HEXOSE: CHARACTERISTICS, FUNCTIONS, DERIVATIVES - GEOGRAFÍA - 2025

A hexose is a carbohydrate that has six carbon atoms and whose empirical formula is C ₆ H ₁₂ O ₆. Carbohydrates or saccharides (from the Greek, sakcharon = sugar) are polyhydroxy-aldehydes or polyhydroxy-ketones.

In nature, the most abundant monosaccharide is glucose, a six-carbon sugar, also called dextrose. Glucose biosynthesis takes place from carbon dioxide and water through photosynthesis.

Source: NEUROtiker

In plants, from glucose, the synthesis of cellulose, a structural polysaccharide, and of starch, a reserve polysaccharide, occurs. In heterotrophic organisms, glucose oxidation is the central metabolic pathway for energy production.

characteristics

Hexoses can be of two types: 1) aldoses (or aldohexoses), in which carbon 1 (C-1) is an aldehyde function; or 2) ketoses (or aldocetoses) in which carbon 2 (C-2) is a keto function. The rest of the carbons are secondary or primary alcohols.

In aldohexoses, all carbons are chiral, except carbon 1 (C-1) and carbon 6 (C-6), that is, they have four asymmetric centers. In ketohexoses there are three asymmetric centers, which are C-3, C-4, and C-5.

In nature, sugars such as hexoses with the L configuration are less abundant than sugars with the D configuration.

The aldehyde function or keto function of hexoses reacts with a secondary hydroxyl group, in an intramolecular reaction, to form cyclic hemiacetals or hemiketals. The six-membered cyclic sugars are pyranous and the five-membered sugars are furanose.

In cyclic sugar, the carbonyl carbon of the aldehyde and keto groups becomes a new chiral center, called the anomeric carbon. The configuration of this carbon can be alpha or beta, that is, it produces two anomers.

Hexoses have different conformations

The six atoms that make up pyranoses are not planar but have two chair-like conformations in which the bulky substituents occupy: a) equatorial positions or b) axial positions. These conformations can be interconverted without breaking the covalent bonds.

Stereochemical interactions between ring substituents influence the relative stability of these conformations. Thus, the most stable conformation is the one in which the largest group occupies an equatorial position.

The chemical reactivity of a certain group is influenced by its conformational location. An example is the hydroxyl group (-OH) which, when occupying the equatorial position, is more easily esterified than when occupying the axial position.

Β-D-glucose, an aldohexose, has all the substituents in the equatorial position, which makes them more susceptible to esterification. This reaction is important for the formation of covalent bonds between sugars. This could explain why β-D-glucose is the most abundant sugar in nature.

Hexoses can form glycosidic bonds

Monosaccharide units, such as hexoses, can be linked covalently through O-glycosidic bonds formed when the anomeric carbon of one sugar molecule reacts with the hydroxyl group of another sugar molecule. The result of this reaction is the formation of an acetal from a hemiacetal.

An example is the reaction of the C-1, anomeric carbon of α-D-glucopyranose with the hydroxyl group of C-4 of another β-D-glucopyranose. From it, α -D-glucopyranosyl- (1®4) -D-glucopyranose is formed.

The glycosidic bond formation reaction involves the removal of a water molecule, called the condensation reaction. The reverse reaction is hydrolysis and breaking of the glycosidic bond.

Hexoses and oxidation-reduction reactions

Sugars whose anomeric carbon atom has not formed glycosidic bonds are called reducing sugars. All monosaccharides, such as the hexoses glucose, mannose, and galactose, are reducing sugars. This is because aldoses or ketoses can donate electrons, or reduce, to an oxidizing agent.

A classic test for reducing sugars is performed with Fehling (or Benedict) and Tollens reagents. For example, a reducing sugar can reduce Ag ⁺ present in an ammonium solution (Tollens' reagent). This reaction produces metallic silver at the bottom of the vessel where the reaction took place.

Through a reaction catalyzed by the enzyme glucose oxidase, the anomeric carbon of D-glucose is oxidized by losing one pair of electrons, and oxygen is reduced by receiving a pair of electrons. This reaction has two products: D-glucono-d-lactone and hydrogen peroxide.

Currently, the glucose concentration in the blood is determined by a test that uses glucose oxidase and peroxidase. This last enzyme catalyzes an oxidation-reduction reaction.

The substrates of peroxidase are hydrogen peroxide and a chromogenic substance, which is oxidized. This reaction can be quantified using a spectrophotometer.

Derivatives of hexoses

There are many derivatives of hexoses whose hydroxyl group is replaced by another substituent. For example, the C-2 hydroxyl group of glucose, galactose, and mannose is replaced with an amino group, forming glucosamine, galactosamine, and mannosamine, respectively.

Frequently, the amino group condenses with acetic acid, forming N-acetylglucosamine. This derivative of glucosamine is found in the cell wall of bacteria.

A derivative of N-acetylmannosamine is N-acetylneuraminic acid, known as sialic acid. The latter is present in glycoproteins and glycolipids on the surface of cells, having a role in recognition by other cells.

Specific oxidation of the primary alcohol group, C-6, of the aldohexoses glucose, galactose, and mannose produces uronic acids. These products are D-glucuronic acid, D-galacturonic acid and D-mannuronic acid, which are part of many polysaccharides.

Uronic acids can undergo intramolecular esterification. It forms lactones of five or six atoms. For example, ascorbic acid (vitamin C) is synthesized by plants.

Substitution of the hydroxyl group (-OH) by a hydrogen atom at C-6 of L-galactose or L-mannose produces L-fucose or L-rhamnose, respectively. L-fucose is found in glycoproteins and glycolipids. L-rhamnose is found in polysaccharides in plants.

Hexoses most common in nature and their functions

Glucose

Symbol: Glc. It is an aldohexose or glucohexose. The D-glucose enantiomer (symbol D-Glu) is more common than the L-Glc enantiomer. D-Glc is present in plants, honey, grapes, and in the blood of animals. It is a source of energy for living beings. It serves as a precursor for the synthesis of glycogen, cellulose, starch and lactose.

Fructose

Symbol: Fru. It is a ketohexose or fructohexose. The D-fructose enantiomer is commonly known as fructose. This sugar is found, for example, in fruits, honey, and semen.

Galactose

Gal symbol. It is an aldohexose or galatohexose. D-galactose is more common than L-galactose. D-galactose is the brain sugar. It is seldom free. It is generally found in plants, animals, and microorganisms in the form of oligosaccharides and polysaccharides.

Mannose

Symbol: Man. It is an aldohexose or mannohexose. The D-mannose form is widely distributed in manna and hemicellulose. It is found as an N-linked oligosaccharide to glycoproteins, forming branches.

Ramnosa

Symbol: Rha. It is an aldohexose that is found in the glycosides of plants, in the polysaccharides of gums and mucilages, as well as in the cell wall of plants and in flavonoids.

References

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