Xylose ( , , "wood") is a common monosaccharide, i.e. a simple sugar. Xylose is classified as aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group, at least in its open-chain form. It is abundant in biomass, and is one of the most abundant sugars in nature.
Structure
The acyclic form of xylose has
chemical formula . Cyclic
hemiacetal isomers are more prevalent in solution. These cyclic isomers include the
feature six-membered rings, and the
, which feature five-membered rings (with a pendant group). Each of these rings is subject to further isomerism, depending on the relative orientation of the anomeric hydroxy group.
The dextrorotary form, -xylose, is the one that usually occurs endogenously in life. A levorotary form, -xylose, can be synthesized.
Occurrence
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
of most edible plants. It was first isolated from wood by Finnish scientist, Koch, in 1881,
but first became commercially viable, with a price close to
sucrose, in 1930.
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 such as heparan sulfate and chondroitin sulfate.
Xylose is also found in some species of Chrysolinina beetles, including Chrysolina coerulans. They have cardiac glycosides (including xylose) in their defensive glands.
Applications
Chemicals
The acid-catalysed degradation of hemicellulose gives
furfural,
a precursor to synthetic polymers and to
tetrahydrofuran.
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Human consumption
Xylose is not a major human nutrient and is largely excreted by the kidneys.
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
Animal medicine
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.
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.
Derivatives
Reduction of xylose by catalytic
hydrogenation produces the
sugar substitute xylitol.
See also
