KETOSES: CHARACTERISTICS, FUNCTIONS, EXAMPLES - BIOLOGY - 2025

Ketose is the term used to denote monosaccharides that contain at least one "ketone" group in their molecular structure, that is, a group characterized as RC (= O) R ', which represents the most oxidized functional group of the molecule.

Monosaccharides are the simplest sugars. They are generally solid, crystalline and colorless compounds; they mostly have a sweet taste and are highly soluble in water and insoluble in non-polar solvents.

Some known ketones (Source: http://www.bionova.org.es/biocast/tema07.htm via Wikimedia Commons)

Structurally speaking, most of the monosaccharides that are present in nature exist in one of two forms: aldose or ketosa; which are molecules that are distinguished by the presence of an aldehyde group or a "keto" group, respectively.

The most common examples of ketose sugars are dihydroxyacetone, erythrulose, xylulose, and ribulose, fructose, sorbose, or isomaltulose, among others.

characteristics

As is true of most monosaccharides, ketoses are molecules made up of carbon, hydrogen, and oxygen atoms linked together through single, unbranched bonds.

In their "open" chain configuration, the general characteristic of all monosaccharides is that they have a carbon atom double bonded to an oxygen atom, forming a carbonyl group.

Structure of dihydroacetone, the simplest ketose (Source: Emeldir via Wikimedia Commons)

Ketoses differ from other closely related monosaccharides, aldoses (which have an aldehyde group, R-HC = O), in that the carbonyl group is not found at the end of the carbon chain, but can be in any other position of the monosaccharide, so it forms a "keto" group, also known as RC (= O) R '.

It is a general rule that the simplest monosaccharides are the “trioses”, that is, those sugars that have only three carbon atoms. Thus, the simplest ketose that can be found in nature is the ketotriose dihydroxyacetone.

Nomenclature

Depending on the number of carbon atoms, the ketoses can be:

- Ketotrioses: the ketoses with three carbon atoms, such as dihydroxyacetone.

- Ketotetrose: ketoses with 4 carbon atoms, such as erythrulose.

- Ketopentoses: ketoses with five carbon atoms, such as ribulose.

- Ketohexoses: ketoses with six carbon atoms, such as fructose.

- Ketoheptoses: ketoses with seven carbon atoms, such as sedoheptulose.

The D- and L- forms

With the sole exception of dihydroxyacetone, all monosaccharides (whether aldoses or ketoses) possess one or more "asymmetric" carbon "centers" or atoms. Therefore, they can be found in two forms or isomers that are "optically active", and that are called enantiomers, which are non-superimposable stereoisomers (mirror images).

Fisher's projection for sedoheptulose, a ketoheptose (Source: Yikrazuul via Wikimedia Commons)

The two possible forms, then, are conventionally known as the D- isomers and the L- isomers and the amount of these enantiomers that a monosaccharide molecule possesses depends on the number of chiral centers or carbons (n), that is, each monosaccharide possesses 2 to the power n stereoisomers.

The α and β forms, ketofuranose and ketopyranous

In aqueous solution, ketoses with 5 or more carbon atoms (also aldoses) are found as cyclic or ring structures, where the carbonyl group is covalently bonded to the oxygen atom of some hydroxyl group in the carbon chain, which forms a derivative compound known as "hemiketal".

Hemicetals are characterized by the presence of an additional asymmetric carbon atom, so that there may be two more stereoisomers for each ketosa, known by the Greek letters α and β, which are called anomers.

In addition, ketoses can be found in cyclic forms of 5 or 6 carbon atoms, which are known respectively as ketofuranose and ketopyranose.

Features

The most common monosaccharides in nature are hexoses, either aldohexoses or ketohexoses. An important example of a ketohexose is fructose, which is a fundamental part of the diet of many animals, insects, fungi and bacteria, as it is mainly found in fruits, honey and vegetables.

Sucrose, which is the sugar that man consumes daily, is a disaccharide made up of a molecule of fructose and another of glucose.

Comparison in the structure of two hexose sugars: glucose (an aldohexose) and fructose (a ketohexose) (Source: Prokaryote2 via Wikimedia Commons)

Since a significant proportion of isomerization between fructose and glucose can occur, this ketohexose is very important from the cellular metabolic point of view, since glucose is one of the main substrates that cells use to obtain energy in the form of ATP.

In the structural context, ketoses are also essential, since some ketopentoses function as intermediates or precursors in the synthesis of the sugars used in the carbon skeletons of nucleic acids, which are present in all living beings and are the molecules that contain their Genetic information.

Examples

Fructose, as already mentioned, is perhaps the most representative example among the ketosis sugars, since it is extremely common in plant tissues and in many of the prepared foods that we consume daily.

However, there are other important ketones that have some significance from an industrial point of view, as they are easy and inexpensive to obtain. Furthermore, like the rest of the known monosaccharides, they are polyfunctional, polar and water-soluble compounds, which implies that they can be subject to multiple chemical transformations.

Among these monosaccharides are:

L-sorbose

It is a ketohexose that is 5-epimer of fructose. This ketose is an intermediate in the industrial production of vitamin C from glucose.

Isomaltulose

This is a disaccharide that is the product of the bacterial fermentation of sucrose (composed of glucose and fructose). Its industrial importance has to do with its possible conversion to D-mannitol or "isomalt", widely used in gastronomy.

Lactulose

This ketose is obtained as a "by-product" of the dairy processing industry and can be artificially converted into N-acetylactosamine, which is a disaccharide present in many biologically important oligosaccharides. Furthermore, it is commercially available as an osmotic laxative called "laevulac."

References

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2. Mathews, CK, Van Holde, KE, & Ahern, KG (2000). Biochemistry. Add. Wesley Longman, San Francisco.
3. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
4. Ouellette, RJ, & Rawn, JD (2014). Organic chemistry: structure, mechanism, and synthesis. Elsevier.
5. Stenesh, J. (1989). Dictionary of biochemistry and molecular biology. John Wiley.
6. Stick, RV, & Williams, S. (2010). Carbohydrates: the essential molecules of life. Elsevier.