|CAS number||ESIS], (racemate) [ESIS], (L-isomer) [|
|Jmol-3D images||Image 1|
|Molar mass||150.13 g/mol|
|Appearance||monoclinic needles or prisms, colourless|
|Density||1.525 g/cm3 (20 °C)|
|Melting point||144–145 °C|
|Chiral rotation [α]D||+22.5° (CHCl3)|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Xylose (cf. Greek ξύλον, xylon, "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes a formyl functional group. It is derived from hemicellulose, one of the main constituents of biomass. Like most sugars, it can adopt several structures depending on conditions. With its free carbonyl group, it is a reducing sugar.
The acyclic form of xylose has chemical formula HOCH2(CH(OH))3CHO. The cyclic hemiacetal isomers are more prevalent in solution and are of two types: the pyranoses, which feature six-membered C5O rings, and the furanoses, which feature five-membered C4O rings (with a pendant CH2OH group). Each of these rings subject to further isomerism, depending on the relative orientation of the anomeric hydroxy group.
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 Koch in 1881.
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.
Xylose is metabolised by humans, though it is not a major human nutrient and largely excreted by the kidneys. Humans must obtain xylose from their diet. An oxido-reductase pathway is present in eukaryotic microorganisms. Humans have an enzyme called xylosyltransferase, which transfers xylose from UDP to a serine in the core protein of proteoglycans.
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. Reduction of xylose by catalytic hydrogenation produces the non-cariogenic sugar substitute xylitol.
In 2014 a low-temperature 50 °C (122 °F), atmospheric-pressure enzyme-driven process to convert xylose into hydrogen with nearly 100% of the theoretical yield was announced. The process employs 13 enzymes, including a novel polyphosphate xylulokinase (XK).
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