RIBULOSE: CHARACTERISTICS, STRUCTURE AND FUNCTIONS - BIOLOGY - 2025

The ribulose is a monosaccharide sugar or carbohydrate containing five carbon atoms and ketone functional group in its structure, so that it is included in the group of ketopentoses.

The four- and five-carbon ketoses are named by inserting the infix "ul" into the name of the corresponding aldose. So, D-ribulose is the ketopentose which corresponds to D-ribose, an aldopentose.

Fisher's projection for Ribulose (Source: NEUROtiker via Wikimedia Commons)

This sugar participates in the D-ribulose form as an intermediary in various metabolic pathways, as in the Calvin cycle, for example. While only in some bacteria of genera such as Acetobacter and Gluconobacter is L-ribose obtained as the final metabolic product. For this reason, these microorganisms are used for their synthesis at an industrial level.

Some compounds derived from ribulose are one of the main intermediate compounds in the pentose phosphate pathway. This pathway is intended to generate NADPH, an important cofactor that functions in nucleotide biosynthesis.

There are industrial mechanisms to synthesize L-ribulose as an isolated compound. The first isolation method with which it was obtained consisted of the Levene and La Forge isolation method of ketoses from L-xylose.

Despite the great advances in industrial methods for the synthesis and purification of chemical compounds, L-ribulose is not obtained as an isolated monosaccharide, being obtained in combined fractions of L-ribose and L-arabinose.

The method to obtain L-ribulose most used at present is purification from G luconobacte frateurii IFO 3254. This species of bacteria is capable of surviving in acidic conditions and has an oxidation pathway from ribitol to L-ribulose.

characteristics

Ribulose as a synthesized, extracted and purified reagent that is frequently found as L-ribulose, is a solid, white and crystalline organic substance. Like all carbohydrates, this monosaccharide is soluble in water and has the typical characteristics of polar substances.

As is common for the rest of the saccharides, ribulose has the same number of carbon and oxygen atoms, and twice this amount in hydrogen atoms.

The most common form in which ribulose can be found in nature is in association with different substituents and forming complex structures, generally phosphorylated, such as ribulose 5-phosphate, ribulose 1,5-bisphosphate, among others.

These compounds generally act as intermediaries and transporters or "vehicles" for phosphate groups in the various cellular metabolic pathways in which they participate.

Structure

The ribulose molecule has a central skeleton of five carbon atoms and a ketone group on the carbon at the C-2 position. As previously stated, this functional group positions it within ketoses as ketopentose.

It has four hydroxyl groups (-OH) attached to the four carbons that are not attached to the ketone group and these four carbons are saturated with hydrogen atoms.

The ribulose molecule can be represented according to the Fisher projection in two forms: D-ribulose or L-ribulose, the L-form being the stereoisomer and enantiomer of the D-form and vice versa.

The classification of the D or L form depends on the orientation of the hydroxyl groups of the first carbon atom after the ketone group. If this group is oriented towards the right side, the molecule representing Fisher corresponds to D-ribulose, otherwise if it is towards the left side (L-ribulose).

In the Haworth projection, ribulose can be represented in two additional structures depending on the orientation of the hydroxyl group of the anomeric carbon atom. In the β position the hydroxyl is oriented towards the upper part of the molecule; while the position α orients the hydroxyl towards the bottom.

Thus, according to the Haworth projection, there can be four possible forms: β-D-ribulose, α-D-ribulose, β-L-ribulose or α-L-ribulose.

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

Features

Pentose phosphate pathway

Most cells, especially those that are constantly and rapidly dividing such as bone marrow, intestinal mucosa, and tumor cells, use ribulose-5-phosphate, which is isomerized to ribose-5-phosphate in the oxidative pathway of pentose phosphate, to produce nucleic acids (RNA and DNA) and coenzymes such as ATP, NADH, FADH2 and coenzyme A.

This oxidative phase of pentose phosphate includes two oxidations that convert glucose 6-phosphate to ribulose 5-phosphate, reducing NADP + to NADPH.

Additionally, ribulose-5-phosphate indirectly activates phosphofruct kinase, an essential enzyme of the glycolytic pathway.

Calvin cycle

The Calvin cycle is the carbon fixation cycle that takes place in photosynthetic organisms after the first reactions of photosynthesis.

It has been proven by marking methods in tests carried out by different researchers, that by marking the carbon in the C-1 position of ribulose-1,5-bisphosphate, carbon dioxide is fixed in this intermediate during the Calvin cycle giving origin to two 3-phosphoglycerate molecules: one labeled and one unlabeled.

RuBisCO (Ribulose 1,5-bisphosphate carboxylase / oxygenase) is considered the most abundant enzyme on the planet and uses ribulose 1,5-bisphosphate as a substrate to catalyze the incorporation of carbon dioxide and the production of 1,3-diphosphoglycerate. in the Calvin cycle.

The breakdown of this unstable intermediate, 1,3-diphosphoglycerate, of six carbon atoms, is also catalyzed by RuBisCO, which is what mediates the formation of two molecules of 3 carbon atoms (3-phosphoglycerate).

Functions in bacteria

Enol-1-O-carboxyphenylamino-1-deoxyribulose phosphate participates as an intermediate metabolite in the biosynthesis of tryptophan from chorismate in bacteria and plants. In this step, one molecule of carbon dioxide and one of water are released, also producing a molecule of indole-3-glycerol-phosphate.

Bacteria use L-ribulose also in the pathways used for ethanol metabolism. Furthermore, these microorganisms possess an enzyme known as L-arabinose isomerase, which modifies arabinose to synthesize L-ribulose.

L-ribulose kinase phosphorylates this downstream metabolite to form L-ribulose-5-phosphate, which can enter the pentose phosphate pathway for the production of the sugars for the nucleic acid backbones and other essential molecules.

References

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