Xylose

Xylose (cf. Greek ξυλος, xylos, "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 an aldehyde functional group. It is the precursor to 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.

Structure
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.

Occurrence
Xylose is the main building block for hemicellulose, which comprises about 30% of plant matter. 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.

Use
The acid-catalysed degradation of hemicellulose gives furfural, a specialty solvent in industry and a precursor to synthetic polymers. xylose is not metabolised by humans. It is completely absorbed and secreted from the kidneys.

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.