The primary structure of an Entamoeba histolytica beta-hexosaminidase A subunit

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Molecular cloning and functional characterization of beta-N-acetylglucosaminidase genes from Sf9 cells

Sf9, a cell line derived from the lepidopteran insect, Spodoptera frugiperda, is widely used as a host for recombinant glycoprotein expression and purification by baculovirus vectors. Previous studies have shown that this cell line has one or more beta-N-acetylglucosaminidase activities that may be involved in the degradation and/or processing of N-glycoprotein glycans. However, these enzymes and their functions remain poorly characterized. Therefore, the goal of this study was to isolate beta-N-acetylglucosaminidase genes from Sf9 cells, over-express the gene products, and characterize their enzymatic activities. A degenerate PCR approach yielded three Sf9 cDNAs, which appeared to encode two distinct beta-N-acetylglucosaminidases, according to bioinformatic analyses. Baculovirus-mediated expression of these two cDNA products induced membrane-associated beta-N-acetylglucosaminidase activities in Sf9 cells, which cleaved terminal N-acetylglucosamine residues from the alpha-3 and -6 branches of a biantennary N-glycan substrate with acidic pH optima and completely hydrolyzed chitotriose to its constituent N-acetylglucosamine monomers. GFP-tagged forms of both enzymes exhibited punctate cytoplasmic fluorescence, which did not overlap with either lysosomal or Golgi-specific dyes. Together, these results indicated that the two new Sf9 genes identified in this study encode broad-spectrum beta-N-acetylglucosaminidases that appear to have unusual intracellular distributions. Their relative lack of substrate specificity and acidic pH optima are consistent with a functional role for these enzymes in glycoprotein glycan and chitin degradation, but not with a role in N-glycoprotein glycan processing.

The beta-N-acetylhexosaminidase of Entamoeba histolytica is composed of two homologous chains and has been localized to cytoplasmic granules

We have purified a beta-N-acetylhexosaminidase from trophozoites of Entamoeba histolytica to homogeneity. In SDS-PAGE, the enzyme yielded a single protein band at an apparent M(r) of 64,000. The elution behaviour of the native enzyme upon molecular sieve chromatography corresponded to a molecular mass of approximately 132,000 suggesting that the enzyme is a dimer. Upon sedimentation velocity centrifugation, hexosaminidase activity sedimented at 12S, implying aggregation to a higher molecular mass complex with an apparent M(r) of approximately 400,000. Based on the N-terminal sequence of the purified enzyme and on data extracted from the E. histolytica genomic data base, we amplified and cloned two genes (EhHEXA and EhHEXB) coding for two presumptive, highly similar hexosaminidase chains which we designated as Ehhexalpha and Ehhexbeta. Northern blot analysis indicated that the two genes were expressed to a similar level, and Western blotting with chain-specific antisera showed that the trophozoites synthesize both proteins. By cell fractionation, the hexosaminidase was found to be a major component of cytoplasmic granules; these contain tissue-destructive factors and are released after collagen-induced exocytosis to the cell surface. In agreement with this observation, immunocytochemistry with an antiserum cross-reacting with both hexosaminidase chains revealed strong fluorescence in surface patches, which we interpret as released granules, and in vesicles throughout the cell. Its localization in cytoplasmic granules strengthens the notion that the hexosaminidase complex may contribute to amoebic pathogenicity.

Compound heterozygosity with two novel mutations in the HEXB gene produces adult Sandhoff disease presenting as a motor neuron disease phenotype

Little information is available on molecular defects involved in adult Sandhoff disease presenting as motor neuron disease phenotype. We studied enzyme activities of beta-hexosaminidase (Hex) and the HEXB gene encoding the beta-subunit of Hex in a family of the Japanese case. Enzyme assay with 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside revealed a reduction in Hex A and B activity in proband's leukocytes. Although the activity of both in the mother were intermediate between those of controls and the proband, only Hex B reduction determined with heat inactivation was found in the father. Analysis of HEXB gene demonstrated two novel point mutations. The first mutation, IVS2-1G>A, was located at the 3'-splice acceptor site of intron 2 derived from the mother, causing exon 3 skipping. The resultant mRNA encoded a shorter beta-chain, which may not form an active enzyme. The second mutation was a G-to-A transition in exon 13 (c.1598G>A) derived from the father and resulted in arginine-to-histidine substitution at amino acid position 533 (R533H). Expression of R533H mutation in COS-1 cells demonstrated a lack of normal Hex activity, indicating that this mutation is pathological. Compound heterozygosity of these two mutations may trigger the development of adult Sandhoff disease with a motor neuron disease phenotype.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Structural and functional characterization of Streptomyces plicatus beta-N-acetylhexosaminidase by comparative molecular modeling and site-directed mutagenesis

