Structural analysis of the endogenous glycoallergen Hev b 2 (endo-β-1,3-glucanase) from Hevea brasiliensis and its recognition by human basophils

Structural and functional analysis of SpGlu64A: a novel glycoside hydrolase family 64 laminaripentaose-producing β-1,3-glucanase from Streptomyces pratensis

Laminaripentaose (L5)-producing β-1,3-glucanases can preferentially cleave the triple-helix curdlan into β-1,3-glucooligosaccharides, especially L5. In this study, a newly identified member of the glycoside hydrolase family 64, β-1,3-glucanase from Streptomyces pratensis (SpGlu64A), was functionally and structurally characterized. SpGlu64A shared highest identity (30%) with a β-1,3-glucanase from Streptomyces matensis. The purified SpGlu64A showed maximal activity at pH 7.5 and 50 °C, and exhibited strict substrate specificity toward curdlan (83.1 U·mg-1). It efficiently hydrolyzed curdlan to produce L5 as the end product. The overall structure of SpGlu64A consisted of a barrel domain and a mixed (α/β) domain, which formed an unusually wide groove with a crescent-like structure. In the two complex structures (SpGlu64A-L3 and SpGlu64A-L4), two oligosaccharide chains were captured and the triple-helical structure was relatively compatible with the wide groove, which suggested the possibility of binding to the triple-helical β-1,3-glucan. A catalytic framework (β6-β9-β10) and the steric hindrance formed by the side chains of residues Y161, N163, and H393 in the catalytic groove were predicted to complete the exotype-like cleavage manner. On the basis of the structure, a fusion protein with the CBM56 domain (SpGlu64A-CBM) and a mutant (Y161F; by site-directed mutation) were obtained, with 1.2- and 1.7-fold increases in specific activity, respectively. Moreover, the combined expression of SpGlu64A-CBM and -Y161F improved the enzyme activity by 2.63-fold. The study will not only be helpful in understanding the reaction mechanism of β-1,3-glucanases but will also provide a basis for further enzyme engineering.

[Hevea brasiliensis como fuente alergénica: revisión bibliográfica]

Abstract

La planta Hevea brasiliensis se utiliza ampliamente en la industria como fuente de extracción de caucho, un elemento empleado en diversas áreas comerciales y médicas. Los estudios inmunológicos de esta especie indican que es una fuente alergénica importante, que puede provocar sensibilización y alergia. Se han identificado diferentes componentes alergénicos de esta planta, con diversas propiedades inmunitarias y bioquímicas, y estudiado más de diez tipos diferentes de alérgenos, cada uno con distinta capacidad de inducir síntomas alérgicos. En esta revisión informamos los avances actuales en el estudio de Hevea brasiliensis.

Characterization of two 1,3-β-glucan-modifying enzymes from Penicillium sumatraense reveals new insights into 1,3-β-glucan metabolism of fungal saprotrophs

BACKGROUND

1,3-β-glucan is a polysaccharide widely distributed in the cell wall of several phylogenetically distant organisms, such as bacteria, fungi, plants and microalgae. The presence of highly active 1,3-β-glucanases in fungi evokes the biological question on how these organisms can efficiently metabolize exogenous sources of 1,3-β-glucan without incurring in autolysis.

RESULTS

To elucidate the molecular mechanisms at the basis of 1,3-β-glucan metabolism in fungal saprotrophs, the putative exo-1,3-β-glucanase G9376 and a truncated form of the putative glucan endo-1,3-β-glucosidase (ΔG7048) from Penicillium sumatraense AQ67100 were heterologously expressed in Pichia pastoris and characterized both in terms of activity and structure. G9376 efficiently converted laminarin and 1,3-β-glucan oligomers into glucose by acting as an exo-glycosidase, whereas G7048 displayed a 1,3-β-transglucanase/branching activity toward 1,3-β-glucan oligomers with a degree of polymerization higher than 5, making these oligomers more recalcitrant to the hydrolysis acted by exo-1,3-β-glucanase G9376. The X-ray crystallographic structure of the catalytic domain of G7048, solved at 1.9 Å of resolution, consists of a (β/α)₈ TIM-barrel fold characteristic of all the GH17 family members. The catalytic site is in a V-shaped cleft containing the two conserved catalytic glutamic residues. Molecular features compatible with the activity of G7048 as 1,3-β-transglucanase are discussed.

CONCLUSIONS

The antagonizing activity between ΔG7048 and G9376 indicates how opportunistic fungi belonging to Penicillium genus can feed on substrates similar for composition and structure to their own cell wall without incurring in a self-deleterious autohydrolysis.

Update on pollen-food allergy syndrome

INTRODUCTION

Allergies affect 20-30% of the population and respiratory allergies are mostly due to pollen grains from anemophilous plants. One to 5% of people suffer from food allergies and clinicians report increasing numbers of pollen-food allergy syndrome (PFAS), such that the symptoms have broadened from respiratory to gastrointestinal, and even to anaphylactic shock in the presence of cofactors. Thirty to 60% of food allergies are associated with pollen allergy while the percentage of pollen allergies associated to food allergy varies according to local environment and dietary habits.

