Hexosaminidase
Encyclopedia
Hexosaminidase is an enzyme
involved in the hydrolysis
of terminal N-acetyl-D-hexosamine residues in N-acetyl-β-D-hexosaminides.
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:
The α and β subunits are encoded by separate genes, HEXA
and HEXB
respectively. Beta-hexosaminidase and the cofactor GM2 activator protein
catalyze the degradation of the GM2 gangliosides and other molecules containing terminal N-acetyl hexosamines. 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
.
-424, and a loop structure that forms from the amino acid sequence in the alpha subunit. The loop in the alpha subunit , consisting of Gly
-280, Ser
-281, Glu
-282, and 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.
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.
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.
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.
Children born with Tay-Sachs usually die between two to 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.
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. The two subunits of hexosaminidase A are shown below:
gene possesses both hexosaminidase and histone acetyltransferase
activities. NCOAT is also known as hexosaminidase C and has distinct substrate specificities compared to lysosomal hexosaminidase A. A single-nucleotide polymorphism in the human O-GlcNAcase gene is linked to diabetes mellitus type 2
.
A fourth mammalian hexosaminidase polypeptide which has been designated hexosaminidase D (HEXDC) has recently been identified.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
involved in the hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of terminal N-acetyl-D-hexosamine residues in N-acetyl-β-D-hexosaminides.
Lysosomal A, B, and S isozymes
Functional lysosomalLysosome
thumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s: [[nucleoli]] [[cell nucleus|nucleus]] [[ribosomes]] [[vesicle |vesicle]] rough [[endoplasmic reticulum]]...
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:
| hexosaminidase isozyme |
subunit composition | function |
|---|---|---|
| A | α/β heterodimer | only isozyme that can hydrolyze GM2 ganglioside in vivo |
| B | β/β homodimer | exists in tissues but no known physiological function |
| S | α/α homodimer | exists in tissues but no known physiological function |
The α and β subunits are encoded by separate genes, HEXA
HEXA
Hexosaminidase A , also known as HEXA, is an enzyme that in humans is encoded by the HEXA gene.Hexosaminidase A and the cofactor GM2 activator protein catalyze the degradation of the GM2 gangliosides and other molecules containing terminal N-acetyl hexosamines...
and HEXB
HEXB
Beta-hexosaminidase subunit beta is an enzyme that in humans is encoded by the HEXB gene.-Further reading:...
respectively. Beta-hexosaminidase and the cofactor GM2 activator protein
GM2A
GM2 ganglioside activator also known as GM2A is a protein which in humans is encoded by the GM2A gene.- Function :The protein encoded by this gene is a small glycolipid transport protein which acts as a substrate specific co-factor for the lysosomal enzyme β-hexosaminidase A...
catalyze the degradation of the GM2 gangliosides and other molecules containing terminal N-acetyl hexosamines. Gene mutations in HEXB often result in Sandhoff disease
Sandhoff disease
Sandhoff disease, also known as Sandhoff-Jatzkewitz disease, variant 0 of GM2-Gangliosidosis or Hexosaminidase A and B deficiency, is a lysosomal genetic, lipid storage disorder caused by the inherited deficiency to create functional beta-hexosaminidases A and B...
; whereas, mutations in HEXA decrease the hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of GM2 gangliosides, which is the main cause of Tay-Sachs disease
Tay-Sachs disease
Tay–Sachs disease is an autosomal recessive genetic disorder...
.
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, ArgArginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
-424, and a loop structure that forms from the amino acid sequence in the alpha subunit. The loop in the alpha subunit , consisting of Gly
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
-280, Ser
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
-281, Glu
Glutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
-282, and Pro
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
-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
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
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.
Mechanism of action
A Michaelis complex consisting of a glutamateGlutamic acid
Glutamic acid is one of the 20 proteinogenic amino acids, and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salts of glutamic acid are known as glutamates...
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.
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 diseaseTay-Sachs disease
Tay–Sachs disease is an autosomal recessive genetic disorder...
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.
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.
Children born with Tay-Sachs usually die between two to four years of age from aspiration and pneumonia
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
. 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.
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
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
and is directed to the lysosome
Lysosome
thumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s: [[nucleoli]] [[cell nucleus|nucleus]] [[ribosomes]] [[vesicle |vesicle]] rough [[endoplasmic reticulum]]...
where it can perform the degradation of GM2 gangliosides. The two subunits of hexosaminidase A are shown below:
Cytosolic C and D isozymes
The bifunctional protein NCOAT (nuclear cytoplasmic O-GlcNAcase and acetyltransferase) that is encoded by the MGEA5MGEA5
Bifunctional protein NCOAT is a protein that in humans is encoded by the MGEA5 gene.-Further reading:...
gene possesses both hexosaminidase and histone acetyltransferase
Histone acetyltransferase
Histone acetyltransferases are enzymes that acetylate conserved lysine amino acids on histone proteins by transferring an acetyl group from acetyl CoA to form ε-N-acetyl lysine....
activities. NCOAT is also known as hexosaminidase C and has distinct substrate specificities compared to lysosomal hexosaminidase A. A single-nucleotide polymorphism in the human O-GlcNAcase gene is linked to diabetes mellitus type 2
Diabetes mellitus type 2
Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...
.
A fourth mammalian hexosaminidase polypeptide which has been designated hexosaminidase D (HEXDC) has recently been identified.