A novel structural fold in polysaccharide lyases: Bacillus subtilis family 11 rhamnogalacturonan lyase YesW with an eight-bladed beta-propeller

An overview of microbial enzymatic approaches for pectin degradation

Pectin, a complex natural macromolecule present in primary cell walls, exhibits high structural diversity. Pectin is composed of a main chain, which contains a high amount of partly methyl-esterified galacturonic acid (GalA), and numerous types of side chains that contain almost 17 different monosaccharides and over 20 different linkages. Due to this peculiar structure, pectin exhibits special physicochemical properties and a variety of bioactivities. For example, pectin exhibits strong bioactivity only in a low molecular weight range. Many different degrading enzymes, including hydrolases, lyases and esterases, are needed to depolymerize pectin due to its structural complexity. Pectin degradation involves polygalacturonases/rhamnogalacturonases and pectate/pectin lyases, which attack the linkages in the backbone via hydrolytic and β-elimination modes, respectively. Pectin methyl/acetyl esterases involved in the de-esterification of pectin also play crucial roles. Many α-L-rhamnohydrolases, unsaturated rhamnogalacturonyl hydrolases, arabinanases and galactanases also contribute to heterogeneous pectin degradation. Although numerous microbial pectin-degrading enzymes have been described, the mechanisms involved in the coordinated degradation of pectin through these enzymes remain unclear. In recent years, the degradation of pectin by Bacteroides has received increasing attention, as Bacteroides species contain a unique genetic structure, polysaccharide utilization loci (PULs). The specific PULs of pectin degradation in Bacteroides species are a new field to study pectin metabolism in gut microbiota. This paper reviews the scientific information available on pectin structural characteristics, pectin-degrading enzymes, and PULs for the specific degradation of pectin.

Discovery of new functions of food proteins and their structural development for multifunctional applications

Proteins and peptides derived from various food sources are used in a variety of applications, including functional foods, pharmaceuticals, and cosmetics. The three-dimensional structure of proteins provides useful insights into their functions and essential information for the creation of proteins with new functions. In this review, a series of functional conversion technologies based on protein structural information derived from foods traditionally consumed in Japan, such as natto (fermented soybeans) and rice, are introduced. For natto, we first identified 2 types of Bacillus subtilis-derived endolytic and exolytic enzymes with different modes of action on soybean cell wall polysaccharides and then focused on the technology used to create an endolytic enzyme from an exolytic enzyme. By applying this technology, a method for creating novel bioactive peptides from rice seed proteins was established. The modified peptides created could provide diverse options for the production of substances such as pharmaceuticals and cosmetic materials.

New β-Propellers Are Continuously Amplified From Single Blades in all Major Lineages of the β-Propeller Superfamily

β-Propellers are toroidal folds, in which consecutive supersecondary structure units of four anti-parallel β-strands-called blades-are arranged radially around a central axis. Uniquely among toroidal folds, blades span the full range of sequence symmetry, from near identity to complete divergence, indicating an ongoing process of amplification and differentiation. We have proposed that the major lineages of β-propellers arose through this mechanism and that therefore their last common ancestor was a single blade, not a fully formed β-propeller. Here we show that this process of amplification and differentiation is also widespread within individual lineages, yielding β-propellers with blades of more than 60% pairwise sequence identity in most major β-propeller families. In some cases, the blades are nearly identical, indicating a very recent amplification event, but even in cases where such recently amplified β-propellers have more than 80% overall sequence identity to each other, comparison of their DNA sequence shows that the amplification occurred independently.

