Galactose

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For the EP by The Sweet Science, see Galactose (EP).
d-Galactose
Haworth projection of
β-d-galactopyranose
Fischer projection of
d-galactose
Names
IUPAC name
Galactose
Systematic IUPAC name
(2S,3R,4S,5R,6R)-6-(hydroxymethyl)oxane-2,3,4,5-tetrol
Other names
Brain sugar
Identifiers
3D model (
JSmol
)
1724619
ChEBI
ChEMBL
ChemSpider
IUPHAR/BPS
KEGG
MeSH Galactose
UNII
  • InChI=1S/C6H12O6/c7-1-2-3(8)4(9)5(10)6(11)12-2/h2-11H,1H2/t2−,3+,4+,5−,6+/m1/s1 checkY
    Key: WQZGKKKJIJFFOK-PHYPRBDBSA-N checkY
  • InChI=1/C6H12O6/c7-1-2-3(8)4(9)5(10)6(11)12-2/h2-11H,1H2/t2−,3+,4+,5−,6+/m1/s1
    Key: WQZGKKKJIJFFOK-PHYPRBDBBU
  • O[C@H]1[C@@H](O)[C@H](O[C@H](O)[C@@H]1O)CO
Properties
C6H12O6
Molar mass 180.156 g·mol−1
Appearance White solid[1]
Odor Odorless[1]
Density 1.5 g/cm3[1]
Melting point 168–170 °C (334–338 °F; 441–443 K)[1]
650 g/L (20 °C)[1]
-103.00·10−6 cm3/mol
Pharmacology
V04CE01 (WHO) V08DA02 (WHO) (microparticles)
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Galactose (

aldohexose and a C-4 epimer of glucose.[3] A galactose molecule linked with a glucose molecule forms a lactose
molecule.

Galactan is a polymeric form of galactose found in hemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates.[4]

D-Galactose is also known as brain sugar since it is a component of

nerve tissue.[5]

Etymology

The word galactose was coined by Charles Weissman[6] in the mid-19th century and is derived from Greek γαλακτος, galaktos, (of milk) and the generic chemical suffix for sugars -ose.[7] The etymology is comparable to that of the word lactose in that both contain roots meaning "milk sugar". Lactose is a disaccharide of galactose plus glucose.

Structure and isomerism

Galactose exists in both open-chain and cyclic form. The open-chain form has a

carbonyl
at the end of the chain.

Four isomers are cyclic, two of them with a pyranose (six-membered) ring, two with a furanose (five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa,[8][9] and is recognized by a putative chordate immune lectin intelectin through its exocyclic 1,2-diol. In the cyclic form there are two anomers, named alpha and beta, since the transition from the open-chain form to the cyclic form involves the creation of a new stereocenter at the site of the open-chain carbonyl.[10]

The IR spectra for galactose shows a broad, strong stretch from roughly wavenumber 2500 cm−1 to wavenumber 3700 cm−1.[11]

The Proton NMR spectra for galactose includes peaks at 4.7 ppm (D2O), 4.15 ppm (−CH2OH), 3.75, 3.61, 3.48 and 3.20 ppm (−CH2 of ring), 2.79–1.90 ppm (−OH).[11]

Cyclic forms of galactose
Chair conformation of D-galactopyranose

Relationship to lactose

Galactose is a

β-galactosidase. The latter is produced by the lac operon in Escherichia coli.[12]

In nature, lactose is found primarily in milk and milk products. Consequently, various food products made with dairy-derived ingredients can contain lactose.[13] Galactose metabolism, which converts galactose into glucose, is carried out by the three principal enzymes in a mechanism known as the Leloir pathway. The enzymes are listed in the order of the metabolic pathway: galactokinase (GALK), galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4’-epimerase (GALE).[citation needed]

In human

mammary glands to synthesize and secrete lactose. In a study where women were fed a diet containing galactose, 69 ± 6% of glucose and 54 ± 4% of galactose in the lactose they produced were derived directly from plasma glucose, while 7 ± 2% of the glucose and 12 ± 2% of the galactose in the lactose, were derived directly from plasma galactose. 25 ± 8% of the glucose and 35 ± 6% of the galactose was synthesized from smaller molecules such as glycerol or acetate in a process referred to in the paper as hexoneogenesis. This suggests that the synthesis of galactose is supplemented by direct uptake and of use of plasma galactose when present.[14]

Metabolism

Galactose metabolism

Glucose is more stable than galactose and is less susceptible to the formation of nonspecific glycoconjugates, molecules with at least one sugar attached to a protein or lipid. Many speculate that it is for this reason that a pathway for rapid conversion from galactose to glucose has been highly conserved among many species.[15]

The main pathway of galactose metabolism is the Leloir pathway; humans and other species, however, have been noted to contain several alternate pathways, such as the De Ley Doudoroff Pathway. The Leloir pathway consists of the latter stage of a two-part process that converts β-D-galactose to UDP-glucose. The initial stage is the conversion of β-D-galactose to α-D-galactose by the enzyme, mutarotase (GALM). The Leloir pathway then carries out the conversion of α-D-galactose to UDP-glucose via three principal enzymes: Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P; Galactose-1-phosphate uridyltransferase (GALT) transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose; and finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway.[16]

