Galactose-1-phosphate uridylyltransferase
Encyclopedia
Galactose-1-phosphate uridylyltransferase (or GALT) is an enzyme
responsible for converting ingested galactose
to glucose
.
Galactose-1-phosphate uridylyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose metabolism, namely:
The expression of GALT is controlled by the actions of the FOXO3 gene. The absence of this enzyme results in classic galactosemia in humans and can be fatal in the newborn period if lactose is not removed from the diet. The pathophysiology of galactosemia has not been clearly defined.
metabolism through ping pong bi-bi kinetics
with a double displacement mechanism. This means that the net reaction consists of two reactants and two products (see reaction above), and it proceeds by the following mechanism: the enzyme reacts with one substrate to generate one product and a modified enzyme, which goes on to react with the second substrate to make the second product while regenerating the original enzyme. In the case of GALT, the His166 residue acts as a potent nucleophile
to facilitate transfer of a nucleotide between UDP-hexoses and hexose-1-phosphates.
) of GALT was discovered by Wedekind, Frey, and Rayment, and their structural analysis has found key amino acids essential for GALT function.
The important amino acids that Wedekind et al. found in their structural analysis of GALT, such as Leu4, Phe75, Asn77, Asp78, Phe79, and Val108, are consistent with residues that have been implicated both in point mutation experiments as well as in clinical screening to play a role in human galactosemia.
is a childhood disease of hereditary nature. The autosomal recessive trait affects approximately 1 in every 40,000-60,000 live-born infants. Classical galactosemia (G/G) is caused by a deficiency in GALT activity, whereas the more common clinical affliction, Duarte/Classica (D/G) arises from attenuation of GALT activity. Symptoms include ovarian failure, dyspraxia
(difficulty speaking correctly and consistently), and neurologic deficits. A single mutation in any of several amino acids can lead to attenuation or deficiency in GALT activity. For example, a single mutation from A to G in exon 6 of the GALT gene changes Glu188 to an arginine
, and a mutation from A to G in exon 10 converts Asn314 to an aspartic acid
. These two mutations also add new restriction enzyme
cut sites, which enable detection by and large-scale population screening with PCR (polymerase chain reaction
). Screening has mostly eliminated neonatal death by G/G galactosemia, but the disease, due to GALT’s role in the biochemical metabolism
of ingested galactose
(which is toxic when accumulated) to the energetically useful glucose
, can certainly be fatal. However, those afflicted with galactosemia can live relatively normal lives by avoiding milk products and anything else containing galactose (since it cannot be metabolized), although there is the potential for problems in neurological development, or other complications, even in those who avoid galactose.
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...
responsible for converting ingested galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
to glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
.
Galactose-1-phosphate uridylyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose metabolism, namely:
- UDP-glucose + galactose 1-phosphate
glucose 1-phosphate + UDP-galactose
The expression of GALT is controlled by the actions of the FOXO3 gene. The absence of this enzyme results in classic galactosemia in humans and can be fatal in the newborn period if lactose is not removed from the diet. The pathophysiology of galactosemia has not been clearly defined.
Mechanism
GALT catalyzes the second reaction of the Leloir pathway of galactoseGalactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
metabolism through ping pong bi-bi kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
with a double displacement mechanism. This means that the net reaction consists of two reactants and two products (see reaction above), and it proceeds by the following mechanism: the enzyme reacts with one substrate to generate one product and a modified enzyme, which goes on to react with the second substrate to make the second product while regenerating the original enzyme. In the case of GALT, the His166 residue acts as a potent nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
to facilitate transfer of a nucleotide between UDP-hexoses and hexose-1-phosphates.
- UDP-glucose + E-His Glucose-1-phosphate + E-His-UMP
- Galactose-1-phosphate + E-His-UMP UDP-galactose + E-His
Structural studies
The three-dimensional structure at 1.8 angstrom resolution (x-ray crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
) of GALT was discovered by Wedekind, Frey, and Rayment, and their structural analysis has found key amino acids essential for GALT function.
The important amino acids that Wedekind et al. found in their structural analysis of GALT, such as Leu4, Phe75, Asn77, Asp78, Phe79, and Val108, are consistent with residues that have been implicated both in point mutation experiments as well as in clinical screening to play a role in human galactosemia.
Clinical significance
Deficiency of GALT causes classic galactosemia. GalactosemiaGalactosemia
Galactosemia is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galactose properly. Although the sugar lactose can metabolize to galactose, galactosemia is not related to and should not be confused with lactose intolerance...
is a childhood disease of hereditary nature. The autosomal recessive trait affects approximately 1 in every 40,000-60,000 live-born infants. Classical galactosemia (G/G) is caused by a deficiency in GALT activity, whereas the more common clinical affliction, Duarte/Classica (D/G) arises from attenuation of GALT activity. Symptoms include ovarian failure, dyspraxia
Dyspraxia
Developmental dyspraxia is a motor learning difficulty that can affect planning of movements and co-ordination as a result of brain messages not being accurately transmitted to the body...
(difficulty speaking correctly and consistently), and neurologic deficits. A single mutation in any of several amino acids can lead to attenuation or deficiency in GALT activity. For example, a single mutation from A to G in exon 6 of the GALT gene changes Glu188 to an arginine
Arginine
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...
, and a mutation from A to G in exon 10 converts Asn314 to an aspartic acid
Aspartic acid
Aspartic acid is an α-amino acid with the chemical formula HOOCCHCH2COOH. The carboxylate anion, salt, or ester of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins...
. These two mutations also add new restriction enzyme
Restriction enzyme
A Restriction Enzyme is an enzyme that cuts double-stranded DNA at specific recognition nucleotide sequences known as restriction sites. Such enzymes, found in bacteria and archaea, are thought to have evolved to provide a defense mechanism against invading viruses...
cut sites, which enable detection by and large-scale population screening with PCR (polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
). Screening has mostly eliminated neonatal death by G/G galactosemia, but the disease, due to GALT’s role in the biochemical metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
of ingested galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
(which is toxic when accumulated) to the energetically useful glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, can certainly be fatal. However, those afflicted with galactosemia can live relatively normal lives by avoiding milk products and anything else containing galactose (since it cannot be metabolized), although there is the potential for problems in neurological development, or other complications, even in those who avoid galactose.