequencing technologies generate new predictions and hypotheses about the functional roles of environmental microorganisms. Yet, until we can test these predictions at a scale that matches our ability to generate them, most of them will remain as hypotheses. Function-based mining of metagenomic libraries can provide direct linkages between genes, metabolic traits and microbial taxa and thus bridge this gap between sequence data generation and functional predictions. Here we developed high-throughput screening assays for function-based characterization of activities involved in plant polymer decomposition from environmental metagenomic libraries. The multiplexed assays use fluorogenic and chromogenic substrates, combine automated liquid handling and use a genetically modified expression host to enable simultaneous screening of 12,160 clones for 14 activities in a total of 170,240 reactions. Using this platform we identified 374 (0.26%) cellulose, hemicellulose, chitin, starch, phosphate and protein hydrolyzing clones from fosmid libraries prepared from decomposing leaf litter. Sequencing on the Illumina MiSeq platform, followed by assembly and gene prediction of a subset of 95 fosmid clones, identified a broad range of bacterial phyla, including Actinobacteria, Bacteroidetes, multiple Proteobacteria sub-phyla in addition to some Fungi. Carbohydrate-active enzyme genes from 20 different glycoside hydrolase (GH) families were detected. Using tetranucleotide frequency (TNF) binning of fosmid sequences, multiple enzyme activities from distinct fosmids were linked, demonstrating how biochemically-confirmed functional traits in environmental metagenomes may be attributed to groups of specific organisms. Overall, our results demonstrate how functional screening of metagenomic libraries can be used to connect microbial functionality to community composition and, as a result, complement large-scale metagenomic sequencing efforts.

Replacement of the catalytic nucleophile aspartyl residue of dextran glucosidase by cysteine sulfinate enhances transglycosylation activity

Dextran glucosidase from Streptococcus mutans (SmDG) catalyzes the hydrolysis of an α-1,6-glucosidic linkage at the nonreducing end of isomaltooligosaccharides and dextran. This enzyme has an Asp-194 catalytic nucleophile and two catalytically unrelated Cys residues, Cys-129 and Cys-532. Cys-free SmDG was constructed by replacement with Ser (C129S/C532S (2CS), the activity of which was the same as that of the wild type, SmDG). The nucleophile mutant of 2CS was generated by substitution of Asp-194 with Cys (D194C-2CS). The hydrolytic activity of D194C-2CS was 8.1 × 10(-4) % of 2CS. KI-associated oxidation of D194C-2CS increased the activity up to 0.27% of 2CS, which was 330 times higher than D194C-2CS. Peptide-mapping mass analysis of the oxidized D194C-2CS (Ox-D194C-2CS) revealed that Cys-194 was converted into cysteine sulfinate. Ox-D194C-2CS and 2CS shared the same properties (optimum pH, pI, and substrate specificity), whereas Ox-D194C-2CS had much higher transglucosylation activity than 2CS. This is the first study indicating that a more acidic nucleophile (-SOO(-)) enhances transglycosylation. The introduction of cysteine sulfinate as a catalytic nucleophile could be a novel approach to enhance transglycosylation.

Dimerization mediates thermo-adaptation, substrate affinity and transglycosylation in a highly thermostable maltogenic amylase of Geobacillus thermoleovorans

BACKGROUND

Maltogenic amylases belong to a subclass of cyclodextrin-hydrolyzing enzymes and hydrolyze cyclodextrins more efficiently than starch unlike typical α-amylases. Several bacterial malto-genic amylases with temperature optima of 40-60°C have been previously characterized. The thermo-adaption, substrate preferences and transglycosylation aspects of extremely thermostable bacterial maltogenic amylases have not yet been reported.

METHODOLOGY/PRINCIPAL FINDINGS

The recombinant monomeric and dimeric forms of maltogenic α-amylase (Gt-Mamy) of the extremely thermophilic bacterium Geobacillus thermoleovorans are of 72.5 and 145 kDa, which are active optimally at 80°C. Extreme thermostability of this enzyme has been explained by analyzing far-UV CD spectra. Dimerization increases T1/2 of Gt-Mamy from 8.2 h to 12.63 h at 90°C and mediates its enthalpy-driven conformational thermostabilization. Furthermore, dime-rization regulates preferential substrate binding of the enzyme. The substrate preference switching of Gt-Mamy upon dimerization has been confirmed from the substrate-binding affinities of the enzyme for various high and low molecular weight substrates. There is an alteration in Km and substrate hydrolysis efficiency (Vmax/Km) of the enzyme (for cyclodex-trins/starch) upon dimerization. N-terminal truncation indicated the role of N-terminal 128 amino acids in the thermostabilization and modulation of substrate-binding affinity. This has been confirmed by molecular docking of β-cyclodextrin to Gt-Mamy that indicated the requirement of homodimer formation by the interaction of a few N-terminal residues of chain A with the catalytic residues of (α/β)8 barrel of chain B and vice-versa for stable cyclodextrin binding. Site directed mutagenesis provided evidence for the role of N-terminal D109 at the dimeric interface in substrate affinity modulation and thermostabilization. The dimeric Gt-Mamy transglycosylates hydrolytic products of G4/G5 and acarbose, while the truncated form does not because of the lack of extra sugar-binding space formed due to dimerization.