The above mechanisms for galactose metabolism are necessary because the human body cannot directly convert galactose into energy, and must first go through one of these processes in order to utilize the sugar.[17]

Galactosemia is an inability to properly break down galactose due to a genetically inherited mutation in one of the enzymes in the Leloir pathway. As a result, the consumption of even small quantities is harmful to galactosemics.[18]

Sources

Galactose is found in

tissues; and is a by-product from the third-generation ethanol production process (from macroalgae).[citation needed
]

Clinical significance

Chronic systemic exposure of

mice, rats, and Drosophila to D-galactose causes the acceleration of senescence (aging). It has been reported that high dose exposure of D-galactose (120 mg/kg) can cause reduced sperm concentration and sperm motility in rodents and has been extensively used as an aging model when administered subcutaneously.[19][20][21]
Two studies have suggested a possible link between galactose in milk and
Harvard School of Public Health showed no specific correlation between lactose-containing foods and ovarian cancer, and showed statistically insignificant increases in risk for consumption of lactose at 30 g/day.[26] More research is necessary to ascertain possible risks.[citation needed
]

Some ongoing studies suggest galactose may have a role in treatment of focal segmental glomerulosclerosis (a kidney disease resulting in kidney failure and proteinuria).[27] This effect is likely to be a result of binding of galactose to FSGS factor.[28]

Galactose is a component of the antigens (chemical markers) present on blood cells that distinguish blood type within the ABO blood group system. In O and A antigens, there are two monomers of galactose on the antigens, whereas in the B antigens there are three monomers of galactose.[29]

A disaccharide composed of two units of galactose,

lone star tick bites.[30]

Galactose in sodium saccharin solution has also been found to cause conditioned flavor avoidance in adult female rats within a laboratory setting when combined with intragastric injections.[31] The reason for this flavor avoidance is still unknown, however it is possible that a decrease in the levels of the enzymes required to convert galactose to glucose in the liver of the rats could be responsible.[31]

History

In 1855, E. O. Erdmann noted that hydrolysis of lactose produced a substance besides glucose.[32][33]

Galactose was first isolated and studied by

Emil Fischer and Robert Morrell determined the configuration of galactose.[37]

See also

References

  1. ^ a b c d e Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
  2. ISBN 9781845691646
    .
  3. ISBN 9788122417661
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  4. ISBN 9780203216514
    .
  5. ^ "16.3 Important Hexoses | The Basics of General, Organic, and Biological Chemistry". courses.lumenlearning.com. Retrieved 2022-05-06.
  6. ^ "Charles Weismann in the 1940 Census". Ancestry. Retrieved 26 December 2017.
  7. ISBN 9783540740155
    . Retrieved 26 December 2017.
  8. PMID 8576037
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  9. PMID 21940757
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  10. ^ "Ophardt, C. Galactose". Archived from the original on 2006-09-08. Retrieved 2015-11-26.
  11. ^
    S2CID 195764874
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  12. PMID 33232031
    , retrieved 2023-10-24
  13. ^ Staff (June 2009). "Lactose Intolerance – National Digestive Diseases Information Clearinghouse". digestive.niddk.nih.gov. Archived from the original on November 25, 2011. Retrieved January 11, 2014.
  14. PMID 12519857
    .
  15. ^ Fridovich-Keil JL, Walter JH. "Galactosemia". In Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson KM, Mitchell G (eds.). The Online Metabolic and Molecular Bases of Inherited Disease.
    a 4 b 21 c 22 d 22
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    a 517 b 516 c 519
  17. ISBN 978-3-8274-2988-9
    .
  18. PMID 20301691
    . Retrieved 17 May 2015.
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  20. S2CID 13641006
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  22. PMID 2510499
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  23. S2CID 34304536
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  27. PMID 19509024
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  28. PMID 20966123
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  29. ISBN 978-0-697-22213-8
    .
  30. ^ "Alpha-gal syndrome - Symptoms and causes". Mayo Clinic. Retrieved 2022-02-25.
  31. ^
    S2CID 37225025
    .
  32. ^ Erdmann EO (1855). Dissertatio de saccharo lactico et amylaceo [Dissertation on milk sugar and starch] (Thesis) (in Latin). University of Berlin.
  33. ^ "Jahresbericht über die Fortschritte der reinen, pharmaceutischen und technischen Chemie" [Annual report on progress in pure, pharmaceutical, and technical chemistry] (in German). 1855. pp. 671–673. see especially p. 673.
  34. ^ Pasteur L (1856). "Note sur le sucre de lait" [Note on milk sugar]. Comptes rendus (in French). 42: 347–351. From page 348: Je propose de le nommer lactose. (I propose to name it lactose.)
  35. ^ Berthelot M (1860). "Chimie organique fondée sur la synthèse" [Organic chemistry based on synthesis]. Mallet-Bachelier (in French). 2. Paris, France: 248–249.
  36. ^ "Galactose" — from the Ancient Greek γάλακτος (gálaktos, “milk”).
  37. doi:10.1002/cber.18940270177
    . The configuration of galactose appears on page 385.

External links

  • Media related to Galactose at Wikimedia Commons
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