Galactose: Difference between revisions

{{chembox

| verifiedrevid = 456363141

| ImageFile = Galactose-3D-balls.png

| ImageFileL1 = Beta-D-Galactopyranose.svg

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| ImageFileR1 = DL-Galactose num.svg

| ImageSizeR1 = 150 px

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| OtherNames =

| Section1 = {{Chembox Identifiers

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChEMBL = <!-- blanked - oldvalue: 300520 -->

| KEGG_Ref = {{keggcite|correct|kegg}}

| KEGG = <!-- blanked - oldvalue: D04291 -->

| StdInChI = 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

| CASNo = <!-- blanked - oldvalue: 26566-61-0 -->

| PubChem = 439357

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = X2RN3Q8DNE

| MeSHName = Galactose

| Section2 = {{Chembox Properties

| Formula = C₆H₁₂O₆

| MolarMass = 180.156 g mol⁻¹

| Appearance =

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| MeltingPt = 167°C

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| Solubility = 683.0 g/L

| Section3 = {{Chembox Hazards

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'''Galactose''' (from [[Greek (language)|Greek]] γάλακτος ''galaktos'' "milk"), sometimes abbreviated '''Gal''', is a type of [[Carbohydrate|sugar]] that is less [[sweetness|sweet]] than [[glucose]]. It is a C-4 [[epimer]] of [[glucose]].

'''Galactan''' is a [[polymer]] of the sugar galactose found in [[hemicellulose]]. Galactan can be converted to galactose by [[hydrolysis]].

==Structure and isomerism==

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.

<ref>Nassau et al. [http://jb.asm.org/cgi/reprint/178/4/1047.pdf Galactofuranose Biosynthesis in Escherichia coli K-12:...] JOURNAL OF BACTERIOLOGY, Feb. 1996, p. 1047–1052</ref>

{{clear}}

Galactose is a [[monosaccharide]]. When combined with [[glucose]] (monosacccharide), through a [[dehydration reaction]], the result is the [[disaccharide]] [[lactose]]. The [[hydrolysis]] of lactose to glucose and galactose is [[catalysis|catalyzed]] by the [[enzyme]]s [[lactase]] and [[beta-galactosidase|β-galactosidase]]. The latter is produced by the [[Lac operon|''lac'' operon]] in ''[[Escherichia coli]]''.

Lactose is found primarily in milk and milk products. 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).

In the human body, glucose is changed into galactose via hexoneogenesis to enable the [[mammary gland]]s to secrete lactose. However, most galactose in [[breast milk]] is synthesized from galactose taken up from the blood, and only 35±6% is made by ''de novo'' synthesis.

<ref name=sunehag>{{cite journal |author=Sunehag A, Tigas S, Haymond MW |title=Contribution of plasma galactose and glucose to milk lactose synthesis during galactose ingestion |journal=J. Clin. Endocrinol. Metab. |volume=88 |issue=1 |pages=225–9 |year=2003 |month=January |pmid=12519857 |doi= 10.1210/jc.2002-020768|url=http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=12519857}}</ref> Glycerol also contributes some to the mammary galactose production.<ref>{{cite journal |author=Sunehag AL, Louie K, Bier JL, Tigas S, Haymond MW |title=Hexoneogenesis in the human breast during lactation |journal=J. Clin. Endocrinol. Metab. |volume=87 |issue=1 |pages=297–301 |year=2002 |month=January |pmid=11788663 |doi= 10.1210/jc.87.1.297|url=http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=11788663}}</ref>

==Galactose metabolism==

Glucose is the primary metabolic fuel for humans. It 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 [[Conservation (genetics)|highly conserved]] among many species.<ref name=OMMBID72>{{cite web |url=http://www.ommbid.com/OMMBID/the_online_metabolic_and_molecular_bases_of_inherited_disease/b/abstract/part7/ch72 |title= Galactosemia |chapter=72 |author=Fridovich-Keil JL, Walter JH |format= |work=The Online Metabolic and Molecular Bases of Inherited Disease |accessdate=}}<br />a 4 b 21 c 22 d 22</ref>

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 [[Uridine diphosphate glucose|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 principle enzymes. Galactokinase (GALK) phosphorylates α-D-galactose to galactose-1-phosphate, or Gal-1-P. Galactose-1-phosphate uridyltransferase (GALT) then transfers a UMP group from UDP-glucose to Gal-1-P to form UDP-galactose. Finally, UDP galactose-4’-epimerase (GALE) interconverts UDP-galactose and UDP-glucose, thereby completing the pathway.<ref name=Bosch07>{{cite journal |author=Bosch AM |title=Classical galactosaemia revisited |journal=J. Inherit. Metab. Dis. |volume=29 |issue=4 |pages=516–25 |year=2006 |month=August |pmid=16838075 |doi=10.1007/s10545-006-0382-0 |url=}}<br /> a 517 b 516 c 519</ref>

[[File:Leloir_pathway_of_galactose_metabolism.gif]]

Galactose is found in [[dairy product]]s, [[sugar beet]]s, and other [[natural gum|gum]]s and [[mucilage]]s. It is also [[Biosynthesis|synthesized]] by the body, where it forms part of [[glycolipid]]s and [[glycoprotein]]s in several [[Biological tissue|tissue]]s.

