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Basirli H, Can M, Sengul T, Seyrantepe V. Lithium treatment rescues dysfunctional autophagy in the cell models of Tay-Sachs disease. Mol Genet Metab 2024; 141:108140. [PMID: 38262289 DOI: 10.1016/j.ymgme.2024.108140] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/18/2023] [Revised: 01/08/2024] [Accepted: 01/10/2024] [Indexed: 01/25/2024]
Abstract
Tay-Sachs disease is a rare lysosomal storage disorder (LSD) caused by a mutation in the HexA gene coding β-hexosaminidase A enzyme. The disruption of the HexA gene causes the accumulation of GM2 ganglioside resulting in progressive neurodegeneration in humans. Surprisingly, Hexa-/- mice did not show neurological phenotypes. Our group recently generated a murine model of Tay-Sachs disease exhibiting excessive GM2 accumulation and severe neuropathological abnormalities mimicking Tay-Sachs patients. Previously, we reported impaired autophagic flux in the brain of Hexa/-Neu3-/- mice. However, regulation of autophagic flux using inducers has not been clarified in Tay-Sachs disease cells. Here, we evaluated the effects of lithium treatment on dysfunctional autophagic flux using LC3 and p62 in the fibroblast and neuroglia of Hexa-/-Neu3-/- mice and Tay-Sachs patients. We discovered the clearance of accumulating autophagosomes, aggregate-prone metabolites, and GM2 ganglioside under lithium-induced conditions. Our data suggest that targeting autophagic flux with an autophagy inducer might be a rational therapeutic strategy for the treatment of Tay-Sachs disease.
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Affiliation(s)
- Hande Basirli
- İzmir Institute of Technology, Department of Molecular Biology and Genetics, İzmir, Turkey
| | - Melike Can
- İzmir Institute of Technology, Department of Molecular Biology and Genetics, İzmir, Turkey
| | - Tugce Sengul
- İzmir Institute of Technology, Department of Molecular Biology and Genetics, İzmir, Turkey
| | - Volkan Seyrantepe
- İzmir Institute of Technology, Department of Molecular Biology and Genetics, İzmir, Turkey; İzmir Institute of Technology, IYTEDEHAM, İzmir, Turkey.
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2
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Ibrahim DMA, Ali OSM, Nasr H, Fateen E, AbdelAleem A. Biochemical and mutational analyses of HEXA in a cohort of Egyptian patients with infantile Tay-Sachs disease. Expansion of the mutation spectrum. Orphanet J Rare Dis 2023; 18:52. [PMID: 36907859 PMCID: PMC10009996 DOI: 10.1186/s13023-023-02637-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2022] [Accepted: 02/06/2023] [Indexed: 03/14/2023] Open
Abstract
BACKGROUND Tay-Sachs disease (TSD), an autosomal recessively inherited neurodegenerative lysosomal storage disease, reported worldwide with a high incidence among population of Eastern European and Ashkenazi Jewish descent. Mutations in the alpha subunit of HEXA that encodes for the β-hexosaminidase-A lead to deficient enzyme activity and TSD phenotype. This study is the first to highlight the HEXA sequence variations spectrum in a cohort of Egyptian patients with infantile TSD. RESULTS This study involved 13 Egyptian infant/children patients presented with the infantile form of TSD, ten of the 13 patients were born to consanguineous marriages. β-hexosaminidase-A enzyme activity was markedly reduced in the 13 patients with a mean activity of 3 µmol/L/h ± 1.56. Sanger sequencing of the HEXA' coding regions and splicing junctions enabled a detection rate of ~ 62% (8/13) in our patients revealing the molecular defects in eight patients; six homozygous-mutant children (five of them were the product of consanguineous marriages) and two patients showed their mutant alleles in heterozygous genotypes, while no disease-causing mutation was identified in the remaining patients. Regulatory intragenic mutations or del/dup may underlie the molecular defect in those patients showing no relevant pathogenic sequencing variants or in the two patients with a heterozygous genotype of the mutant allele. This research identified three novel, likely pathogenic variants in association with the TSD phenotype; two missense, c.920A > C (E307A) and c.952C > G (H318D) in exon 8, and a single base deletion c.484delG causing a frameshift E162Rfs*37 (p.Glu162ArgfsTer37) in exon 5. Three recurrent disease-causing missense mutations; c.1495C > T (R499C), c.1511G > A(R504H), and c.1510C > T(R504C) in exon 13 were identified in five of the eight patients. None of the variants was detected in 50 healthy Egyptians' DNA. Five variants, likely benign or of uncertain significance, S3T, I436V, E506E, and T2T, in exons 1, 11,13, & 1 were detected in our study. CONCLUSIONS For the proper diagnostics, genetic counseling, and primary prevention, our study stresses the important role of Next Generation Sequencing approaches in delineating the molecular defect in TSD-candidate patients that showed negative Sanger sequencing or a heterozygous mutant allele in their genetic testing results. Interestingly, the three recurrent TSD associated mutations were clustered on chromosome 13 and accounted for 38% of the HEXA mutations detected in this study. This suggested exon 13 as the first candidate for sequencing screening in Egyptian patients with infantile TSD. Larger studies involving our regional population are recommended, hence unique disease associated pathogenic variations could be identified.
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Affiliation(s)
- Doaa M A Ibrahim
- Department of Medical Molecular Genetics, Human Genetics and Genome Research Institute, National Research Centre, Cairo, Egypt
| | - Ola S M Ali
- Department of Biochemistry, Faculty of Pharmacy (Girls), Al-Azhar University, Cairo, Egypt
| | - Hala Nasr
- Department of Medical Molecular Genetics, Human Genetics and Genome Research Institute, National Research Centre, Cairo, Egypt
| | - Ekram Fateen
- Department of Biochemical Genetics, Human Genetics and Genome Research Institute, National Research Centre, Cairo, Egypt.
| | - Alice AbdelAleem
- Department of Medical Molecular Genetics, Human Genetics and Genome Research Institute, National Research Centre, Cairo, Egypt.
