Xylose

The acyclic form of xylose has chemical formula {{chem|HOCH|2|(CH(OH))|3C|H|O}}. The cyclic hemiacetal isomers are more prevalent in solution and are of two types: the pyranoses, which feature six-membered {{chem|C|5|O}} rings, and the furanoses, which feature five-membered {{chem|C|4|O}} rings (with a pendant {{chem|CH|2|OH}} group). Each of these rings is subject to further isomerism, depending on the relative orientation of the anomeric hydroxy group.

The dextrorotary form, {{sm|d}}-xylose, is the one that usually occurs endogenously in living things. A levorotary form, {{sm|l}}-xylose, can be synthesized.

Xylose is the main building block for the hemicellulose xylan, which comprises about 30% of some plants (birch for example), far less in others (spruce and pine have about 9% xylan). Xylose is otherwise pervasive, being found in the embryos of most edible plants. It was first isolated from wood by Finnish scientist, Koch, in 1881,{{cite book |editor-first=C.S. |editor-last=Hudson |editor2-first=S.M. |editor2-last=Cantor |title=Advances in Carbohydrate Chemistry |publisher=Elsevier |date=2014 |orig-date=1950 |volume=5 |isbn=9780080562643 |pages=278 |url=https://books.google.com/books?id=rUapoxXf-v4C}} but first became commercially viable, with a price close to sucrose, in 1930.{{cite journal |first1=Mabel M. |last1=Miller |first2=Howard B. |last2=Lewis |title=Pentose Metabolism: I. The Rate of Absorption of d-Xylose and the Formation of Glycogen in the Organism of the White Rat after Oral Administration of d-Xylose |journal=Journal of Biological Chemistry |volume=98 |issue=1 |pages=133–140 |date=1932 |doi=10.1016/S0021-9258(18)76145-0 |doi-access=free }}

Xylose is also the first saccharide added to the serine or threonine in the proteoglycan type O-glycosylation, and, so, it is the first saccharide in biosynthetic pathways of most anionic polysaccharides such as heparan sulfate and chondroitin sulfate.{{citation | last1 = Buskas | first2 = Sampat | last2 = Ingale | first3 = Geert-Jan | last3 = Boons | title = Glycopeptides as versatile tool for glycobiology | journal = Glycobiology | volume = 16 | issue = 8 | pages = 113R–36R | year = 2006 | doi = 10.1093/glycob/cwj125 | pmid = 16675547 | first1 = Therese| doi-access = free }}

Xylose is also found in some species of Chrysolinina beetles, including Chrysolina coerulans. They have cardiac glycosides (including xylose) in their defensive glands.{{cite book |first=E. David |last=Morgan |title=Biosynthesis in Insects |year=2004 |chapter=§ 7.3.1 Sterols in Insects |chapter-url={{google books|AbXid2E-17YC |page=112 |plainurl=yes}} |page=112 |publisher=Royal Society of Chemistry |isbn=9780854046911}}

The acid-catalysed degradation of hemicellulose gives furfural,{{OrgSynth | title = Furfural | volume = 1 | pages = 49 | year = 1921 | collvol = 1 | collvolpages = 280 |first= Roger |last=Adams |first2=V. |last2=Voorhees | prep = cv1p0280 |doi=10.15227/orgsyn.001.0049}}{{cite journal|last=Gómez Millán |first=Gerardo|author2=Hellsten, Sanna|author3=King, Alistair W.T. |author4=Pokki, Juha-Pekka|author5=Llorca, Jordi|author6=Sixta, Herbert |title=A comparative study of water-immiscible organic solvents in the production of furfural from xylose and birch hydrolysate |journal=Journal of Industrial and Engineering Chemistry |date=25 April 2019|volume=72 |pages=354–363|doi=10.1016/j.jiec.2018.12.037|hdl=10138/307298|s2cid=104358224 |hdl-access=free}} a precursor to synthetic polymers and to tetrahydrofuran.{{Ullmann|doi=10.1002/14356007.a12_119.pub2|chapter=Furfural and Derivatives|year=2007|last1=Hoydonckx|first1=H. E.|last2=Van Rhijn|first2=W. M.|last3=Van Rhijn|first3=W.|last4=De Vos|first4=D. E.|last5=Jacobs|first5=P. A.|isbn=978-3527306732}}

Xylose is metabolised by humans, although it is not a major human nutrient and is largely excreted by the kidneys.{{cite thesis |url=https://repository.up.ac.za/handle/2263/27501 |title=Physiological and microbiological studies of nectar xylose metabolism in the Namaqua rock mouse, Aethomys namaquensis (A. Smith, 1834) |last=Johnson |first=S.A. |date=2007-08-24 |type=PhD |hdl=2263/27501}} Humans can obtain xylose only from their diet. An oxidoreductase pathway is present in eukaryotic microorganisms. Humans have enzymes called protein xylosyltransferases (XYLT1, XYLT2) which transfer xylose from UDP to a serine in the core protein of proteoglycans.

Xylose contains 2.4 calories per gram{{Cite patent|title=Method of producing xylose|gdate=1999-08-06 |number=US6239274B1 |country=US |url=https://patents.google.com/patent/US6239274B1/en}} (lower than glucose or sucrose, approx. 4 calories per gram).

In animal medicine, xylose is used to test for malabsorption by administration in water to the patient after fasting. If xylose is detected in blood and/or urine within the next few hours, it has been absorbed by the intestines.{{citation | url = https://www.nlm.nih.gov/medlineplus/ency/article/003606.htm | title = D-xylose absorption | work = MedlinePlus | publisher = U.S. National Library of Medicine | date = July 2008 | access-date = 2009-09-06}}

High xylose intake on the order of approximately 100 g/kg of animal body weight is relatively well tolerated in pigs, and in a similar manner to results from human studies, a portion of the xylose ingested is excreted in urine unmodified.{{cite journal |vauthors=Schutte JB, de Jong J, Polziehn R, Verstegen MW |title=Nutritional implications of D-xylose in pigs |journal=Br J Nutr |volume=66 |issue=1 |pages=83–93 |date=July 1991 |pmid=1931909 |doi=10.1079/bjn19910012 |s2cid=27670020 |doi-access=free }}

Reduction of xylose by catalytic hydrogenation produces the sugar substitute xylitol.

• Saccharophagus degradans
• Xylonic acid
• Xylose metabolism

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{{Carbohydrates}}

Category:Aldopentoses

Category:Pyranoses

Category:Furanoses