Induction of β-N-acetylhexosaminidase in Aspergillus oryzae

Sequencing, cloning and high-yield expression of a fungal β-N-acetylhexosaminidase in Pichia pastoris

The β-N-acetylhexosaminidase from Talaromyces flavus has a remarkable synthetic ability, processing even carbohydrates with various functionalities. Its broader use is partially hampered by low-yield production in the native fungus. Here, we present an optimized 3-day production of this enzyme in the eukaryotic host of Pichia pastoris, in ca 10-fold higher volume activity (10 U/ml) and close-to-perfect purity (one chromatographic step needed). Importantly, the recombinant enzyme features the same biochemical and catalytic properties, including the syntheses with derivatized carbohydrate substrates. This is the first example of the overexpression of a fungal β-N-acetylhexosaminidase by a single-cell producer in liquid medium. It represents a promising solution for wider biotechnological applications of this outstanding enzyme.

Crystallization and diffraction analysis of β-N-acetylhexosaminidase from Aspergillus oryzae

Fungal β-N-acetylhexosaminidases are enzymes that are used in the chemoenzymatic synthesis of biologically interesting oligosaccharides. The enzyme from Aspergillus oryzae was produced and purified from its natural source and crystallized using the hanging-drop vapour-diffusion method. Diffraction data from two crystal forms (primitive monoclinic and primitive tetragonal) were collected to resolutions of 3.2 and 2.4 Å, respectively. Electrophoretic and quantitative N-terminal protein-sequencing analyses confirmed that the crystals are formed by a complete biologically active enzyme consisting of a glycosylated catalytic unit and a noncovalently attached propeptide.

Structure of the dimeric N-glycosylated form of fungal beta-N-acetylhexosaminidase revealed by computer modeling, vibrational spectroscopy, and biochemical studies

Background

Fungal β-N-acetylhexosaminidases catalyze the hydrolysis of chitobiose into its constituent monosaccharides. These enzymes are physiologically important during the life cycle of the fungus for the formation of septa, germ tubes and fruit-bodies. Crystal structures are known for two monomeric bacterial enzymes and the dimeric human lysosomal β-N-acetylhexosaminidase. The fungal β-N-acetylhexosaminidases are robust enzymes commonly used in chemoenzymatic syntheses of oligosaccharides. The enzyme from Aspergillus oryzae was purified and its sequence was determined.

Results

The complete primary structure of the fungal β-N-acetylhexosaminidase from Aspergillus oryzae CCF1066 was used to construct molecular models of the catalytic subunit of the enzyme, the enzyme dimer, and the N-glycosylated dimer. Experimental data were obtained from infrared and Raman spectroscopy, and biochemical studies of the native and deglycosylated enzyme, and are in good agreement with the models. Enzyme deglycosylated under native conditions displays identical kinetic parameters but is significantly less stable in acidic conditions, consistent with model predictions. The molecular model of the deglycosylated enzyme was solvated and a molecular dynamics simulation was run over 20 ns. The molecular model is able to bind the natural substrate – chitobiose with a stable value of binding energy during the molecular dynamics simulation.

Conclusion

Whereas the intracellular bacterial β-N-acetylhexosaminidases are monomeric, the extracellular secreted enzymes of fungi and humans occur as dimers. Dimerization of the fungal β-N-acetylhexosaminidase appears to be a reversible process that is strictly pH dependent. Oligosaccharide moieties may also participate in the dimerization process that might represent a unique feature of the exclusively extracellular enzymes. Deglycosylation had only limited effect on enzyme activity, but it significantly affected enzyme stability in acidic conditions. Dimerization and N-glycosylation are the enzyme's strategy for catalytic subunit stabilization. The disulfide bridge that connects Cys⁴⁴⁸ with Cys⁴⁸³ stabilizes a hinge region in a flexible loop close to the active site, which is an exclusive feature of the fungal enzymes, neither present in bacterial nor mammalian structures. This loop may play the role of a substrate binding site lid, anchored by a disulphide bridge that prevents the substrate binding site from being influenced by the flexible motion of the loop.

Large Propeptides of Fungal β-N-Acetylhexosaminidases Are Novel Enzyme Regulators That Must Be Intracellularly Processed to Control Activity, Dimerization, and Secretion into the Extracellular Environment

Filamentous fungi produce and secrete beta-N-acetylhexosaminidases, Hex, as important components of the binary chitinolytic systems involved in the formation of septa and hyphenation. Enzyme reconstitution experiments published previously indicate that Hex can occur in the form of two molecular species containing either one or two molecules of the propeptide noncovalently associated with the enzyme dimer. Here, we describe a novel mechanism for the regulation of the activity of Hex based on the association of their catalytic subunits with the large N-terminal propeptides in vivo. We show that the enzyme precursor is processed early in the biosynthesis, shortly after the addition of N-glycans through the action of a dibasic peptidase, cleaving both before and after the dibasic sequence. The processing site for this unique dibasic peptidase, different from that of kexins, is conserved among the beta-N-acetylhexosaminidases from filamentous fungi, and inhibition of the dibasic peptidase abrogates enzyme folding and activation. Binding of the released propeptide to the catalytic subunit of Hex is essential for its activation. An examination of the kinetics of Hex activation and dimerization in vitro allowed us to understand the unusually high efficiency of the assembly of this enzyme. We also report that the fungus is able to actively regulate the concentration of the processed propeptide in endoplasmic reticulum and thus the specific activity of the produced Hex. This novel regulatory mechanism enables the control of the catalytic activity and architecture of the secreted enzyme according to the needs of the producing cell at various stages of its growth cycle.

