Hexosaminidase

2018-10-22T01:04:08Z

130.132.173.239: copyedit

{{enzyme
| Name = β-N-acetylhexosaminidase
| EC_number = 3.2.1.52
| CAS_number = 9012-33-3
| IUBMB_EC_number = 3/2/1/52
| GO_code = 0016231
| image = Hexosaminidase A (heterodimer, with van der Waals interactions).jpg
| width =
| caption = Hexosaminidase A (Hex A)
}}
'''Hexosaminidase''' ({{EC number|3.2.1.52}}, ''beta-acetylaminodeoxyhexosidase'', ''N-acetyl-beta-D-hexosaminidase'', ''N-acetyl-beta-hexosaminidase'', ''N-acetyl hexosaminidase'', ''beta-hexosaminidase'', ''beta-acetylhexosaminidinase'', ''beta-D-N-acetylhexosaminidase'', ''beta-N-acetyl-D-hexosaminidase'', ''beta-N-acetylglucosaminidase'', ''hexosaminidase A'', ''N-acetylhexosaminidase'', ''beta-D-hexosaminidase'') is an [[enzyme]] involved in the [[hydrolysis]] of terminal N-acetyl-D-[[hexosamine]] residues in N-acetyl-β-D-hexosaminides.<ref name="pmid2529847">{{cite journal | author = Cabezas JA | title = Some comments on the type references of the official nomenclature (IUB) for β-''N''-acetylglucosaminidase, β-''N''-acetylhexosaminidase and β-''N''-acetylgalactosaminidase | journal = Biochem. J. | volume = 261 | issue = 3 | pages = 1059–60 |date=August 1989 | pmid = 2529847 | pmc = 1138940 | doi = }}</ref><ref name="pmid33660">{{cite journal |vauthors=Calvo P, Reglero A, Cabezas JA | title = Purification and properties of β-''N''-acetylhexosaminidase from the mollusc ''Helicella ericetorum'' Müller | journal = Biochem. J. | volume = 175 | issue = 2 | pages = 743–50 |date=November 1978 | pmid = 33660 | pmc = 1186125 | doi = }}</ref><ref name="pmid6055190">{{cite journal |vauthors=Frohwein YZ, Gatt S | title = Isolation of β-''N''-acetylhexosaminidase, β-''N''-acetylglucosaminidase, and β-''N''-acetylgalactosaminidase from calf brain | journal = Biochemistry | volume = 6 | issue = 9 | pages = 2775–82 |date=September 1967 | pmid = 6055190 | doi = 10.1021/bi00861a018}}</ref><ref name="pmid5506280">{{cite journal |vauthors=Li SC, Li YT | title = Studies on the glycosidases of jack bean meal. 3. Crystallization and properties of β-''N''-acetylhexosaminidase | journal = J. Biol. Chem. | volume = 245 | issue = 19 | pages = 5153–60 |date=October 1970 | pmid = 5506280 | doi = }}</ref>

== Isozymes and genes ==

=== Lysosomal A, B, and S isozymes ===

Functional [[lysosome|lysosomal]] β-hexosaminidase enzymes are dimeric in structure. Three isozymes are produced through the combination of α and β subunits to form any one of three active dimers:<ref name="pmid8672428">{{cite journal |vauthors=Hou Y, Tse R, Mahuran DJ | title = Direct determination of the substrate specificity of the alpha-active site in heterodimeric beta-hexosaminidase | journal = Biochemistry | volume = 35 | issue = 13 | pages = 3963–9 |date=April 1996 | pmid = 8672428 | doi = 10.1021/bi9524575 | url = }}</ref>

{| class="wikitable" border="1"
|-
! hexosaminidase isozyme
! subunit composition
! function
|-
| align="center" | A
| align="center" | α/β heterodimer
| align="center" | only isozyme that can hydrolyze GM2 ganglioside in vivo
|-
| align="center" | B
| align="center" | β/β homodimer
| align="center" | exists in tissues but no known physiological function
|-
| align="center" | S
| align="center" | α/α homodimer
| align="center" | exists in tissues but no known physiological function
|}

