The arabinose is a monosaccharide five carbon atoms, and since it has an aldehyde functional group in its structure, it is classified within the group of aldopentoses. Its name derives from gum arabic, from where it was isolated for the first time.
It is a sugar exclusive to plant organisms and some studies indicate that it represents between 5 and 10% of the saccharides of the cell wall in the model plants Arabidopsis thaliana and Oryza sativa (rice).
Fisher projection for D - (-) and L - (+) - Arabinose (Source: Wikimedia Commons)
It is part of the composition of pectin and hemicellulose, two biopolymers of great importance from a natural and industrial point of view.
Sugar beet pulp is a good example of industrial waste used for the extraction of arabinose, which has various applications in the field of microbiology and medicine for diagnostic purposes, in the synthesis of antineoplastic and antiviral drugs, among others.
Since it is a highly abundant saccharide in plant preparations, there is currently great interest in research regarding its separation from mixtures of saccharides by various methods.
This is especially true when the preparations are used for fermentative processes that achieve the production of ethanol, since few commercially available microorganisms are capable of producing this alcohol from arabinose.
Characteristics and structure
L-arabinose is found commercially as a white crystalline powder that is often used as a sweetener in the food industry. Its chemical formula is C5H10O5 and it has a molecular weight of around 150 g / mol.
Unlike most monosaccharides in nature, this sugar is predominantly found as the L-arabinose isomer.
In general, L-isomers are common components in membrane glycoconjugates, which are molecules of diverse nature that are linked by glycosidic bonds to carbohydrate residues, so L-arabinose is no exception.
Haworth projection of the Arabinosa (Source: NEUROtiker via Wikimedia Commons)
The L-isomeric form of arabinose has two ring-shaped structures: L-arabinopyranose and L-arabinofuranose. Free arabinose exists in solution as L-arabinopyranose, since this form is more stable than furanose.
However, when observing the polysaccharide components of the plant cell wall, as well as the glycoproteins and proteoglycans where arabinose is included, the predominant form is L-arabinofuranose.
It is prudent for the reader to remember that the terms "pyran" and "furan" refer to the possible cyclic hemiacetal configurations that a sugar can acquire, with rings of 6 or 5 bonds, respectively.
L-arabinose in plants
Arabinose is widely distributed among land plants, including liverworts, mosses, and many chlorophyte and chlorophyte algae, green and brown algae, respectively. This fact suggests that the metabolic pathway for its synthesis was acquired early in "primitive" plants.
Most of the polysaccharides, proteoglycans, glycoproteins, and secreted peptides containing polysaccharides with L-arabinose in plants are synthesized in the Golgi complex, although small glycoconjugates can be synthesized in the cytosol.
The only known L-arabinose generation pathway in plants is one in which it is synthesized as UDP-L-arabinopyranose from UDP-xylose, with the participation of a UDP-xylose 4-epimerase, which catalyzes epimerization in the C-4 position of UDP-xylose.
This reaction is part of the de novo synthesis pathway for nucleotide sugars or UDP-sugars, which begins with UDP-glucose synthesized from sucrose and UDP by sucrose synthase, or from glucose 1-P and UTP by UDP-glucose pyrophosphorylase.
Other mechanisms have been proposed for the production of UDP-L-arabinopyranose from UDP-galacturonic acid by decarboxylation of the C-6 carbon, however, the enzyme UDP-galacturonic acid decarboxylase in charge of catalyzing said reaction has not been found in plants.
L-arabinose in bacteria
From a structural point of view, authors point to L-arabinose as a constituent of the cell wall of many bacteria. However, its importance is seen from a more anthropic point of view:
Humans are not able to absorb the plant L-arabinose they consume in the diet from the gut. However, E. coli, a naturally resident bacterium in the human intestine, is able to survive at the expense of this monosaccharide as its sole source of carbon and energy.
This species of bacteria and other related ones, is capable of metabolizing L-arabinose through the use of the enzymatic products of the araBAD operon. Once these microorganisms take L-arabinose from the medium, they are able to convert it intracellularly into D-xylulose-5-P, which they use, among other things, for the pentose phosphate pathway.
In experimental biology this operon has been used in genetic constructions for the controlled expression of homologous and heterologous genes in bacterial expression systems.
Features
Depending on the context in which it is contemplated, L-arabinose has different functions. In addition to some of those that have been named in the previous points, reference may be made to the following:
-One of the molecules with the highest proportion of L-arabinose in plants is pectic arabinan, from which the polymer complex of pectin, which is found in the cell wall of plants, is enriched.
-Pectic arabinane is involved with the regulation of stomatal closure and opening, crucial processes for gas exchange between plants and their surrounding environment.
-Another example of the presence and functionality of L-arabinose in plants is the family of arabinogalactan proteins, which are proteoglycans composed of a large carbohydrate region rich in L-arabinose and galactose residues.
-Many secondary flavonoid-type plant compounds are L-arabinopyranosylated, that is, they have linked L-arabinopyranose residues, especially in A. thaliana.
-The utility of L-arabinose has been proposed as a natural drug, since its monomeric units inhibit intestinal maltase and sucrase activities in vitro. Sucrase activity is important for lowering blood sugar levels.
-The inclusion of L-arabinose in the diet of rats kept in the laboratory seems to contribute significantly to the decrease of insulin and triacylglycerol levels in blood plasma and liver.
-In 1973 this monosaccharide was used by Bilik and Caplovic for the synthesis of L-ribose by epimerization of L-arabinose catalyzed by molybdate.
-Finally, L-arabinose is used in numerous media formulations for the in vitro culture of various microorganisms.
References
- Garrett, R., & Grisham, C. (2010). Biochemistry (4th ed.). Boston, USA: Brooks / Cole. CENGAGE Learning.
- Kotake, T., Yamanashi, Y., Imaizumi, C., & Tsumuraya, Y. (2016). Metabolism of L-arabinose in plants. Journal of Plant Research, 1–12.
- Nelson, DL, & Cox, MM (2009). Lehninger Principles of Biochemistry. Omega Editions (5th ed.).
- Schleif, R. (2000). Regulation of the L -arabinose operon of Escherichia coli. Trends in Genetics, 16, 559–565.
- Spagnuolo, M., Crecchio, C., Pizzigallo, MDR, & Ruggiero, P. (1999). Fractionation of Sugar Beet Pulp into Pectin, Cellulose, and Arabinose by Arabinases Combined with Ultrafiltration. Biotechnology and Bioengineering, 64, 686–691.
- Voet, D., & Voet, J. (2006). Biochemistry (3rd ed.). Editorial Médica Panamericana.
- Yurkanis Bruice, P. (2003). Organic Chemistry. Pearson.