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3
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Robb M, Robb CS, Higgins MA, Hobbs JK, Paton JC, Boraston AB. A Second β-Hexosaminidase Encoded in the Streptococcus pneumoniae Genome Provides an Expanded Biochemical Ability to Degrade Host Glycans. J Biol Chem 2015; 290:30888-900. [PMID: 26491009 DOI: 10.1074/jbc.m115.688630] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2015] [Indexed: 12/19/2022] Open
Abstract
An important facet of the interaction between the pathogen Streptococcus pneumoniae (pneumococcus) and its human host is the ability of this bacterium to process host glycans. To achieve cleavage of the glycosidic bonds in host glycans, S. pneumoniae deploys a wide array of glycoside hydrolases. Here, we identify and characterize a new family 20 glycoside hydrolase, GH20C, from S. pneumoniae. Recombinant GH20C possessed the ability to hydrolyze the β-linkages joining either N-acetylglucosamine or N-acetylgalactosamine to a wide variety of aglycon residues, thus revealing this enzyme to be a generalist N-acetylhexosaminidase in vitro. X-ray crystal structures were determined for GH20C in a ligand-free form, in complex with the N-acetylglucosamine and N-acetylgalactosamine products of catalysis and in complex with both gluco- and galacto-configured inhibitors O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino N-phenyl carbamate (PUGNAc), O-(2-acetamido-2-deoxy-D-galactopyranosylidene)amino N-phenyl carbamate (GalPUGNAc), N-acetyl-D-glucosamine-thiazoline (NGT), and N-acetyl-D-galactosamine-thiazoline (GalNGT) at resolutions from 1.84 to 2.7 Å. These structures showed N-acetylglucosamine and N-acetylgalactosamine to be recognized via identical sets of molecular interactions. Although the same sets of interaction were maintained with the gluco- and galacto-configured inhibitors, the inhibition constants suggested preferred recognition of the axial O4 when an aglycon moiety was present (Ki for PUGNAc > GalPUGNAc) but preferred recognition of an equatorial O4 when the aglycon was absent (Ki for GalNGT > NGT). Overall, this study reveals GH20C to be another tool that is unique in the arsenal of S. pneumoniae and that it may implement the effort of the bacterium to utilize and/or destroy the wide array of host glycans that it may encounter.
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Affiliation(s)
- Melissa Robb
- From the Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada V8W 3P6 and
| | - Craig S Robb
- From the Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada V8W 3P6 and
| | - Melanie A Higgins
- the Research Centre for Infectious Diseases, School of Biological Sciences, University of Adelaide, South Australia 5005, Australia
| | - Joanne K Hobbs
- From the Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada V8W 3P6 and
| | - James C Paton
- the Research Centre for Infectious Diseases, School of Biological Sciences, University of Adelaide, South Australia 5005, Australia
| | - Alisdair B Boraston
- From the Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada V8W 3P6 and
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4
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Sinici I, Yonekawa S, Tkachyova I, Gray SJ, Samulski RJ, Wakarchuk W, Mark BL, Mahuran DJ. In cellulo examination of a beta-alpha hybrid construct of beta-hexosaminidase A subunits, reported to interact with the GM2 activator protein and hydrolyze GM2 ganglioside. PLoS One 2013; 8:e57908. [PMID: 23483939 PMCID: PMC3587417 DOI: 10.1371/journal.pone.0057908] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2012] [Accepted: 01/29/2013] [Indexed: 11/19/2022] Open
Abstract
The hydrolysis in lysosomes of GM2 ganglioside to GM3 ganglioside requires the correct synthesis, intracellular assembly and transport of three separate gene products; i.e., the alpha and beta subunits of heterodimeric beta-hexosaminidase A, E.C. # 3.2.1.52 (encoded by the HEXA and HEXB genes, respectively), and the GM2-activator protein (GM2AP, encoded by the GM2A gene). Mutations in any one of these genes can result in one of three neurodegenerative diseases collectively known as GM2 gangliosidosis (HEXA, Tay-Sachs disease, MIM # 272800; HEXB, Sandhoff disease, MIM # 268800; and GM2A, AB-variant form, MIM # 272750). Elements of both of the hexosaminidase A subunits are needed to productively interact with the GM2 ganglioside-GM2AP complex in the lysosome. Some of these elements have been predicted from the crystal structures of hexosaminidase and the activator. Recently a hybrid of the two subunits has been constructed and reported to be capable of forming homodimers that can perform this reaction in vivo, which could greatly simplify vector-mediated gene transfer approaches for Tay-Sachs or Sandhoff diseases. A cDNA encoding a hybrid hexosaminidase subunit capable of dimerizing and hydrolyzing GM2 ganglioside could be incorporated into a single vector, whereas packaging both subunits of hexosaminidase A into vectors, such as adeno-associated virus, would be impractical due to size constraints. In this report we examine the previously published hybrid construct (H1) and a new more extensive hybrid (H2), with our documented in cellulo (live cell- based) assay utilizing a fluorescent GM2 ganglioside derivative. Unfortunately when Tay-Sachs cells were transfected with either the H1 or H2 hybrid construct and then were fed the GM2 derivative, no significant increase in its turnover was detected. In vitro assays with the isolated H1 or H2 homodimers confirmed that neither was capable of human GM2AP-dependent hydrolysis of GM2 ganglioside.