The α and β subunits are encoded by separate genes, ''[[HEXA]]'' and ''[[HEXB]]'' respectively. Beta-hexosaminidase and the cofactor [[GM2A|GM2 activator protein]] catalyze the degradation of the GM2 [[gangliosides]] and other molecules containing terminal N-acetyl hexosamines.<ref>{{cite journal |vauthors=Knapp S, Vocadlo D, Gao Z, Kirk B, Lou J, Withers SG | title = NAG-thiazoline, an N-acetylbeta-hexosaminidase inhibitor that implicates acetamido participation | journal = J. Am. Chem. Soc. | volume = 118 | issue = 28 | pages = 6804–6805 | year = 1996 | month = | pmid = | pmc = | doi = 10.1021/ja960826u }}</ref> Gene mutations in ''HEXB'' often result in [[Sandhoff disease]]; whereas, [[mutations]] in ''HEXA'' decrease the [[hydrolysis]] of GM2 gangliosides, which is the main cause of [[Tay–Sachs disease]].<ref name="pmid12662933">{{cite journal |vauthors=Mark BL, Mahuran DJ, Cherney MM, Zhao D, Knapp S, James MN | title = Crystal Structure of Human β-Hexosaminidase B: Understanding the Molecular Basis of Sandhoff and Tay–Sachs Disease | journal = J. Mol. Biol. | volume = 327 | issue = 5 | pages = 1093–109 |date=April 2003 | pmid = 12662933 | pmc = 2910754 | doi = 10.1016/S0022-2836(03)00216-X| url = }}</ref>

{|
|{{infobox protein
| Name = β-hexosaminidase subunit alpha
| caption =
| image =
| width =
| HGNCid = 4878
| Symbol = [[HEXA]]
| AltSymbols =
| EntrezGene = 3073
| OMIM = 606869
| RefSeq = NM_000520
| UniProt = P06865
| PDB =
| ECnumber = 3.2.1.52
| Chromosome = 15
| Arm = q
| Band = 24.1
| LocusSupplementaryData =
}}
|{{infobox protein
| Name = β-hexosaminidase subunit beta
| caption =
| image =
| width =
| HGNCid = 4879
| Symbol = [[HEXB]]
| AltSymbols =
| EntrezGene = 3074
| OMIM = 606873
| RefSeq = NM_000521
| UniProt = P07686
| PDB =
| ECnumber = 3.2.1.52
| Chromosome = 5
| Arm = q
| Band = 13.3
| LocusSupplementaryData =
}}
|}

==== Function ====

Even though the alpha and beta subunits of lysosomal hexosaminidase can both cleave GalNAc residues, only the alpha subunit is able to hydrolyze GM2 gangliosides because of a key residue, [[arginine|Arg]]-424, and a loop structure that forms from the amino acid sequence in the alpha subunit. The loop in the alpha subunit, consisting of [[glycine|Gly]]-280, [[serine|Ser]]-281, [[glutamic acid|Glu]]-282, and [[proline|Pro]]-283 which is absent in the beta subunit, serves as an ideal structure for the binding of the GM2 activator protein (GM2AP), and arginine is essential for binding the N-acetyl-neuraminic acid residue of GM2 gangliosides. The GM2 activator protein transports GM2 gangliosides and presents the [[lipid]]s to hexosaminidase, so a functional hexosaminidase enzyme is able to hydrolyze GM2 gangliosides into GM3 gangliosides by removing the N-acetylgalactosamine (GalNAc) residue from GM2 gangliosides.<ref name=Lemieux/>

==== Mechanism of action ====

A Michaelis complex consisting of a [[glutamic acid|glutamate]] residue, a GalNAc residue on the GM2 ganglioside, and an [[aspartate]] residue leads to the formation of an oxazolinium ion intermediate. A glutamate residue (alpha Glu-323/beta Glu-355) works as an acid by donating its hydrogen to the glycosidic oxygen atom on the GalNAc residue. An aspartate residue (alpha Asp-322/beta Asp-354) positions the C2-acetamindo group so that it can be attacked by the nucleophile (N-acetamido oxygen atom on carbon 1 of the substrate). The aspartate residue stabilizes the positive charge on the nitrogen atom in the oxazolinium ion intermediate. Following the formation of the oxazolinium ion intermediate, water attacks the electrophillic acetal carbon. Glutamate acts as a base by deprotonating the water leading to the formation of the product complex and the GM3ganglioside.<ref name=Lemieux/>

{{multiple image
| align = center
| width = 450
| footer =
| image1 = Gangs_Mechanism.jpg
| alt1 =
| caption1 = Hydrolysis of GM2 ganglioside to GM3 ganglioside catalyzed by hexosaminidase A.<ref name=Lemieux/>
| image2 = Mechanism.jpg
| alt2 =
| caption2 = The mechanism of the hydrolysis of a GM2 ganglioside and removal of a GalNAc residue to produce GM3 ganglioside.<ref name=Lemieux/>
}}