CONCLUSION/SIGNIFICANCE

N-terminal domain controls enthalpy-driven thermostabilization, substrate-binding affinity and transglycosylation activity of Gt-Mamy by homodimer formation.

How protein targeting to primary plastids via the endomembrane system could have evolved? A new hypothesis based on phylogenetic studies

Background

It is commonly assumed that a heterotrophic ancestor of the supergroup Archaeplastida/Plantae engulfed a cyanobacterium that was transformed into a primary plastid; however, it is still unclear how nuclear-encoded proteins initially were imported into the new organelle. Most proteins targeted to primary plastids carry a transit peptide and are transported post-translationally using Toc and Tic translocons. There are, however, several proteins with N-terminal signal peptides that are directed to higher plant plastids in vesicles derived from the endomembrane system (ES). The existence of these proteins inspired a hypothesis that all nuclear-encoded, plastid-targeted proteins initially carried signal peptides and were targeted to the ancestral primary plastid via the host ES.

Results

We present the first phylogenetic analyses of Arabidopsis thaliana α-carbonic anhydrase (CAH1), Oryza sativa nucleotide pyrophosphatase/phosphodiesterase (NPP1), and two O. sativa α-amylases (αAmy3, αAmy7), proteins that are directed to higher plant primary plastids via the ES. We also investigated protein disulfide isomerase (RB60) from the green alga Chlamydomonas reinhardtii because of its peculiar dual post- and co-translational targeting to both the plastid and ES. Our analyses show that these proteins all are of eukaryotic rather than cyanobacterial origin, and that their non-plastid homologs are equipped with signal peptides responsible for co-translational import into the host ES. Our results indicate that vesicular trafficking of proteins to primary plastids evolved long after the cyanobacterial endosymbiosis (possibly only in higher plants) to permit their glycosylation and/or transport to more than one cellular compartment.

Conclusions

The proteins we analyzed are not relics of ES-mediated protein targeting to the ancestral primary plastid. Available data indicate that Toc- and Tic-based translocation dominated protein import into primary plastids from the beginning. Only a handful of host proteins, which already were targeted through the ES, later were adapted to reach the plastid via the vesicular trafficking. They represent a derived class of higher plant plastid-targeted proteins with an unusual evolutionary history.

Reviewers

This article was reviewed by Prof. William Martin, Dr. Philippe Deschamps (nominated by Dr. Purificacion Lopez-Garcia) and Dr Simonetta Gribaldo.

Biochemical characterization of 4-α-glucanotransferase from Saccharophagus degradans 2-40 and its potential role in glycogen degradation

4-α-Glucanotransferase, an enzyme encoded by malQ, transfers 1,4-α-glucan to an acceptor carbohydrate to produce long linear maltodextrins of varying lengths. To investigate the biochemical characteristics of the malQ gene (Sde0986) from Saccharophagus degradans 2-40 and to understand its physiological role in vivo, the malQ gene was cloned and expressed in Escherichia coli. The amino acid sequence of MalQ was found to be 36-47% identical to that of amylomaltases from gammaproteobacteria. MalQ is a monomeric enzyme that belongs to a family of 77 glycoside hydrolases, with a molecular mass of 104 kDa. The optimal pH and temperature for MalQ toward maltotriose were determined to be 8.5 and 35 °C, respectively. Furthermore, the enzyme displayed glycosyl transfer activity on maltodextrins of various sizes to yield glucose and long linear maltodextrins. MalQ, however, could be distinguished from other bacterial and archaeal amylomaltases in that it did not produce maltose and cyclic glucan. Reverse transcription PCR results showed that malQ was not induced by maltose and was highly expressed in the stationary phase. These data suggest that the main physiological role of malQ in S. degradans is in the degradation of glycogen, although the gene is commonly known to be involved in maltose metabolism in E. coli.

Recombinant bacterial amylopullulanases: developments and perspectives

Pullulanases are endo-acting enzymes capable of hydrolyzing α-1, 6-glycosidic linkages in starch, pullulan, amylopectin, and related oligosaccharides, while amylopullulanases are bifunctional enzymes with an active site capable of cleaving both α-1, 4 and α-1, 6 linkages in starch, amylose and other oligosaccharides, and α-1, 6 linkages in pullulan. The amylopullulanases are classified in GH13 and GH57 family enzymes based on the architecture of catalytic domain and number of conserved sequences. The enzymes with two active sites, one for the hydrolysis of α-1, 4- glycosidic bond and the other for α-1, 6-glycosidic bond, are called α-amylase-pullulanases, while amylopullulanases have only one active site for cleaving both α-1, 4- and α-1, 6-glycosidic bonds. The amylopullulanases produced by bacteria find applications in the starch and baking industries as a catalyst for one step starch liquefaction-saccharification for making various sugar syrups, as antistaling agent in bread and as a detergent additive.