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Affiliation(s)
- Incilay Sinici
- Department of Biochemistry, Hacettepe University, Faculty of Medicine, Ankara, Turkey
| | - Sayuri Yonekawa
- Genetics and Genome Biology, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada
| | - Ilona Tkachyova
- Genetics and Genome Biology, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada
| | - Steven J. Gray
- Gene Therapy Center, University of North Carolina, Chapel Hill, North Carolina, United States of America
| | - R. Jude Samulski
- Gene Therapy Center, University of North Carolina, Chapel Hill, North Carolina, United States of America
| | - Warren Wakarchuk
- Ryerson University, Department of Chemistry and Biology, Toronto, Canada
| | - Brian L. Mark
- Department of Microbiology, University of Manitoba, Winnipeg, Manitoba, Canada
| | - Don J. Mahuran
- Genetics and Genome Biology, Research Institute, Hospital for Sick Children, Toronto, Ontario, Canada
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada
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5
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Sanders DN, Zeng R, Wenger DA, Johnson GS, Johnson GC, Decker JE, Katz ML, Platt SR, O'Brien DP. GM2 gangliosidosis associated with a HEXA missense mutation in Japanese Chin dogs: a potential model for Tay Sachs disease. Mol Genet Metab 2013; 108:70-5. [PMID: 23266199 DOI: 10.1016/j.ymgme.2012.11.008] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/16/2012] [Accepted: 11/17/2012] [Indexed: 12/25/2022]
Abstract
GM2 gangliosidosis is a fatal lysosomal storage disease caused by a deficiency of β-hexosaminidase (EC 3.2.1.52). There are two major isoforms of the enzyme: hexosaminidase A composed of an α and a β subunit (encoded by HEXA and HEXB genes, respectively); and, hexosaminidase B composed of two β subunits. Hexosaminidase A requires an activator protein encoded by GM2A to catabolize GM2 ganglioside, but even in the absence of the activator protein, it can hydrolyze the synthetic substrates commonly used to assess enzyme activity. GM2 gangliosidosis has been reported in Japanese Chin dogs, and we identified the disease in two related Japanese Chin dogs based on clinical signs, histopathology and elevated brain GM2 gangliosides. As in previous reports, we found normal or elevated hexosaminidase activity when measured with the synthetic substrates. This suggested that the canine disease is analogous to human AB variant of G(M2) gangliosidosis, which results from mutations in GM2A. However, only common neutral single nucleotide polymorphisms were found upon sequence analysis of the canine ortholog of GM2A from the affected Japanese Chins. When the same DNA samples were used to sequence HEXA, we identified a homozygous HEXA:c967G>A transition which predicts a p.E323K substitution. The glutamyl moiety at 323 is known to make an essential contribution to the active site of hexosaminidase A, and none of the 128 normal Japanese Chins and 92 normal dogs of other breeds that we tested was homozygous for HEXA:c967A. Thus it appears that the HEXA:c967G>A transition is responsible for the GM2 gangliosidosis in Japanese Chins.
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Affiliation(s)
- Douglas N Sanders
- Mason Eye Institute, University of Missouri School of Medicine, Columbia, MO 65211, USA
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6
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Rempel BP, Withers SG. Covalent inhibitors of glycosidases and their applications in biochemistry and biology. Glycobiology 2008; 18:570-86. [PMID: 18499865 DOI: 10.1093/glycob/cwn041] [Citation(s) in RCA: 146] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Glycoside hydrolases are important enzymes in a number of essential biological processes. Irreversible inhibitors of this class of enzyme have attracted interest as probes of both structure and function. In this review we discuss some of the compounds used to covalently modify glycosidases, their use in residue identification, structural and mechanistic investigations, and finally their applications, both in vitro and in vivo, to complex biological systems.
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Affiliation(s)
- Brian P Rempel
- Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
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7
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Borzym-Kluczyk M, Olszewska E, Radziejewska I, Lewszuk A, Zwierz K. Isoenzymes of N-acetyl-β-hexosaminidase in human pleomorphic adenoma and healthy salivary glands: a preliminary study. Clin Chem Lab Med 2008; 46:131-6. [DOI: 10.1515/cclm.2008.018] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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8
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LeCleir GR, Buchan A, Maurer J, Moran MA, Hollibaugh JT. Comparison of chitinolytic enzymes from an alkaline, hypersaline lake and an estuary. Environ Microbiol 2007; 9:197-205. [PMID: 17227424 DOI: 10.1111/j.1462-2920.2006.01128.x] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
We examined the genetic and physiological characteristics of chitin degrading enzymes expressed by fosmids cloned from two strains of chitinolytic gammaproteobacteria isolated from alkaline, hypersaline Mono Lake, California; and from a metagenomic library derived from an estuarine bacterial community (Dean Creek, Sapelo Island, GA, USA). The Mono Lake chitinolytic enzymes presented unique adaptations in terms of halo- and alkalitolerance. The sequence from one of the Mono Lake isolates (strain 12A) was a conventional family 18 glycosyl hydrolase; however, the expressed protein had a novel secondary activity peak at pH 10. We obtained a novel family 20 glycosyl hydrolase sequence from Mono Lake strain AI21. The activity of the expressed protein had a pH optimum of 10, several pH units higher than any other enzyme currently assigned to this family, and the enzyme retained 80% of its activity at pH 11. The enzyme was also halotolerant, retaining activity in salt solutions of up to 225 g l(-1). Sequence analysis indicated a molecular weight of approximately 90 kDa for the protein, and that it contained two active sites. Culture supernatant contained two chitinolytic proteins, 45 and 31 kDa, suggesting possible post-expression modification of the gene product. In contrast, the sequence found in the estuarine metagenomic library and the functional characteristics of the protein expressed from it were those of a conventional family 18 glycosyl hydrolase.