==== Gene mutations resulting in Tay–Sachs disease ====
There are numerous mutations that lead to hexosaminidase deficiency including gene deletions, nonsense mutations, and missense mutations. [[Tay–Sachs disease]] occurs when hexosaminidase A loses its ability to function. People with Tay–Sachs disease are unable to remove the GalNAc residue from the GM2 ganglioside, and as a result, they end up storing 100 to 1000 times more GM2 gangliosides in the brain than the normal person. Over 100 different mutations have been discovered just in infantile cases of Tay–Sachs disease alone.<ref name="Nyhan">{{cite book |vauthors=Ozand PT, Nyhan WL, Barshop BA | title = Atlas of metabolic diseases | edition = | language = | publisher = Hodder Arnold | location = London | year = 2005 | origyear = | pages = 539–546 | quote = | isbn = 0-340-80970-1 | oclc = | doi = | url = | accessdate = | chapter = Part Thirteen Lipid Storage Disorders: Tay-Sachs disease/hexosaminidase A deficiency }}</ref>

The most common mutation, which occurs in over 80 percent of Tay–Sachs patients, results from a four base pair addition (TATC) in exon 11 of the Hex A gene. This insertion leads to an early stop [[codon]], which causes the Hex A deficiency.<ref name="pmid7887427">{{cite journal |vauthors=Boles DJ, Proia RL | title = The molecular basis of HEXA mRNA deficiency caused by the most common Tay–Sachs disease mutation | journal = Am. J. Hum. Genet. | volume = 56 | issue = 3 | pages = 716–24 |date=March 1995 | pmid = 7887427 | pmc = 1801160 | doi = | url = }}</ref>

Children born with Tay–Sachs usually die between two and four years of age from aspiration and [[pneumonia]]. Tay–Sachs causes cerebral degeneration and blindness. Patients also experience flaccid extremities and seizures. At this point in time, there has been no cure or effective treatment of Tay–Sachs disease.<ref name=Nyhan/>

NAG-thiazoline, NGT, acts as mechanism based inhibitor of hexosaminidase A. In patients with Tay–Sachs disease (misfolded hexosaminidase A), NGT acts as a molecular chaperone by binding in the active site of hexosaminidase A which helps create a properly folded hexosaminidase A. The stable dimer conformation of hexosaminidase A has the ability to leave the [[endoplasmic reticulum]] and is directed to the [[lysosome]] where it can perform the degradation of GM2 gangliosides.<ref name="Lemieux">{{cite journal |vauthors=Lemieux MJ, Mark BL, Cherney MM, Withers SG, Mahuran DJ, James MN | title = Crystallographic Structure of Human β-Hexosaminidase A: Interpretation of Tay-Sachs Mutations and Loss of GM2 Ganglioside Hydrolysis | journal = J. Mol. Biol. | volume = 359 | issue = 4 | pages = 913–29 |date=June 2006 | pmid = 16698036 | pmc = 2910082 | doi = 10.1016/j.jmb.2006.04.004 | url = }}</ref> The two subunits of hexosaminidase A are shown below:
{{multiple image
| align = center
| width = 400
| footer =
| image1 = Final_Alpha_Active_Site.jpg
| alt1 =
| caption1 = The alpha subunit active site shown bound to NAG-thiazoline (NGT) in beta-hexosaminidase. {{PDB|2GK1}} The light green outline surrounding NGT represents the Van der Waals surface of NGT. The critical amino acids in the active site that are able to hydrogen bond with NGT include Arginine 178 and Glutamate 462.<ref name=Lemieux/>
| image2 = Beta_Active_Site.jpg
| alt2 =
| caption2 = The beta subunit active site shown bound to NAG-thiazoline (NGT) in beta-hexosaminidase. {{PDB|2GK1}} The light blue outline surrounding NGT represents the Van der Waals surface of NGT. The critical amino acids in the active site that are able to hydrogen bond with NGT include Glutamate 491 and Aspartate 452.<ref name=Lemieux/>
}}