Biochemical Characterization of Two Rhamnogalacturonan Lyases From Bacteroides ovatus ATCC 8483 With Preference for RG-I Substrates

Rhamnogalacturonan lyase (RGL) cleaves backbone α-1,4 glycosidic bonds between L-rhamnose and D-galacturonic acid residues in type I rhamnogalacturonan (RG-I) by β-elimination to generate RG oligosaccharides with various degrees of polymerization. Here, we cloned, expressed, purified and biochemically characterized two RGLs (Bo3128 and Bo4416) in the PL11 family from Bacteroides ovatus ATCC 8483. Bo3128 and Bo4416 displayed maximal activity at pH 9.5 and pH 6.5, respectively. Whereas the activity of Bo3128 could be increased 1.5 fold in the presence of 5 mM Ca²⁺, Bo4416 required divalent metal ions to show any enzymatic activity. Both of RGLs showed a substrate preference for RG-I compared to other pectin domains. Bo4416 and Bo3128 primarily yielded unsaturated RG oligosaccharides, with Bo3128 also producing them with short side chains, with yields of 32.4 and 62.4%, respectively. Characterization of both RGLs contribute to the preparation of rhamnogalacturonan oligosaccharides, as well as for the analysis of the fine structure of RG-I pectins.

Bacteria with a mouth: Discovery and new insights into cell surface structure and macromolecule transport

A bacterium with a "mouth"-like pit structure isolated for the first time in the history of microbiology was a Gram-negative rod, containing glycosphingolipids in the cell envelope, and named Sphingomonas sp. strain A1. The pit was dynamic, with repetitive opening and closing during growth on alginate, and directly included alginate concentrated around the pit, particularly by flagellins, an alginate-binding protein localized on the cell surface. Alginate incorporated into the periplasm was subsequently transferred to the cytoplasm by cooperative interactions of periplasmic solute-binding proteins and an ATP-binding cassette transporter in the cytoplasmic membrane. The mechanisms of assembly, functions, and interactions between the above-mentioned molecules were clarified using structural biology. The pit was transplanted into other strains of sphingomonads, and the pitted recombinant cells were effectively applied to the production of bioethanol, bioremediation for dioxin removal, and other tasks. Studies of the function of the pit shed light on the biological significance of cell surface structures and macromolecule transport in bacteria.

Rhamnogalacturonan I modifying enzymes: an update

Rhamnogalacturonan I (RGI) modifying enzymes catalyse the degradation of the RGI backbone and encompass enzymes specific for either the α1,2-bond linking galacturonic acid to rhamnose or the α1,4-bond linking rhamnose to galacturonic acid in the RGI backbone. The first microbial enzyme found to be able to catalyse the degradation of the RGI backbone, an endo-hydrolase (EC 3.2.1.171) derived from Aspergillus aculeatus, was discovered 25 years ago. Today the group of RGI modifying enzymes encompasses endo- and exo-hydrolases as well as lyases. The RGI hydrolases, EC 3.2.1.171-EC 3.2.1.174, have been described to be produced by Aspergillus spp. and Bacillus subtilis and are categorized in glycosyl hydrolase families 28 and 105. The RGI lyases, EC 4.2.2.23-EC 4.2.2.24, have been isolated from different fungi and bacterial species and are categorized in polysaccharide lyase families 4 and 11. This review brings together the available knowledge of the RGI modifying enzymes and provides a detailed overview of biocatalytic reaction characteristics, classification, structure-function traits, and analyses the protein properties of these enzymes by multiple sequence alignments in neighbour-joining phylogenetic trees. Some recently detected unique structural features and dependence of calcium for activity of some of these enzymes (notably the lyases) are discussed and newly published results regarding improvement of their thermostability by protein engineering are highlighted. Knowledge of these enzymes is important for understanding microbial plant cell wall degradation and for advancing enzymatic processing and biorefining of pectinaceous plant biomass.