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Affiliation(s)
- Gary R LeCleir
- Department of Marine Sciences, University of Georgia, Athens, GA, USA
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9
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Lemieux MJ, Mark BL, Cherney MM, Withers SG, Mahuran DJ, James MNG. Crystallographic structure of human beta-hexosaminidase A: interpretation of Tay-Sachs mutations and loss of GM2 ganglioside hydrolysis. J Mol Biol 2006; 359:913-29. [PMID: 16698036 PMCID: PMC2910082 DOI: 10.1016/j.jmb.2006.04.004] [Citation(s) in RCA: 142] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2006] [Revised: 03/30/2006] [Accepted: 04/01/2006] [Indexed: 11/21/2022]
Abstract
Lysosomal beta-hexosaminidase A (Hex A) is essential for the degradation of GM2 gangliosides in the central and peripheral nervous system. Accumulation of GM2 leads to severely debilitating neurodegeneration associated with Tay-Sachs disease (TSD), Sandoff disease (SD) and AB variant. Here, we present the X-ray crystallographic structure of Hex A to 2.8 A resolution and the structure of Hex A in complex with NAG-thiazoline, (NGT) to 3.25 A resolution. NGT, a mechanism-based inhibitor, has been shown to act as a chemical chaperone that, to some extent, prevents misfolding of a Hex A mutant associated with adult onset Tay Sachs disease and, as a result, increases the residual activity of Hex A to a level above the critical threshold for disease. The crystal structure of Hex A reveals an alphabeta heterodimer, with each subunit having a functional active site. Only the alpha-subunit active site can hydrolyze GM2 gangliosides due to a flexible loop structure that is removed post-translationally from beta, and to the presence of alphaAsn423 and alphaArg424. The loop structure is involved in binding the GM2 activator protein, while alphaArg424 is critical for binding the carboxylate group of the N-acetyl-neuraminic acid residue of GM2. The beta-subunit lacks these key residues and has betaAsp452 and betaLeu453 in their place; the beta-subunit therefore cleaves only neutral substrates efficiently. Mutations in the alpha-subunit, associated with TSD, and those in the beta-subunit, associated with SD are discussed. The effect of NGT binding in the active site of a mutant Hex A and its effect on protein function is discussed.
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Affiliation(s)
- M. Joanne Lemieux
- CIHR Group in Protein, Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alta., Canada, T6G 2H7
| | - Brian L. Mark
- CIHR Group in Protein, Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alta., Canada, T6G 2H7
| | - Maia M. Cherney
- CIHR Group in Protein, Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alta., Canada, T6G 2H7
| | - Stephen G. Withers
- Chemistry Department, University of British Columbia, Vancouver, BC, Canada, V6T 1Z1
| | - Don J. Mahuran
- Department of Laboratory, Medicine and Pathobiology, Sick Kids Hospital, 555, University Avenue, University of Toronto, Toronto, Ont., Canada M5G 1X8
| | - Michael N. G. James
- CIHR Group in Protein, Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alta., Canada, T6G 2H7
- Corresponding author:
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10
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Koyanagi R, Honegger TG. Molecular cloning and sequence analysis of an ascidian egg beta-N-acetylhexosaminidase with a potential role in fertilization. Dev Growth Differ 2003; 45:209-18. [PMID: 12828682 DOI: 10.1046/j.1524-4725.2003.689.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Beta-N-acetylhexosaminidase, which is found almost ubiquitously in sperm of invertebrates and vertebrates, supposedly mediates a carbohydrate-based transient sperm-egg coat binding. In ascidians and mammals, beta-hexosaminidase released at fertilization from eggs has been proposed to modify sperm receptor glycoproteins of the egg envelope, thus setting up a block to polyspermy. Previously, it was shown that in potential sperm receptor glycoproteins of the ascidian Phallusia mammillata, N-acetylglucosamine is the prevailing glycoside residue and that the egg harbors three active molecular forms of beta-hexosaminidase. In the present study, P. mammillata beta-hexosaminidase cDNA was isolated from an ovarian cDNA library and characterized. The deduced amino acid sequence showed a high similarity with other known beta-hexosaminidases; however, P. mammillata beta-hexosaminidase had a unique potential N-glycosylation site. A phylogenetic analysis suggested that P. mammillata beta-hexosaminidase developed independently after having branched off from the common ancestor gene of the chordate enzyme before two isoforms of the mammalian enzyme appeared. In situ hybridization revealed stage-specific expression of beta-hexosaminidase mRNA during oogenesis in the oocyte and in the accessory test and follicle cells. This suggests that the three egg beta-hexosaminidase forms are specific for the oocyte, test cells and follicle cells.
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Affiliation(s)
- Ryo Koyanagi
- Zoological Institute, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
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11
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Mark BL, Mahuran DJ, Cherney MM, Zhao D, Knapp S, James MNG. Crystal structure of human beta-hexosaminidase B: understanding the molecular basis of Sandhoff and Tay-Sachs disease. J Mol Biol 2003; 327:1093-109. [PMID: 12662933 PMCID: PMC2910754 DOI: 10.1016/s0022-2836(03)00216-x] [Citation(s) in RCA: 164] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
In humans, two major beta-hexosaminidase isoenzymes exist: Hex A and Hex B. Hex A is a heterodimer of subunits alpha and beta (60% identity), whereas Hex B is a homodimer of beta-subunits. Interest in human beta-hexosaminidase stems from its association with Tay-Sachs and Sandhoff disease; these are prototypical lysosomal storage disorders resulting from the abnormal accumulation of G(M2)-ganglioside (G(M2)). Hex A degrades G(M2) by removing a terminal N-acetyl-D-galactosamine (beta-GalNAc) residue, and this activity requires the G(M2)-activator, a protein which solubilizes the ganglioside for presentation to Hex A. We present here the crystal structure of human Hex B, alone (2.4A) and in complex with the mechanistic inhibitors GalNAc-isofagomine (2.2A) or NAG-thiazoline (2.5A). From these, and the known X-ray structure of the G(M2)-activator, we have modeled Hex A in complex with the activator and ganglioside. Together, our crystallographic and modeling data demonstrate how alpha and beta-subunits dimerize to form either Hex A or Hex B, how these isoenzymes hydrolyze diverse substrates, and how many documented point mutations cause Sandhoff disease (beta-subunit mutations) and Tay-Sachs disease (alpha-subunit mutations).