N-glycosylated catalytic unit meets O-glycosylated propeptide: complex protein architecture in a fungal hexosaminidase

β-N-Acetylhexosaminidase from a filamentous fungus Aspergillus oryzae is a secreted enzyme known to be an important component of the binary chitinolytic system. Cloning of the hexA gene and sequencing of the enzyme revealed its unique preproprotein structure. While the enzyme's zincin-like and catalytic domain had significant similarities with members of the glycohydrolase 20 family, the propeptide was unique for the fungal enzyme. Detailed pulse–chase and inhibition studies revealed that propeptide was processed during the biosynthesis of the enzyme. Moreover, the presence of propeptide was necessary for enzyme activation, dimerization and secretion. The catalytic unit was N-glycosylated, and the propeptide was O-glycosylated, both in their C-terminal parts. Deglycosylation experiments revealed that the N-glycosylation increased the stability and solubility of the enzyme. In contrast, O-glycosylated propeptide was necessary to attain the full enzymic activity.

Enzymatic synthesis of complex glycosaminotrioses and study of their molecular recognition by hevein domains

Hevein, a protein found in Hevea brasiliensis, has a CRD domain, which is known to bind chitin and GlcNAc-containing oligosaccharides. By using NMR and molecular modeling as major tools we have demonstrated that trisaccharides containing GalNAc and ManNAc residues are also recognized by hevein domains. Thus far unknown trisaccharides GlcNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (1) and GalNAcbeta(1-->4)GlcNAcbeta(1-->4)ManNAc (2) were synthesized with the use of beta-N-acetylhexosaminidase from Aspergillus oryzae. This method is based on the rather unique phenomenon that some fungal beta-N-acetylhexosaminidases cannot hydrolyze disaccharide GlcNAcbeta(1-->4)ManNAc (5) contrary to chitobiose GlcNAcbeta(1-->4)GlcNAc (4) that is cleaved and, therefore, cannot be used as an acceptor for further transglycosylation. Both trisaccharides 1 and 2 were prepared by transglycosylation from disaccharidic acceptor in good yields ranging from 35% to 40%. Our observations strongly indicate that the present nature of the modifications of chitotriose (GlcNAcbeta(1-->lcNAcbeta(1-->4)GlcNAc, 3) at either the non-reducing end (GalNAc instead of GlcNAc) or at the reducing end (ManNAc instead of GlcNAc) do not modify the mode of binding of the trisaccharide to hevein. The association constant values indicate that chitotriose (3) binding is better than that of 1 and 2, and that the binding of (with ManNAc at the reducing end) is favored with respect to that of 2 (with ManNAc at the reducing end with a non-reducing GalNAc moiety).

Fungal beta-N-acetylhexosaminidases with high beta-N-acetylgalactosaminidase activity and their use for synthesis of beta-GalNAc-containing oligosaccharides

About 60 fungal strains were tested for production of extracellular beta-N-acetylhexosaminidases. A unique beta-N-acetylhexosaminidase with the beta-GalNAc-ase/beta-GlcNAc-ase ratio of 2.3-2.8 was found in the culture filtrates of some strains of Penicillium oxalicum. Addition of 20% (w/v) MgSO(4) increased the beta-GalNAc-ase/beta-GlcNAc-ase ratio to the value of 3.35. Cultivation conditions influence this ratio as well. beta-N-Acetylhexosaminidases from P. oxalicum CCF 2430 and Aspergillus oryzae CCF 1066 considerably differing in the GalNAc-ase activity were used for the synthesis of the following structures beta-D-GalpNAc-(1-->4)-D-GlcpNAc, beta-D-GalpNAc-(1-->6)-D-GlcpNAc, beta-D-GalpNAc-(1-->6)-D-GalpNAc, beta-D-GalpNAc-(1-->4)-alpha-D-GlcpNAcOAll and beta-D-GalpNAc-(1-->6)-beta-D-Galp-(1-->4)-alpha-D-GlcpNAcOAll to demonstrate the application of these new enzymes.

Toward an optimal oligosaccharide ligand for rat natural killer cell activation receptor NKR-P1

Aminosugars have a good affinity for the NKR-P1A protein, the major activating receptor at the surface of rat natural killer cells. We have systematically investigated the structural requirements of the recombinant soluble dimeric form of the receptor for its optimal carbohydrate ligands. While N-acetylD-mannosamine was the best neutral monosaccharide ligand, its participation in the context of an extended oligosaccharide sequence was equally important. The IC(50) value for the GalNAcbeta1 --> ManNAc disaccharide was nearly 10(-10) M with a further possible increase depending on the type of the glycosidic linkage and the aglycon nature. From the point of view of its availability, stability, and affinity for the receptor and a potential in vivo use, these studies are pivotal for the design of an oligosaccharide or glycomimetics suitable for further clustering into the multivalent glycodendrimers.

Enzymatic glycosylation using 6-O-acylated sugar donors and acceptors: beta-N-acetylhexosaminidase-catalysed synthesis of 6-O,N,N'-triacetylchitobiose and 6'-O,N,N'-triacetylchitobiose

p-Nitrophenyl 6-O-acetyl-2-acetamido-2-deoxy-beta-D-glucopyranoside (5a) was used as the glycosyl donor in a beta-N-acetylhexosaminidase-catalysed (from Penicillium brasilianum) glycosylation of GlcNAc yielding 6'-O,N,N'-triacetylchitobiose (6), while 6-O-acetyl-2-acetamido-2-deoxy-beta-D-glucopyranose (3a) served as a selectively protected acceptor in a transglycosylation reaction catalysed by the same enzyme to yield 6-O,N,N'-triacetylchitobiose (4).