Role of pectinolytic enzymes identified in Clostridium thermocellum cellulosome

The cloning, expression and characterization of three cellulosomal pectinolytic enzymes viz., two variants of PL1 (PL1A and PL1B) and PL9 from Clostridium thermocellum was carried out. The comparison of the primary sequences of PL1A, PL1B and PL9 revealed that these proteins displayed considerable sequence similarities with family 1 and 9 polysaccharide lyases, respectively. PL1A, PL1B and PL9 are the putative catalytic domains of protein sequence ABN54148.1 and ABN53381.1 respectively. These two protein sequences also contain putative carbohydrate binding module (CBM) and type-I dockerin. The associated putative CBM of PL1A showed strong homology with family 6 CBMs while those of PL1B and PL9 showed homology with family 35 CBMs. Recombinant derivatives of these three enzymes showed molecular masses of approximately 34 kDa, 40 kDa and 32 kDa for PL1A, PL1B and PL9, respectively. PL1A, PL1B and PL9 displayed high activity toward polygalacturonic acid and pectin (up to 55% methyl-esterified) from citrus fruits. However, PL1B showed relatively higher activity towards 55% and 85% methyl-esterified pectin (citrus). PL1A and PL9 showed higher activity on rhamnogalacturonan than PL1B. Both PL1A and PL9 displayed maximum activity at pH 8.5 with optimum temperature of 50°C and 60°C respectively. PL1B achieved highest activity at pH 9.8, under an optimum temperature of 50°C. PL1A, PL1B and PL9 all produced two or more unsaturated galacturonates from pectic substrates as displayed by TLC analysis confirming that they are endo-pectate lyase belonging to family 1 and 9, respectively. This report reveals that pectinolytic activity displayed by Clostridium thermocellum cellulosome is coordinated by a sub-set of at least three multi-modular enzymes.

Calcium binding proteins and calcium signaling in prokaryotes

With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca(2+)) binding motifs including the canonical and pseudo EF-hand motifs, Ca(2+)-binding β-roll, Greek key motif and a novel putative Ca(2+)-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca(2+) in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca(2+) in prokaryotes.

Bacterial pectate lyases, structural and functional diversity

Pectate lyases are enzymes involved in plant cell wall degradation. They cleave pectin using a β-elimination mechanism, specific for acidic polysaccharides. They are mainly produced by plant pathogens and plant-associated organisms, and only rarely by animals. Pectate lyases are also commonly produced in the bacterial world, either by bacteria living in close proximity with plants or by gut bacteria that find plant material in the digestive tract of their hosts. The role of pectate lyases is essential for plant pathogens, such as Dickeya dadantii, that use a set of pectate lyases as their main virulence factor. Symbiotic bacteria produce their own pectate lyases, but they also induce plant pectate lyases to initiate the symbiosis. Pectin degradation products may act as signals affecting the plant–bacteria interactions. Bacterial pectate lyases are also essential for using the pectin of dead or living plants as a carbon source for growth. In the animal gut, Bacteroides pectate lyases degrade the pectin of ingested food, and this is particularly important for herbivores that depend on their microflora for the digestion of pectin. Some human pathogens, such as Yersinia enterocolitica, produce a few intracellular pectate lyases that can facilitate their growth in the presence of highly pectinolytic bacteria, at the plant surface, in the soil or in the animal gut.

Extracellular calcium triggers unique transcriptional programs and modulates staurosporine-induced cell death in Neurospora crassa

Alterations in the intracellular levels of calcium are a common response to cell death stimuli in animals and fungi and, particularly, in the Neurospora crassa response to staurosporine. We highlight the importance of the extracellular availability of Ca²⁺ for this response. Limitation of the ion in the culture medium further sensitizes cells to the drug and results in increased accumulation of reactive oxygen species (ROS). Conversely, an approximately 30-fold excess of external Ca²⁺ leads to increased drug tolerance and lower ROS generation. In line with this, distinct staurosporine-induced cytosolic Ca²⁺ signaling profiles were observed in the absence or presence of excessive external Ca²⁺. High-throughput RNA sequencing revealed that different concentrations of extracellular Ca²⁺ define distinct transcriptional programs. Our transcriptional profiling also pointed to two putative novel Ca²⁺-binding proteins, encoded by the NCU08524 and NCU06607 genes, and provides a reference dataset for future investigations on the role of Ca²⁺ in fungal biology.