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Affiliation(s)
- Brian L. Mark
- Canadian Institutes of Heath Research Group in Protein Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alt.,Canada T6G 2H7
| | - Don J. Mahuran
- The Research Institute, The Hospital for Sick Children, 555 University Ave, Toronto Ont., Canada M5G1X8
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ont., Canada M5G1L6
| | - Maia M. Cherney
- Canadian Institutes of Heath Research Group in Protein Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alt.,Canada T6G 2H7
| | - Dalian Zhao
- Department of Chemistry, Rutgers University, New Brunswick, NJ 08903, USA
| | - Spencer Knapp
- Department of Chemistry, Rutgers University, New Brunswick, NJ 08903, USA
| | - Michael N. G. James
- Canadian Institutes of Heath Research Group in Protein Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alt.,Canada T6G 2H7
- Corresponding author:
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12
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Sharma R, Bukovac S, Callahan J, Mahuran D. A single site in human beta-hexosaminidase A binds both 6-sulfate-groups on hexosamines and the sialic acid moiety of GM2 ganglioside. BIOCHIMICA ET BIOPHYSICA ACTA 2003; 1637:113-8. [PMID: 12527415 PMCID: PMC2910081 DOI: 10.1016/s0925-4439(02)00221-1] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Human beta-hexosaminidase A (Hex A) (alphabeta) is composed of two subunits whose primary structures are approximately 60% identical. Deficiency of either subunit results in severe neurological disease due to the storage of GM2 ganglioside; Tay-Sachs disease, alpha deficiency, and Sandhoff disease, beta deficiency. Whereas both subunits contain active sites only the alpha-site can efficiently bind negatively charged 6-sulfated hexosamine substrates and GM2 ganglioside. We have recently identified the alphaArg(424) as playing a critical role in the binding of 6-sulfate-containing substrates, and betaAsp(452) as actively inhibiting their binding. To determine if these same residues affect the binding of the sialic acid moiety of GM2 ganglioside, an alphaArg(424)Gln form of Hex A was expressed and its kinetics analyzed using the GM2 activator protein:[3H]-GM2 ganglioside complex as a substrate. The mutant showed a approximately 3-fold increase in its K(m) for the complex. Next a form of Hex B (betabeta) containing a double mutation, betaAspLeu(453)AsnArg (duplicating the alpha-aligning sequences), was expressed. As compared to the wild type (WT), the mutant exhibited a >30-fold increase in its ability to hydrolyze a 6-sulfated substrate and was now able to hydrolyze GM2 ganglioside when the GM2 activator protein was replaced by sodium taurocholate. Thus, this alpha-site is critical for binding both types of negatively charge substrates.
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Affiliation(s)
- Rohita Sharma
- Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada M5G 2C4
| | - Scott Bukovac
- Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada M5G 2C4
| | - John Callahan
- Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8
- Department of Biochemistry, University of Toronto, Toronto, Ontario, Canada M5G 2C4
| | - Don Mahuran
- Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8
- Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada M5G 2C4
- Corresponding author. Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. Tel.: +1-416-813-6161; fax: +1-416-813-8700., (D. Mahuran)
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13
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Affiliation(s)
- R J Desnick
- Department of Human Genetics, Mount Sinai School of Medicine of New York University, New York 10029, USA
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14
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Affiliation(s)
- R L Proia
- Genetics of Development and Disease Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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15
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Mahuran DJ, Gravel RA. The beta-hexosaminidase story in Toronto: from enzyme structure to gene mutation. ADVANCES IN GENETICS 2002; 44:145-63. [PMID: 11596980 DOI: 10.1016/s0065-2660(01)44077-6] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/21/2023]
Affiliation(s)
- D J Mahuran
- The Research Institute, The Hospital for Sick Children and Department of Laboratory Medicine and Pathobiology University of Toronto, Ontario, Canada.
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16
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Wells L, Gao Y, Mahoney JA, Vosseller K, Chen C, Rosen A, Hart GW. Dynamic O-glycosylation of nuclear and cytosolic proteins: further characterization of the nucleocytoplasmic beta-N-acetylglucosaminidase, O-GlcNAcase. J Biol Chem 2002; 277:1755-61. [PMID: 11788610 DOI: 10.1074/jbc.m109656200] [Citation(s) in RCA: 185] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
beta-O-linked N-acetylglucosamine (O-GlcNAc) is an abundant and dynamic post-translational modification implicated in protein regulation that appears to be functionally more similar to phosphorylation than to classical glycosylation. There are nucleocytoplasmic enzymes for the attachment and removal of O-GlcNAc. Here, we further characterize the recently cloned beta-N-acetylglucosaminidase, O-GlcNAcase. Both recombinant and purified endogenous O-GlcNAcase rapidly release free GlcNAc from O-GlcNAc-modified peptide substrates. The recombinant enzyme functions as a monomer and has kinetic parameters (K(m) = 1.1 mm for paranitrophenyl-GlcNAc, k(cat) = 1 s(-1)) that are similar to those of lysosomal hexosaminidases. The endogenous O-GlcNAcase appears to be in a complex with other proteins and is predominantly localized to the cytosol. Overexpression of the enzyme in living cells results in decreased O-GlcNAc modification of nucleocytoplasmic proteins. Finally, we show that the enzyme is a substrate for caspase-3 but, surprisingly, the cleavage has no effect on in vitro O-GlcNAcase activity. These studies support the identification of this protein as an O-GlcNAcase and identify important interactions and modifications that may regulate the enzyme and O-GlcNAc cycling.