=== Cytosolic C and D isozymes ===

The bifunctional protein NCOAT ('''n'''uclear '''c'''ytoplasmic '''O'''-GlcNAcase and '''a'''cetyl'''t'''ransferase) that is encoded by the ''[[MGEA5]]'' gene possesses both hexosaminidase and [[histone acetyltransferase]] activities.<ref name="pmid16356930">{{cite journal |vauthors=Toleman CA, Paterson AJ, Kudlow JE | title = The histone acetyltransferase NCOAT contains a zinc finger-like motif involved in substrate recognition | journal = J. Biol. Chem. | volume = 281 | issue = 7 | pages = 3918–25 |date=February 2006 | pmid = 16356930 | doi = 10.1074/jbc.M510485200 | url = }}</ref> NCOAT is also known as hexosaminidase C<ref name="pmid945735">{{cite journal |vauthors=Besley GT, Broadhead DM | title = Studies on human N-acetyl-Beta-d-hexosaminidase C separated from neonatal brain | journal = Biochem. J. | volume = 155 | issue = 1 | pages = 205–8 |date=April 1976 | pmid = 945735 | pmc = 1172820 | doi = | url = }}</ref> and has distinct substrate specificities compared to lysosomal hexosaminidase A.<ref name="pmid11148210">{{cite journal |vauthors=Gao Y, Wells L, Comer FI, Parker GJ, Hart GW | title = Dynamic O-glycosylation of nuclear and cytosolic proteins: cloning and characterization of a neutral, cytosolic beta-N-acetylglucosaminidase from human brain | journal = J. Biol. Chem. | volume = 276 | issue = 13 | pages = 9838–45 |date=March 2001 | pmid = 11148210 | doi = 10.1074/jbc.M010420200 | url = }}</ref> A [[single-nucleotide polymorphism]] in the human O-GlcNAcase gene is linked to [[diabetes mellitus type 2]].<ref name="pmid16882729">{{cite journal |vauthors=Forsythe ME, Love DC, Lazarus BD, Kim EJ, Prinz WA, Ashwell G, Krause MW, Hanover JA | title = Caenorhabditis elegans ortholog of a diabetes susceptibility locus: oga-1 (O-GlcNAcase) knockout impacts O-GlcNAc cycling, metabolism, and dauer | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 103 | issue = 32 | pages = 11952–7 |date=August 2006 | pmid = 16882729 | pmc = 1567679 | doi = 10.1073/pnas.0601931103 | url = }}</ref>

A fourth mammalian hexosaminidase polypeptide which has been designated hexosaminidase D (''HEXDC'') has recently been identified.<ref name="pmid19040401">{{cite journal |vauthors=Gutternigg M, Rendić D, Voglauer R, Iskratsch T, Wilson IB | title = MAMMALIAN CELLS CONTAIN A SECOND NUCLEOCYTOPLASMIC HEXOSAMINIDASE | journal = Biochem. J. | volume = 419 | issue = 1 | pages = 83–90 |date=April 2009 | pmid = 19040401 | pmc = 2850170 | doi = 10.1042/BJ20081630 | url = }}</ref>

{|
|{{infobox protein
| Name = hexosaminidase C
| caption =
| image =
| width =
| HGNCid = 7056
| Symbol = [[MGEA5]]
| AltSymbols =
| EntrezGene = 10724
| OMIM = 604039
| RefSeq = NM_012215
| UniProt = O60502
| PDB =
| ECnumber = 3.2.1.52
| Chromosome = 10
| Arm = q
| Band = 24.1-24.3
| LocusSupplementaryData =
}}
|{{infobox protein
| Name = hexosaminidase D
| caption =
| image =
| width =
| HGNCid = 26307
| Symbol = HEXDC
| AltSymbols = FLJ23825
| EntrezGene = 284004
| OMIM =
| RefSeq = NM_173620
| UniProt = Q8IYN4
| PDB =
| ECnumber = 3.2.1.52
| Chromosome = 17
| Arm = q
| Band = 25.3
| LocusSupplementaryData =
}}
|}

== References ==
{{Reflist}}

== External links ==
* [https://www.ncbi.nlm.nih.gov/books/NBK1218/ GeneReviews/NCBI/NIH/UW entry on hexosaminidase A deficiency, Tay–Sachs disease]
* {{MeshName|hexosaminidase A}}
* {{EC number|3.2.1.52}}

{{Sphingolipid metabolism enzymes}}
{{Glycoprotein catabolism}}
{{Sugar hydrolases}}
{{Enzymes}}
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[[Category:EC 3.2.1]]

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Hexosaminidase