New structural and functional contexts of the Dx[DN]xDG linear motif: insights into evolution of calcium-binding proteins

Binding of calcium ions (Ca²⁺) to proteins can have profound effects on their structure and function. Common roles of calcium binding include structure stabilization and regulation of activity. It is known that diverse families – EF-hands being one of at least twelve – use a Dx[DN]xDG linear motif to bind calcium in near-identical fashion. Here, four novel structural contexts for the motif are described. Existing experimental data for one of them, a thermophilic archaeal subtilisin, demonstrate for the first time a role for Dx[DN]xDG-bound calcium in protein folding. An integrin-like embedding of the motif in the blade of a β-propeller fold – here named the calcium blade – is discovered in structures of bacterial and fungal proteins. Furthermore, sensitive database searches suggest a common origin for the calcium blade in β-propeller structures of different sizes and a pan-kingdom distribution of these proteins. Factors favouring the multiple convergent evolution of the motif appear to include its general Asp-richness, the regular spacing of the Asp residues and the fact that change of Asp into Gly and vice versa can occur though a single nucleotide change. Among the known structural contexts for the Dx[DN]xDG motif, only the calcium blade and the EF-hand are currently found intracellularly in large numbers, perhaps because the higher extracellular concentration of Ca²⁺ allows for easier fixing of newly evolved motifs that have acquired useful functions. The analysis presented here will inform ongoing efforts toward prediction of similar calcium-binding motifs from sequence information alone.

Structural and mechanistic classification of uronic acid-containing polysaccharide lyases

Polysaccharide lyases (PLs) have been assigned to 21 families based on their sequences, with ~ 50 singletons awaiting further classification. For 19 of these families, the structure of at least one protein is known. In this review, we have analyzed the available structural information and show that presently known PL families belong to six general folds. Only two general catalytic mechanisms have been observed among these PLs: (1) metal-assisted neutralization of the acidic group of the sugar next to the cleaved bond, with, rather unusually, arginine or lysine playing the role of Brønsted base and (2) neutralization of the acidic group on the sugar by a close approach of an amino or acidic group forcing its protonation and Tyr or Tyr-His acting as the Brønsted base and acid.

Crystal structure of exotype alginate lyase Atu3025 from Agrobacterium tumefaciens

Alginate, a major component of the cell wall matrix in brown seaweeds, is degraded by alginate lyases through a beta-elimination reaction. Almost all alginate lyases act endolytically on substrate, thereby yielding unsaturated oligouronic acids having 4-deoxy-l-erythro-hex-4-enepyranosyluronic acid at the nonreducing end. In contrast, Agrobacterium tumefaciens alginate lyase Atu3025, a member of polysaccharide lyase family 15, acts on alginate polysaccharides and oligosaccharides exolytically and releases unsaturated monosaccharides from the substrate terminal. The crystal structures of Atu3025 and its inactive mutant in complex with alginate trisaccharide (H531A/DeltaGGG) were determined at 2.10- and 2.99-A resolutions with final R-factors of 18.3 and 19.9%, respectively, by x-ray crystallography. The enzyme is comprised of an alpha/alpha-barrel + anti-parallel beta-sheet as a basic scaffold, and its structural fold has not been seen in alginate lyases analyzed thus far. The structural analysis of H531A/DeltaGGG and subsequent site-directed mutagenesis studies proposed the enzyme reaction mechanism, with His(311) and Tyr(365) as the catalytic base and acid, respectively. Two structural determinants, i.e. a short alpha-helix in the central alpha/alpha-barrel domain and a conformational change at the interface between the central and C-terminal domains, are essential for the exolytic mode of action. This is, to our knowledge, the first report on the structure of the family 15 enzyme.