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Affiliation(s)
- Lance Wells
- Department of Biological Chemistry and Medicine, The Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA
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17
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Sharma R, Deng H, Leung A, Mahuran D. Identification of the 6-sulfate binding site unique to alpha-subunit-containing isozymes of human beta-hexosaminidase. Biochemistry 2001; 40:5440-6. [PMID: 11331008 PMCID: PMC2910086 DOI: 10.1021/bi0029200] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
In humans, beta-hexosaminidase A (alphabeta) is required to hydrolyze GM2 ganglioside. A deficiency of either the alpha- or beta-subunit leads to a severe neurological disease, Tay-Sachs or Sandhoff disease, respectively. In mammals beta-hexosaminidase B (betabeta) and S (alphaalpha) are other major and minor isozymes. The primary structures of the alpha- and beta-subunits are 60% identical, but only the alpha-containing isozymes can efficiently hydrolyze beta-linked GlcNAc-6-SO(4) from natural or artificial substrates. Hexosaminidase has been grouped with glycosidases in family 20. A molecular model of the active site of the human hexosaminidase has been generated from the crystal structure of a family 20 bacterial chitobiase. We now use the chitobiase structure to identify residues close to the carbon-6 oxygen of NAG-A, the nonreducing beta-GlcNAc residue of its bound substrate. The chitobiase side chains in the best interactive positions align with alpha-Asn(423)Arg(424) and beta-Asp(453)Leu(454). The change in charge from positive in alpha to negative in beta is consistent with the lower K(m) of hexosaminidase S, and the much higher K(m) and lower pH optimum of hexosaminidase B, toward sulfated versus unsulfated substrates. In vitro mutagenesis, CHO cell expression, and kinetic analyses of an alphaArg(424)Lys hexosaminidase S detected little change in V(max) but a 2-fold increase in K(m) for the sulfated substrate. Its K(m) for the nonsulfated substrate was unaffected. When alphaAsn(423) was converted to Asp, again only the K(m) for the sulfated substrate was changed, increasing by 6-fold. Neutralization of the charge on alphaArg(424) by substituting Gln produced a hexosaminidase S with a K(m) decrease of 3-fold and a V(max) increased by 6-fold for the unsulfated substrate, parameters nearly identical to those of hexosaminidase B at pH 4.2. As well, for the sulfated substrate at pH 4.2 its K(m) was increased 9-fold and its V(max) decreased 1.5-fold, values very similar to those of hexosaminidase B obtained at pH 3.0, where its betaAsp(453) becomes protonated.
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Affiliation(s)
| | | | | | - Don Mahuran
- To whom correspondence should be addressed at The Research Institute, The Hospital for Sick Children, 555 University Ave., Toronto, Ontario, Canada M5G 1X8. Telephone: 416-813-6161. Fax: 416-813-8700.
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18
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Mark BL, Vocadlo DJ, Knapp S, Triggs-Raine BL, Withers SG, James MN. Crystallographic evidence for substrate-assisted catalysis in a bacterial beta-hexosaminidase. J Biol Chem 2001; 276:10330-7. [PMID: 11124970 DOI: 10.1074/jbc.m011067200] [Citation(s) in RCA: 206] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
beta-Hexosaminidase, a family 20 glycosyl hydrolase, catalyzes the removal of beta-1,4-linked N-acetylhexosamine residues from oligosaccharides and their conjugates. Heritable deficiency of this enzyme results in various forms of GalNAc-beta(1,4)-[N-acetylneuraminic acid (2,3)]-Gal-beta(1,4)-Glc-ceramide gangliosidosis, including Tay-Sachs disease. We have determined the x-ray crystal structure of a beta-hexosaminidase from Streptomyces plicatus to 2.2 A resolution (Protein Data Bank code ). beta-Hexosaminidases are believed to use a substrate-assisted catalytic mechanism that generates a cyclic oxazolinium ion intermediate. We have solved and refined a complex between the cyclic intermediate analogue N-acetylglucosamine-thiazoline and beta-hexosaminidase from S. plicatus to 2.1 A resolution (Protein Data Bank code ). Difference Fourier analysis revealed the pyranose ring of N-acetylglucosamine-thiazoline bound in the enzyme active site with a conformation close to that of a (4)C(1) chair. A tryptophan-lined hydrophobic pocket envelopes the thiazoline ring, protecting it from solvolysis at the iminium ion carbon. Within this pocket, Tyr(393) and Asp(313) appear important for positioning the 2-acetamido group of the substrate for nucleophilic attack at the anomeric center and for dispersing the positive charge distributed into the oxazolinium ring upon cyclization. This complex provides decisive structural evidence for substrate-assisted catalysis and the formation of a covalent, cyclic intermediate in family 20 beta-hexosaminidases.
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Affiliation(s)
- B L Mark
- Medical Research Council Group in Protein Structure and Function, Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
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19
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Hou Y, Vocadlo DJ, Leung A, Withers SG, Mahuran D. Characterization of the Glu and Asp residues in the active site of human beta-hexosaminidase B. Biochemistry 2001; 40:2201-9. [PMID: 11329289 PMCID: PMC2910085 DOI: 10.1021/bi002018s] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Human beta-hexosaminidase A (alpha beta) and B (beta beta) are composed of subunits (alpha and beta) that are 60% identical and have been grouped with other evolutionarily related glycosidases into "Family 20". The three-dimensional structure of only one Family 20 member has been elucidated, a bacterial chitobiase. This enzyme shares primary structure homology with both the human subunits only in its active-site region, and even in this restricted area, the level of identity is only 26%. Thus, the validity of the molecular model for the active site of the human enzyme based on chitobiase must be determined experimentally. In this report, we analyze highly purified mutant forms of human hexosaminidase B that have had conservative substitutions made at Glu and Asp residues predicted by the chitobiase model to be part of its active site. Mutation of beta Glu(355) to Gln reduces k(cat) 5000-fold with only a small effect on K(m), while also shifting the pH optimum. These effects are consistent with assignment of this residue as the acid/base catalytic residue. Similarly, mutation of beta Asp(354) to Asn reduced k(cat) 2000-fold while leaving K(m) essentially unaltered, consistent with assignment of this residue as the residue that interacts with the substrate acetamide group to promote its attack on the anomeric center. These data in conjunction with the mutagenesis studies of Asp(241) and Glu(491) indicate that the molecular model is substantially accurate in its identification of catalytically important residues.
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Affiliation(s)
| | | | | | | | - Don Mahuran
- To whom correspondence should be addressed. Phone: (416) 813-6161. Fax: (416) 813-8700.
