XYLOSE: CHARACTERISTICS, STRUCTURE AND FUNCTIONS - BIOLOGY - 2024

The xylose is a monosaccharide five carbon atoms having an aldehyde functional group, so it is classified, along with related as ribose and arabinose, within the group of aldopentoses sugars.

Koch, in 1881, was the first to discover and isolate it from wood. Since then many scientists classified it as one of the most "rare" and uncommon sugars given the difficulties and costs of obtaining it.

Fisher's projection for D- and L-Xylose (Source: akane700 via Wikimedia Commons)

In 1930, however, an American cooperative managed to obtain it from the husk of cottonseed, a much cheaper material, and since then it has become popular as a sugar obtainable at prices comparable to those of sucrose production.

At present, various methods are used to isolate it from the wood of different species of woody plants and from some waste products.

Its derivatives are widely used as sweeteners in foods and beverages developed for diabetics, since they do not contribute to the increase in blood glucose levels. The most synthesized derivative used as a sweetener is xylitol.

The use of xylose as a carbon source in the alcoholic fermentation industry has become in these times one of the most important points of scientific research.

characteristics

Like glucose, xylose has a sweet taste and some studies have shown that it has about 40% of the sweet taste of glucose.

As a reagent it is commercially available as a white crystalline powder. It has, like many other pentose sugars, a molecular weight of around 150.13 g / mol and a molecular formula of C5H10O5.

Given its polar structure, this monosaccharide is easily soluble in water and has a melting point around 150 ° C.

Structure

The most common form or isomer in nature is D-xylose, while the L-xylose form is that obtained by chemical synthesis for commercial use.

This carbohydrate has four OH groups and thanks to its free aldehyde group, it is considered a reducing sugar. Like other sugars, depending on the medium where it is found, it can be found in different ways (with respect to the shape of its ring).

Haworth projection for Xylose (Source: NEUROtiker via Wikimedia Commons)

Cyclic isomers (hemiacetals) can be found in solution as pyrans or furans, that is, as rings of six or five bonds which, in turn, depending on the position of the anomeric hydroxyl group (-OH), can have more isomeric forms.

Features

In cells

Like other saccharides such as glucose, fructose, galactose, mannose, and arabinose, as well as some derived amino sugars, D-xylose is a monosaccharide that can be commonly found as a structural part of large polysaccharides.

It represents more than 30% of the material obtained from the hydrolysis of hemicellulose of vegetable origin and can be fermented to ethanol by some bacteria, yeasts and fungi.

As the main constituent of xylan polymers in plants, xylose is considered one of the most abundant carbohydrates on earth after glucose.

Hemicellulose is composed for the most part by arabinoxylan, a polymer whose backbone is made up of xyloses linked by β-1,4 bonds, where arabinose residues can be linked in the -OH groups at the 2 'or 3' positions. These bonds can be degraded by microbial enzymes.

Through the pentose phosphate metabolic pathway in eukaryotic organisms, xylose is catabolized to xylulose-5-P, which functions as an intermediary in this pathway for subsequent nucleotide synthesis.

Xylose in nectar

Until just over a decade ago, the main sugars found in floral nectar were glucose, fructose, and sucrose. Despite them, two genera of the Proteaceae family possess a fourth monosaccharide: xylose.

The Protea and Faurea genera have this saccharide in concentrations of up to 40% in their nectar, a fact that has been difficult to explain since it seems to be unpalatable (showy or tasty) for most of the natural pollinators of these plants.

Some authors consider this characteristic as a prevention mechanism for nonspecific flower visitors, while others think that its presence has more to do with the degradation of the cell walls of nectaries by fungi or bacteria.

In medicine

D-xylose is also used as an intermediate in the manufacture of drugs with therapeutic functions. It is used as a substitute for sugar for anti-caries (anti-caries) purposes.

In the field of veterinary medicine, it is used for the test of malabsorption and in the same way it is involved in procedures to evaluate the intestinal absorption capacity of simple sugars in humans.

In the industry

As mentioned before, one of the most common uses of xylose, commercially speaking, is as a low-calorie sweetener nutritional supplement and its use is approved by the FDA (Food and Drug Administration)..

The production of alternative fuels such as ethanol is achieved mainly thanks to the fermentation of carbohydrates present in plant biomass, which represents a long-term source of said alcohol.

Xylose is the second most abundant carbohydrate in nature, since it is part of hemicellulose, a heteropolysaccharide present in the cell wall of plant cells and which is an important part of the fibers in wood.

Much effort is currently being made to achieve the fermentation of this sugar in order to produce a greater amount of ethanol from plant tissue, using genetically modified microorganisms for this purpose (especially bacteria and yeasts).

Effects of xylose on animal metabolism

Xylose seems to be very little used by monogastric animals (animals with a single stomach, different from ruminant animals, with more than one gastric cavity).

In both poultry and pigs, when too much D-xylose is included in their daily diet, a linear decrease can be observed in the average daily weight gain, in feeding efficiency and in the content of dry matter excreted.

This is explained by the inability of most animals for the degradation of hemicellulose polymers, for which different research groups have been given the task of looking for alternatives such as exogenous enzymes, the inclusion of probiotics and microorganisms in the diet, etc.

Very little is known about the metabolic utilization of xylose in vertebrates, however, it is known that its addition as a nutritional supplement usually ends up as an excretion product in the urine.

References

1. Garrett, R., & Grisham, C. (2010). Biochemistry (4th ed.). Boston, USA: Brooks / Cole. CENGAGE Learning.
2. Huntley, NF, & Patience, JF (2018). Xylose: absorption, fermentation, and post-absorptive metabolism in the pig. Journal of Animal Science and Biotechnology, 9 (4), 1–9.
3. Jackson, S., & Nicolson, SW (2002). Xylose as a nectar sugar: from biochemistry to ecology. Comparative Biochemistry and Physiology, 131, 613–620.
4. Jeffries, TW (2006). Engineering yeasts for xylose metabolism. Current Opinion in Biotechnology, 17, 320–326.
5. Kotter, P., & Ciriacy, M. (1993). Xylose fermentation by Saccharomyces cerevisiae. Appl. Microbiol. Biotechnol, 38, 776-783.
6. Mathews, C., van Holde, K., & Ahern, K. (2000). Biochemistry (3rd ed.). San Francisco, California: Pearson.
7. Miller, M., & Lewis, H. (1932). Pentose Metabolism. J. Biol. Chem., 98, 133-140.
8. National Center for Biotechnology Information. PubChem Database. (+) - Xylose, CID = 644160, www.pubchem.ncbi.nlm.nih.gov/compound/644160 (accessed on Apr. 12, 2019)
9. Schneider, H., Wang, P., Chan, Y., & Maleszka, R. (1981). Conversion of D-Xylose into Ethanol by the yeast Pachysolen tannophilus. Biotechnology Letters, 3 (2), 89–92.