Structural determinants responsible for substrate recognition and mode of action in family 11 polysaccharide lyases

A saprophytic Bacillus subtilis secretes two types of rhamnogalacturonan (RG) lyases, endotype YesW and exotype YesX, which are responsible for an initial cleavage of the RG type I (RG-I) region of plant cell wall pectin. Polysaccharide lyase family 11 YesW and YesX with a significant sequence identity (67.8%) cleave glycoside bonds between rhamnose and galacturonic acid residues in RG-I through a beta-elimination reaction. Here we show the structural determinants for substrate recognition and the mode of action in polysaccharide lyase family 11 lyases. The crystal structures of YesW in complex with rhamnose and ligand-free YesX were determined at 1.32 and 1.65 A resolution, respectively. The YesW amino acid residues such as Asn(152), Asp(172), Asn(532), Gly(533), Thr(534), and Tyr(595) in the active cleft bind to rhamnose molecules through hydrogen bonds and van der Waals contacts. Other rhamnose molecules are accommodated at the noncatalytic domain far from the active cleft, revealing that the domain possibly functions as a novel carbohydrate-binding module. A structural comparison between YesW and YesX indicates that a specific loop in YesX for recognizing the terminal saccharide molecule sterically inhibits penetration of the polymer over the active cleft. The loop-deficient YesX mutant exhibits YesW-like endotype activity, demonstrating that molecular conversion regarding the mode of action is achieved by the addition/removal of the loop for recognizing the terminal saccharide. This is the first report on a structural insight into RG-I recognition and molecular conversion of exotype to endotype in polysaccharide lyases.

Investigation of atomic level patterns in protein--small ligand interactions

BACKGROUND

Shape complementarity and non-covalent interactions are believed to drive protein-ligand interaction. To date protein-protein, protein-DNA, and protein-RNA interactions were systematically investigated, which is in contrast to interactions with small ligands. We investigate the role of covalent and non-covalent bonds in protein-small ligand interactions using a comprehensive dataset of 2,320 complexes.

METHODOLOGY AND PRINCIPAL FINDINGS

We show that protein-ligand interactions are governed by different forces for different ligand types, i.e., protein-organic compound interactions are governed by hydrogen bonds, van der Waals contacts, and covalent bonds; protein-metal ion interactions are dominated by electrostatic force and coordination bonds; protein-anion interactions are established with electrostatic force, hydrogen bonds, and van der Waals contacts; and protein-inorganic cluster interactions are driven by coordination bonds. We extracted several frequently occurring atomic-level patterns concerning these interactions. For instance, 73% of investigated covalent bonds were summarized with just three patterns in which bonds are formed between thiol of Cys and carbon or sulfur atoms of ligands, and nitrogen of Lys and carbon of ligands. Similar patterns were found for the coordination bonds. Hydrogen bonds occur in 67% of protein-organic compound complexes and 66% of them are formed between NH- group of protein residues and oxygen atom of ligands. We quantify relative abundance of specific interaction types and discuss their characteristic features. The extracted protein-organic compound patterns are shown to complement and improve a geometric approach for prediction of binding sites.

CONCLUSIONS AND SIGNIFICANCE

We show that for a given type (group) of ligands and type of the interaction force, majority of protein-ligand interactions are repetitive and could be summarized with several simple atomic-level patterns. We summarize and analyze 10 frequently occurring interaction patterns that cover 56% of all considered complexes and we show a practical application for the patterns that concerns interactions with organic compounds.

Crystal structure of the glycosidase family 73 peptidoglycan hydrolase FlgJ

Glycoside hydrolase (GH) categorized into family 73 plays an important role in degrading bacterial cell wall peptidoglycan. The flagellar protein FlgJ contains N- and C-terminal domains responsible for flagellar rod assembly and peptidoglycan hydrolysis, respectively. A member of family GH-73, the C-terminal domain (SPH1045-C) of FlgJ from Sphingomonas sp. strain A1 was expressed in Escherichia coli, purified, and characterized. SPH1045-C exhibited bacterial cell lytic activity most efficiently at pH 6.0 and 37 degrees C. The X-ray crystallographic structure of SPH1045-C was determined at 1.74 A resolution by single-wavelength anomalous diffraction. The enzyme consists of two lobes, alpha and beta. A deep cleft located between the two lobes can accommodate polymer molecules, suggesting that the active site is located in the cleft. Although SPH1045-C shows a structural homology with family GH-22 and GH-23 lysozymes, the arrangement of the nucleophile/base residue in the active site is specific to each peptidoglycan hydrolase.