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20
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Abstract
Enzymatic hydrolysis of glycosides can occur by one of two elementary mechanisms identified by the stereochemical outcome of the reaction, inversion or retention. The key active-site residues involved are a pair of carboxylic acids in each case, and strategies for their identification and for probing the details of their roles in catalysis have been developed through detailed kinetic analysis of mutants. Similarly the roles of other active-site residues have also been probed this way, and mutants have been developed that trap intermediates in catalysis, allowing the determination of the three-dimensional structures of several such key species. By manipulating the locations or even the presence of these carboxyl side chains in the active site, the mechanisms of several glycosidases have been completely changed, and this has allowed the development of "glycosynthases," mutant glycosidases that are capable of synthesizing oligosaccharides but unable to degrade them. Surprisingly little progress has been made on altering specificities through mutagenesis, although recent results suggest that gene shuffling coupled with effective screens will provide the most effective approach.
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Affiliation(s)
- H D Ly
- Department of Chemistry, University of British Columbia, Vancouver, Canada
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21
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Prag G, Papanikolau Y, Tavlas G, Vorgias CE, Petratos K, Oppenheim AB. Structures of chitobiase mutants complexed with the substrate Di-N-acetyl-d-glucosamine: the catalytic role of the conserved acidic pair, aspartate 539 and glutamate 540. J Mol Biol 2000; 300:611-7. [PMID: 10884356 DOI: 10.1006/jmbi.2000.3906] [Citation(s) in RCA: 68] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The catalytic domain of chitobiase (beta-N-1-4 acetylhexosaminidase) from Serratia marcescens, is an alpha/beta TIM-barrel. This enzyme belongs to family 20 of glycosyl hydrolases in which a conserved amino acid pair, aspartate-glutamate, is present (Asp539-Glu540). It was proposed that catalysis by this enzyme family is carried out by glutamate 540 acting as a proton donor and by the acetamido group of the substrate as a nucleophile. We investigated the role of Asp539 and Glu540 by site-directed mutagenesis, biochemical characterization and by structural analyses of chitobiase -substrate co-crystals. We found that both residues are essential for chitobiase activity. The mutations, however, led to subtle changes in the catalytic site. Our results support the model that Glu540 acts as the proton donor and that Asp539 acts in several different ways. Asp539 restrains the acetamido group of the substrate in a specific orientation by forming a hydrogen bond with N2 of the non-reduced (-1) sugar. In addition, this residue participates in substrate binding. It is also required for the correct positioning of Glu540 and may provide additional negative charge at the active site. Thus, these biochemical and structural studies provide a molecular explanation for the functional importance and conservation of these residues.
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Affiliation(s)
- G Prag
- The Department of Molecular Genetics and Biotechnology, The Hebrew University-Hadassah Medical School, Jerusalem, 91120, Israel
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22
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Karpati M, Peleg L, Gazit E, Akstein E, Goldman B. A novel mutation in the HEXA gene specific to Tay-Sachs disease carriers of Jewish Iraqi origin. Clin Genet 2000; 57:398-400. [PMID: 10852376 DOI: 10.1034/j.1399-0004.2000.570512.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
An increased frequency of carriers of 1:140, as defined by reduced hexosaminidase A (HexA) activity, was observed among Iraqi Jews participating in the Tay-Sachs disease (TSD) carrier detection program. Prior to this finding, TSD among Jews had been restricted to those of Eastern European (Ashkenazi) and Moroccan descent with carrier frequencies of 1:29 and 1:110 for Jews of Ashkenazi and Moroccan extraction, respectively. A general, pan-ethnic frequency of approximately 1:280 has been observed among other Jewish Israeli populations. Analysis of 48 DNA samples from Iraqi Jews suspected, by enzymatic assay, to be carriers revealed a total of five mutations, one of which was novel. In nine carriers (19%), a known mutation typical to either Ashkenazi or Moroccan Jews was identified. DeltaF304/ 305 was detected in four individuals, and + 1278TATC in three. G269S and R170Q each appeared in a single person. The new mutation, G749T, resulting in a substitution of glycine to valine at position 250 has been found in 19 of the DNA samples (40%). This mutation was not detected among 100 non-carrier, Iraqi Jews and 65 Ashkenazi enzymatically determined carriers. Aside from Ashkenazi and Moroccan Jews, a specific mutation in the HEXA gene has now also been identified in Jews of Iraqi descent.
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Affiliation(s)
- M Karpati
- Genetic Institute, Sheba Medical Center, Tel Hashomer, Israel
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23
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Kaplan F. Tay-Sachs disease carrier screening: a model for prevention of genetic disease. GENETIC TESTING 1999; 2:271-92. [PMID: 10464605 DOI: 10.1089/gte.1998.2.271] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Tay-Sachs disease (TSD) is an autosomal-recessive, progressive, and ultimately fatal neurodegenerative disorder. Within the last 30 years, the discovery of the enzymatic basis of the disease, namely deficiency of the enzyme hexosaminidase A, made possible both enzymatic diagnosis of TSD and heterozygote identification. In the last decade, the cloning of the HEXA gene and the identification of more than 80 associated TSD-causing mutations has permitted molecular diagnosis in many instances. TSD was the first genetic condition for which community-based screening for carrier detection was implemented. As such, the TSD experience can be viewed as a prototypic effort for public education, carrier testing, and reproductive counseling for avoiding fatal childhood disease. More importantly, the outcome of TSD screening over the last 28 years offers convincing evidence that such an effort can dramatically reduce incidence of the disease.
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Affiliation(s)
- F Kaplan
- McGill University-Montreal Children's Hospital Research Institute, Quebec, Canada.
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24
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25
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Mark BL, Wasney GA, Salo TJ, Khan AR, Cao Z, Robbins PW, James MN, Triggs-Raine BL. Structural and functional characterization of Streptomyces plicatus beta-N-acetylhexosaminidase by comparative molecular modeling and site-directed mutagenesis. J Biol Chem 1998; 273:19618-24. [PMID: 9677388 DOI: 10.1074/jbc.273.31.19618] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
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.