We have sequenced the Streptomyces plicatus beta-N-acetylhexosaminidase (SpHex) gene and identified the encoded protein as a member of family 20 glycosyl hydrolases. This family includes human beta-N-acetylhexosaminidases whose deficiency results in various forms of GM2 gangliosidosis. Based upon the x-ray structure of Serratia marcescens chitobiase (SmChb), we generated a three-dimensional model of SpHex by comparative molecular modeling. The overall structure of the enzyme is very similar to homology modeling-derived structures of human beta-N-acetylhexosaminidases, with differences being confined mainly to loop regions. From previous studies of the human enzymes, sequence alignments of family 20 enzymes, and analysis of the SmChb x-ray structure, we selected and mutated putative SpHex active site residues. Arg162 --> His mutation increased Km 40-fold and reduced Vmax 5-fold, providing the first biochemical evidence for this conserved Arg residue (Arg178 in human beta-N-acetylhexosaminidase A (HexA) and Arg349 in SmChb) as a substrate-binding residue in a family 20 enzyme, a finding consistent with our three-dimensional model of SpHex. Glu314 --> Gln reduced Vmax 296-fold, reduced Km 7-fold, and altered the pH profile, consistent with it being the catalytic acid residue as suggested by our model and other studies. Asp246 --> Asn reduced Vmax 2-fold and increased Km only 1.2-fold, suggesting that Asp246 may play a lesser role in the catalytic mechanism of this enzyme. Taken together with the x-ray structure of SmChb, these studies suggest a common catalytic mechanism for family 20 glycosyl hydrolases.

The effects of Entamoeba histolytica lysates on human colonic mucins

Detergent lysates of Entamoeba histolytica trophozoites contained high levels of beta-N acetyl-D-glucosaminidase, beta-N acetyl-D-galactosaminidase and alpha-D-galactosidase activity, and lower but significant levels of five other glycosidases. Although these activities should have been capable of largely degrading the oligosaccharide side-chains of human colonic mucin, in fact only about one third of high MW mucin was degraded in 72 h and trypsin alone produced a similar effect. There was no evidence that these glycosidases were excreted and we conclude that they are unlikely to represent significant virulence factors for E. histolytica.

Beta-N-acetylhexosaminidase: a target for the design of antifungal agents

This review provides biochemical, analytical, and biological background information relating to beta-N-acetylhexosaminidase (HexNAc'ase; EC 3.2.1.52) as an emerging target for the design of low-molecular-weight antifungals. The article includes the following: (1) a biochemical description of HexNAc'ase (reaction catalyzed, nomenclature, and mechanism of action) that sets it apart from other, similar enzymes; (2) an overview and a critical evaluation of methods to assay the enzyme, including in crude extracts (photo- and fluorometric procedures with model substrates; HPLC/pulsed amperometric detection of N-acetylglucosamine and chito-oligomers; end-point vs. rate measurements); (3) a summary of some general characteristics of HexNAc'ases from fungi and organisms of other types (Km values, substrate preference, and glycoconjugation); (4) an hypothesis of a specific target function of wall-associated HexNAc'ase (a component of the assembly of surface-located enzymes effecting a continuous turnover and remodelling of the wall fabric through its combined hydrolytic and transglycosylating activities, and a mediator enzyme acting in concert with chitinase and chitin synthase to provide for the controlled lysis and synthesis of chitin during growth); (5) a tabulation of the structural formulae of reaction-based HexNAc'ase inhibitors with Ki values < or = 100 microM (some of them representing transition state mimics that could serve as leads for the development of new antifungals); and (6) an outline of approaches towards the establishment of a three-dimensional model of HexNAc'ase suitable for a truly rational design of antimycotics as well as agricultural fungicides.

Identification of an active acidic residue in the catalytic site of beta-hexosaminidase

Human beta-hexosaminidases A and B (EC 3.2.1.52) are dimeric lysosomal glycosidases composed of evolutionarily related alpha and/or beta subunits. Both isozymes hydrolyze terminal beta-linked GalNAc or GlcNAc residues from numerous artificial and natural substrates; however, in vivo GM2 ganglioside is a substrate for only the heterodimeric A isozyme. Thus, mutations in either gene encoding its alpha or beta subunits can result in GM2 ganglioside storage and Tay-Sachs or Sandhoff disease, respectively. All glycosyl hydrolases ae believed to have one or more acidic residues in their catalytic site. We demonstrate that incubation of hexosaminidase with a chemical modifier specific for carboxyl side chains produces a time-dependent loss of activity, and that this effect can be blocked by the inclusion of a strong competitive inhibitor in the reaction mix. We hypothesized that the catalytic acid residue(s) should be located in a region of overall homology and be invariant within the aligned deduced primary sequences of the human alpha and beta subunits, as well as hexosaminidases from other species, including bacteria. Such a region is encoded by exons 5-6 of the HEXA and HEXB genes. This region includes beta Arg211 (invariant in 15 sequences), which we have previously shown to be an active residue. This region also contains two invariant and one conserved acidic residues. A fourth acidic residue, Asp alpha 258, beta 290, in exon 7 was also investigated because of its association with the B1 variant of Tay-Sachs disease. Conservative substitutions were made at each candidate residue by in vitro mutagenesis of a beta cDNA, followed by cellular expression. Of these, only the beta Asp196Asn substitution decreased the kcat (350-910-fold) without any noticeable effect on the K(m). Mutagenesis of either beta Asp240 or beta Asp290 to Asn decreased kcat by 10- or 1.4-fold but also raised the K(m) of the enzyme 11- of 3-fold, respectively. The above results strongly suggest that beta Asp196 is a catalytic acid residue in beta-hexosaminidase.