Chronic systemic exposure of [[mice]], [[rats]], and ''[[Drosophila]]'' to D-galactose causes the acceleration of [[senescence]] and has been used as an aging model.<ref>{{cite pmid|16710848}}</ref>

Two studies have suggested a possible link between galactose in milk and [[ovarian cancer]].<ref>{{cite journal |author=Cramer D |title=Lactase persistence and milk consumption as determinants of ovarian cancer risk |journal=Am J Epidemiol |volume=130 |issue=5 |pages=904–10 |year=1989 |pmid=2510499}}</ref><ref>{{cite journal |author=Cramer D, Harlow B, Willett W, Welch W, Bell D, Scully R, Ng W, Knapp R |title=Galactose consumption and metabolism in relation to the risk of ovarian cancer |journal=Lancet |volume=2 |issue=8654 |pages=66–71 |year=1989 |pmid=2567871 |doi=10.1016/S0140-6736(89)90313-9}}</ref> Other studies show no correlation, even in the presence of defective galactose metabolism.<ref>{{cite journal |author= Marc T. Goodman , Anna H. Wu , Ko-Hui Tung , Katharine McDuffie , Daniel W. Cramer , Lynne R. Wilkens , Keith Terada , Juergen K. V. Reichardt , and Won G. Ng |title=Association of Galactose-1-Phosphate Uridyltransferase Activity and N314D Genotype with the Risk of Ovarian Cancer |journal=Am. J. Epidemiol |volume=156 |issue=8 |pages=693–701 |year=2002 |pmid=12370157 |doi=10.1093/aje/kwf104}}</ref><ref>{{cite journal |author=Fung, W. L. Alan, Risch, Harvey, McLaughlin, John, Rosen, Barry, Cole, David, Vesprini, Danny, Narod, Steven A. |title=The N314D Polymorphism of Galactose-1-Phosphate Uridyl Transferase Does Not Modify the Risk of Ovarian Cancer |journal= Cancer Epidemiol Biomarkers Prev |volume=12 |issue=7 |pages=678–80 |year=2003 |pmid=12869412}}</ref> More recently, pooled analysis done by the [[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/d.<ref>{{cite journal|author=Genkinger, Jeanine M., Hunter, David J., Spiegelman, Donna, Anderson, Kristin E., Arslan, Alan, Beeson, W. Lawrence, Buring, Julie E., Fraser, Gary E., Freudenheim, Jo L., Goldbohm, R. Alexandra, Hankinson, Susan E., Jacobs, David R., Jr., Koushik, Anita, Lacey, James V., Jr., Larsson, Susanna C., Leitzmann, Michael, McCullough, Marji L., Miller, Anthony B., Rodriguez, Carmen, Rohan, Thomas E., Schouten, Leo J., Shore, Roy, Smit, Ellen, Wolk, Alicja, Zhang, Shumin M., Smith-Warner, Stephanie A. |title=Dairy Products and Ovarian Cancer: A Pooled Analysis of 12 Cohort Studies|journal=Cancer Epidemiol Biomarkers Prev |volume=15 |pages=364–372 |year=2006 |pmid=16492930 | doi = 10.1158/1055-9965.EPI-05-0484|issue=2}}</ref> More research is necessary to ascertain possible risks.

Some ongoing studies suggest galactose may have a role in treatment of [[focal segmental glomerulosclerosis]] (a kidney disease resulting in kidney failure and proteinuria).{{Citation needed|date=February 2007}} This effect is likely to be a result of binding of galactose to FSGS factor.{{Citation needed|date=February 2007}}

Galactose is a component of the antigens present on blood cells that determine blood type within the [[ABO blood group system]].<ref name="Raven and Johnson">{{cite book|title=Understanding Biology|edition=3rd|author=Peter H. Raven & George B. Johnson|pages=203|isbn=0-697-22213-6|year=1995|editor=Carol J. Mills (ed)|publisher=WM C. Brown}}</ref>

==See also==

==References==

{{reflist|2}}

[[Category:Galactose]]

[[Category:Sweeteners]]

[[ar:جالاكتوز]]

[[bg:Галактоза]]

[[ca:Galactosa]]

[[cs:Galaktosa]]

[[da:Galaktose]]

[[de:Galactose]]

[[el:Γαλακτόζη]]

[[es:Galactosa]]

[[eu:Galaktosa]]

[[fa:گالاکتوز]]

[[fr:Galactose]]

[[fy:Galaktoaze]]

[[gl:Galactosa]]

[[ko:갈락토스]]

[[it:Galattosio]]

[[he:גלקטוז]]

[[kk:Галактоза]]

[[la:Galactosum]]

[[hu:Galaktóz]]

[[ms:Galaktosa]]

[[nl:Galactose]]

[[ja:ガラクトース]]

[[no:Galaktose]]

[[oc:Galactòsa]]

[[pl:Galaktoza]]

[[pt:Galactose]]

[[ro:Galactoză]]

[[ru:Галактоза]]

[[sr:Галактоза]]

[[sh:Galaktoza]]

[[fi:Galaktoosi]]

[[sv:Galaktos]]

[[th:กาแล็กโทส]]

[[tr:Galaktoz]]

[[uk:Галактоза]]

[[zh:半乳糖]]