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Affiliation(s)
- B L Mark
- Department of Biochemistry and Molecular Biology, University of Manitoba, Winnipeg, Manitoba, R3E 0W3, Canada
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26
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Affiliation(s)
- S V Hodgson
- South Thames Regional Genetics Centre (East), Guy's Hospital, Division of Medical and Molecular Genetics, London, UK
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27
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Abstract
Glycosphingolipids are ubiquitous membrane components of eukaryotic cells. They participate in various cell recognition events and can regulate enzymes and receptors within the plasma membrane. Sphingolipidoses are due to an impaired lysosomal digestion of these substances. Glycosphingolipids are degraded by the action of exohydrolases, which are supported, in the case of glycosphingolipids with short oligosaccharide chains, by sphingolipid activator proteins. Five sphingolipid activator proteins are known so far, the GM2-activator and the SAPs, SAP-A to D (also called saposins). Degradation of glycosphingolipids requires endocytic membrane flow of plasma membrane derived glycosphingolipids into the lysosomes. Recent research focused on the topology of this process and on the mechanism and physiological function of sphingolipid activator proteins. Limited knowledge is available about enzymology and topology of glycosphingolipid biosynthesis. Recently, intermediates of this metabolic pathway have been identified as novel signalling molecules. Inhibition of glycosphingolipid biosynthesis has been shown to be beneficial in the animal model of Tay-Sachs disease. Mice with disrupted genes for lysosomal hydrolases and activator proteins are useful models for known human diseases and are valuable tools for the study of glycosphingolipid metabolism, the pathogenesis of sphingolipidoses and novel therapeutic approaches.
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Affiliation(s)
- Thomas Kolter
- KekuléInstitut für Organische Chemie und Biochemie der Universität, Bonn, Germany
| | - Konrad Sandhoff
- KekuléInstitut für Organische Chemie und Biochemie der Universität, Bonn, Germany
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28
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Yadao F, Hechtman P, Kaplan F. Formation of a ternary complex between GM2 activator protein, GM2 ganglioside and hexosaminidase A. BIOCHIMICA ET BIOPHYSICA ACTA 1997; 1340:45-52. [PMID: 9217013 DOI: 10.1016/s0167-4838(97)00027-7] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The GM2 activator is a 17 kDa protein required for the hydrolysis of GM2 ganglioside by the lysosomal enzyme hexosaminidase A (HexA). The activator behaves as a substrate binding protein, solubilizing GM2 ganglioside monomers from micelles (in vitro) or membranes (in vivo). However, the activator also shows a high order of specificity for activation of lysosomal hydrolases and has been predicted to form a ternary complex with the heterodimeric enzyme (alphabeta) Hex A and GM2 ganglioside. We demonstrated a transient interaction between HexA and the GM2 activator. A chimeric protein containing the FLAG epitope sequence upstream of the GM2 activator was expressed in Escherichia coli and purified using the M1 immunoaffinity (anti-FLAG) column. Binding of the FLAG-GM2 activator (FLAG-AP) fusion protein to the M1 column led to the specific retardation of Hex A applied to the column. Other proteins were not retarded by the column nor did they compete with Hex A for binding to FLAG-AP. Hex A and GM2 ganglioside could be simultaneously bound to the column, but the binding of each ligand was independent of the other. The homodimeric (beta beta) isozyme Hex B did not bind to the immobilized activator. The alpha alpha homodimer, HexS, bound weakly, confirming that a hexosaminidase alpha subunit is required for interaction of enzyme and activator.
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Affiliation(s)
- F Yadao
- McGill University-Montreal Children's Hospital Research Institute, Montreal, Canada
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29
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Pennybacker M, Schuette CG, Liessem B, Hepbildikler ST, Kopetka JA, Ellis MR, Myerowitz R, Sandhoff K, Proia RL. Evidence for the involvement of Glu-355 in the catalytic action of human beta-hexosaminidase B. J Biol Chem 1997; 272:8002-6. [PMID: 9065471 DOI: 10.1074/jbc.272.12.8002] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
In a previous study the photoactivable affinity probe, 3-azi-1-[([6-3H]2-acetamido-2-deoxy-1-beta-D-galactopyranosyl)thio ]-b utane, was used to identify the active site of beta-hexosaminidase B, a beta-subunit dimer (Liessem, B., Glombitza, G. J., Knoll, F., Lehmann, J., Kellermann, J., Lottspeich, F., and Sandhoff, K. (1995) J. Biol. Chem. 270, 23693-23699). The probe predominately labeled Glu-355, a highly conserved residue among hexosaminidases. To determine if Glu-355 has a role in catalysis, beta-subunit mutants were prepared with the Glu-355 codon altered to either Ala, Gln, Asp, or Trp. After expression of mutant proteins using recombinant baculovirus, the enzyme activity associated with the beta-subunits was found to be reduced to background levels. Although catalytic activity was lost, the mutations did not otherwise affect the folding or assembly of the subunits. The mutant beta-subunits could be isolated using substrate affinity chromatography, indicating they contained intact substrate binding sites. As shown by cross-linking with disuccinimidyl suberate, the mutant beta-subunits were properly assembled. They could also participate in the formation of functional beta-hexosaminidase A activity as indicated by activator-dependent GM2 ganglioside degradation activity produced by co-expression of the mutant beta-subunits with the alpha-subunit. Finally, the mutant subunits showed normal lysosomal processing in COS-1 cells, demonstrating that a transport-competent protein conformation had been attained. Collectively the results provide strong support for the intimate involvement of Glu-355 in beta-hexosaminidase B-mediated catalysis.
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Affiliation(s)
- M Pennybacker
- Section on Biochemical Genetics, Genetics and Biochemistry Branch, NIDDK, National Institutes of Health, Bethesda, Maryland 20892, USA
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