Xylose Reductase
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Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a
monosaccharide Monosaccharides (from Greek ''monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solub ...
of the
aldopentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.carbon atoms and includes an aldehyde functional group. It is derived from hemicellulose, one of the main constituents of
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
. Like most sugars, it can adopt several structures depending on conditions. With its free
aldehyde group In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
, it is a reducing sugar.


Structure

The acyclic form of xylose has chemical formula . The cyclic hemiacetal isomers are more prevalent in solution and are of two types: the
pyranose Pyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarity ...
s, which feature six-membered rings, and the furanoses, 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 living things. 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
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s 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 Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...
, in 1930. Xylose is also the first saccharide added to the
serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
or
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO ...
in the
proteoglycan Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to whic ...
type O-glycosylation, and, so, it is the first saccharide in biosynthetic pathways of most anionic
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s 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 Furfural is an organic compound with the formula C4H3OCHO. It is a colorless liquid, although commercial samples are often brown. It has an aldehyde group attached to the 2-position of furan. It is a product of the dehydration of sugars, as occurs ...
, a precursor to synthetic polymers and to tetrahydrofuran.


Human consumption

Xylose is metabolised by humans, although it 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 microorganism A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms an ...
s. Humans have enzymes called
protein xylosyltransferase In enzymology, a protein xylosyltransferase () is an enzyme that catalyzes the chemical reaction in which a beta-D-xylosyl residue is transferred from UDP-D-xylose to the sidechain oxygen atom of a serine residue in a protein. This enzyme belongs ...
s (
XYLT1 Xylosyltransferase 1 is an enzyme that in humans is encoded by the ''XYLT1'' gene. Xylosyltransferase (XT; EC 2.4.2.26) catalyzes the transfer of UDP-xylose to serine residues within XT recognition sequences of target proteins. Addition of this ...
, XYLT2) which transfer xylose from UDP to a serine in the core protein of proteoglycans. Xylose contains 2.4 calories per gram (lower than glucose or sucrose, approx. 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 intake is passed out in urine undigested.


Hydrogen production

In 2014 a low-temperature , 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 In enzymology, a xylulokinase () is an enzyme that catalyzes the chemical reaction : ATP + D-xylulose ADP + D-xylulose 5-phosphate Thus, the two substrates of this enzyme are ATP and D-xylulose, whereas its two products are ADP and D-xylul ...
(XK).


Derivatives

Reduction of xylose by catalytic hydrogenation produces the
sugar substitute A sugar substitute is a food additive that provides a sweetness like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie () or low-calorie sweetener. Artificial sweeteners may be d ...
xylitol.


See also

*
Saccharophagus degradans ''Saccharophagus degradans'' (formerly ''Microbulbifer degradans'') is a gram-negative marine bacterium known to degrade a number of complex polysaccharides as energy source. ''S. degradans'' have also been shown to ferment xylose to ethanol. I ...
*
Xylonic acid Xylonic acid is a sugar acid that can be obtained by oxidation of the hemiacetal/aldehyde group of xylose. The C-2 epimer is known as lyxonic acid. References Sugar acids Monosaccharides {{organic-compound-stub ...
*
Xylose metabolism D-Xylose is a five-carbon aldose (pentose, monosaccharide) that can be catabolized or metabolized into useful products by a variety of organisms. There are at least four different pathways for the catabolism of D-xylose: An oxido-reductase pathw ...


References

{{Carbohydrates Aldopentoses