AREAS COVERED

Articles published in peer-reviewed journals, covered by PubMed databank, clinical data are discussed including symptoms, diagnosis, and management. A chapter emphasizes the role of six well-known allergen families involved in PFAS: PR10 proteins, profilins, lipid transfer proteins, thaumatin-like proteins, isoflavone reductases, and β-1,3 glucanases. The relevance in PFAS of three supplementary allergen families is presented: oleosins, polygalacturonases, and gibberellin-regulated proteins. To support the discussion a few original relevant results were added.

EXPERT OPINION

Both allergenic sources, pollen and food, are submitted to the same stressful environmental changes resulting in an increase of pathogenesis-related proteins in which numerous allergens are found. This might be responsible for the potential increase of PFAS.

Ligustrum pollen: New insights into allergic disease

Respiratory allergies are important medical conditions because they affect nearly 20% of the population worldwide, with higher prevalence in industrialized cities. Aeroallergens such as pollen are responsible for up to 40% of respiratory allergies. The pollen from Ligustrum (privet hedge) is a great source of inhalant allergens associated with allergic respiratory diseases around the world. However, it has been underestimated as a sensitization factor. Interestingly, over the last few years a number of novel allergens have been identified from Ligustrum using immunoproteomics technologies. Cross-linking of IgE and Ligustrum allergens could lead to the rapid release of inflammatory mediators by mast cells and basophils. These will promote a late response characterized by activation of T cells and overproduction of Th2 cytokines such as IL-4, IL-5, IL-9, and IL-13. These inflammatory changes cause respiratory diseases like asthma and allergic rhinitis in sensitized subjects. Here, we review Ligustrum pollen allergens and focus on their clinical and immunological significance in allergic disease as well as the use of hypoallergenic derivatives in personalized therapy.

Modulating the function of a β-1,3-glucanosyltransferase to that of an endo-β-1,3-glucanase by structure-based protein engineering

A glycoside hydrolase (GH) family 17 β-1,3-glucanosyltransferase (RmBgt17A) from Rhizomucor miehei CAU432 (CGMCC No. 4967) shared very low sequence homology (∼20 % identity) with that of other β-1,3-glucanases,despite their similar structural folds. Structural comparison and sequence alignment between RmBgt17A and GH family 17 β-1,3-glucanases suggested important roles for three residues (Tyr102, Trp157, and Glu158) located in the substrate-binding cleft of RmBgt17A in transglycosylation activity. A series of site-directed mutagenesis studies indicated that a single Glu-to-Ala mutation (E158A) modulates the function of RmBgt17A to that of a β-1,3-glucanase. Mutant E158A exhibited high hydrolytic activity (39.95 U/mg) toward reduced laminarin, 348.5-fold higher than the wild type. Optimal pH and temperature of the purified RmBgt17A-E158A were 4.5 and 55 °C, respectively. TLC analysis suggested that RmBgt17A-E158A is an endo-β-1,3-glucanase. Our study provides novel insight into protein engineering of the substrate-binding cleft of glycoside hydrolases to modulate the function of transglycosylation and hydrolysis.

The concept of allergen-associated molecular patterns (AAMP)

For proteins to become allergenic, they need to acquire features enabling them to induce B cell activation and isotype switch to IgE production. Crosslinking of the B-cell receptor (BCR) is the most efficient way to productively activate B-cells. The IgE-crosslinking capability of allergens is equally crucial in the effector phase of immediate type allergy. Antigens, which acquire enhanced crosslinking capacity by oligomerization, aggregation, or the expression of repetitive epitopes may therefore gain allergenic potency. The accumulated evidence for repetitive epitope display by allergens suggests the existence of allergen-associated molecular patterns.

The first crystal structure of a glycoside hydrolase family 17 β-1,3-glucanosyltransferase displays a unique catalytic cleft

β-1,3-Glucanosyltransferase (EC 2.4.1.-) plays an important role in the formation of branched glucans, as well as in cell-wall assembly and rearrangement in fungi and yeasts. The crystal structures of a novel glycoside hydrolase (GH) family 17 β-1,3-glucanosyltransferase from Rhizomucor miehei (RmBgt17A) and the complexes of its active-site mutant (E189A) with two substrates were solved at resolutions of 1.30, 2.30 and 2.27 Å, respectively. The overall structure of RmBgt17A had the characteristic (β/α)8 TIM-barrel fold. The structures of RmBgt17A and other GH family 17 members were compared: it was found that a conserved subdomain located in the region near helix α6 and part of the catalytic cleft in other GH family 17 members was absent in RmBgt17A. Instead, four amino-acid residues exposed to the surface of the enzyme (Tyr135, Tyr136, Glu158 and His172) were found in the reducing terminus of subsite +2 of RmBgt17A, hindering access to the catalytic cleft. This distinct region of RmBgt17A makes its catalytic cleft shorter than those of other reported GH family 17 enzymes. The complex structures also illustrated that RmBgt17A can only provide subsites -3 to +2. This structural evidence provides a clear explanation of the catalytic mode of RmBgt17A, in which laminaribiose is released from the reducing end of linear β-1,3-glucan and the remaining glucan is transferred to the end of another β-1,3-glucan acceptor. The first crystal structure of a GH family 17 β-1,3-glucanosyltransferase may be useful in studies of the catalytic mechanism of GH family 17 proteins, and provides a basis for further enzymatic engineering or antifungal drug screening.