Introduction

Ribose is the most important monosaccharide in the life of living organisms after glucose. The main importance of ribose is because of its role in the structure of nucleotides. It is present even in the smallest organisms like viruses and bacteria. 

The metabolic pathways of ribose hold prime importance in the living organisms. In this article, we will discuss different aspects of ribose like its structure, isomers, properties, sources, metabolism, etc. Keep reading to completely understand the concepts related to this sugar. 

Structure

Ribose is a monosaccharide having five carbons, thus called a pentose sugar. Ribose is the most important pentose present in living organisms. It is an aldose sugar, having an aldehydic functional group. Its molecular formula is represented as C₅H₁₀O₅.

The structural formula of ribose can be represented in two forms;

• Linear chain 
• Closed ring

Both these structural forms of ribose coexist in equilibrium with each other in an aqueous solution. They are readily interconvertible. 

Linear chain

The aliphatic or open chain form of ribose is made up of five carbon atoms that are arranged in the form of a linear chain. As monosaccharides are polyhydroxy aldehydes or ketones, each carbon atom in ribose is having a hydroxyl group except the first carbon. Like all other aldose sugar, the first carbon of ribose is also a part of aldehydic functional group. Ribose is also called an aldopentose.

Ring form 

 In an aqueous solution, ribose can form two types of closed rings;

• Ribofuranose
• Ribopyranose

Ribofuranose is a five cornered ring. Four corners of this pentagon are occupied by carbon atoms, while the fifth corner, the apical one, is formed by an oxygen atom of the carbonyl functional group. One carbon hangs outside the ring. This is the least abundant closed ring form of ribose in aqueous solutions. 

Ribopyranose ring has a hexagonal structure. Its structure is similar to glucose with five corners occupied by carbon atoms and one corner by an oxygen atom. However, unlike glucose, none of its carbon atom hangs outside the ring as all the five carbon atoms participate in ring formation. This is the most abundant form of ribose in aqueous solution with more than 80% abundance. 

It should always be kept in mind that all these forms are interchangeable. 

Isomers

The property of isomerization is common is all monosaccharides. The number of isomers of a monosaccharide is dependent on the number of chiral carbons present in it. Remember a chiral carbon is one that is attached to four different atoms or functional groups. The formula to determine the number of isomers from chiral carbons is as follows;

N = 2ⁿ 

(here N=number of isomers, and n=number of chiral carbons)

Except for the first and the last carbon, the other three carbon atoms in ribose show chiral behavior. The ribose has eight isomers.

 Each of these eight isomers can have either an alpha or beta orientation when dissolved in water. Thus, a total of 16 different structural forms. 

Optical isomers

Optical isomers are the molecules that differ in the optical properties. Ribose exists in the form of two optical isomers that are the mirror images of one another. 

• D-ribose
• L-ribose

D-ribose is dextrorotatory. A beam of light is bent in the right direction when passed through its aqueous solution. While writing its structural formula, the hydroxyl groups are written on the right side of carbon atoms. 

On the other hand, L-ribose is levorotatory. It bends the light rays to the left when passed through its aqueous solution. The hydroxyl groups are written on the left side of carbon atoms in the structural formula of L-ribose. 

Only D-ribose is found naturally in living organisms. The L-ribose is a synthetic molecule and does not occur in living structures. 

Epimers of ribose

Epimers are the isomers differing in the structural arrangement of atoms around a single carbon atom. Ribose have four epimers;

• Ribose
• Arabinose (C2 epimer of ribose)
• Xylose (C3 epimer of ribose)
• Lyxose (C2 epimer of xylose)

All these epimers have two optical isomers, an L-isomer and a D-isomer.  

Alpha and beta forms

All the isomers of ribose can have either an alpha or beta ring when dissolved in an aqueous solution. 

Remember that in alpha rings, the hydroxyl group of carbonyl group (of carbon 1) is directed below the plane of the ring, while in beta rings, it is directed above the plan of the ring. 

Alpha or beta orientation can be readily found by looking at the structural formula of the molecule. 

Both the alpha and beta forms exist in equilibrium with each other and are interconvertible. 

Properties

Ribose resembles all other monosaccharides in its properties. The common properties are as follows;

• It is sweet,  but less sweet than glucose 
• It is also a polar compound that readily dissolves in water
• It is a reducing sugar having a free functional group
• It cannot by hydrolyzed
• It can undergo reactions like oxidation, reduction and ester formation
• Repetitive units of ribose can also be combined by glycosidic bonds to form polymers.

Occurrence

Ribose is only rarely present in nature in its free form. Surprisingly, ribose in free form was found by scientist in meteorites along with other sugar molecules.

In living organisms, ribose is present in combined form as a part of other molecules. It abundantly occurs in nucleic acids of living organisms 

Being an integral component of ribonucleotides, it is present in all forms of RNA present in living organisms from viruses to large mammals. 

It is also present in ATP, ADP, NADH, NADPH, FADH₂, and other nucleotides present in living cells. 

Synthesis 

Ribose is synthesized from glucose and other monosaccharide molecules in living cells by a process known as pentose phosphate pathway. 

The pentose phosphate pathway is a series of chemical reactions taking place in the cytosol of cells. The process begins with a molecule of glucose and is divided into two phases; 

• Phase 1 involves irreversible reactions
• Phase 2 involves reversible conversions

Irreversible reactions

Phase one of the pentose phosphate pathway involves three irreversible oxidative reactions. Beginning with a molecule of glucose-6-phosphate, these reactions oxidize glucose into ribulose-5-phosphate and a molecule of carbon dioxide is released. 

• In the first reaction, glucose-6-phosphate is converted to 6-phosphogluconolactone by glucose-6-phosphate dehydrogenase. 
• The second reaction involves the hydrolysis of 6-phosphogluconolactone into 6-phosphogluconate by a hydrolase enzyme
• In the third reaction, 6-phosphogluconate undergoes oxidative decarboxylation to yield ribulose-5-phosphate. 

The first and the third reaction are the oxidative reaction that use NADP⁺ as an oxidizing agent. As a result, one molecule of NADPH is produced in each of these two reactions. 

Reversible conversions

These are the non-oxidative reactions involving the interconversions of sugars containing 3 to 7 carbon atoms. It begins with the tautomerization of ribulose-5-phosphate into ribose-5-phosphate or xylulose-5-phosphate. 

After this step of tautomerization, the ribose-5-phosphate have two fates;

• It can either leave the reversible conversions to participate in nucleic acids synthesis 
• Or it can undergo further conversions resulting in the synthesis of glyceraldehyde-3-phosphate or fructose-6-phosphate

Two types of enzymes are involved in these reversible conversions;

• Transketolase, it transfers a keto group from one sugar molecule to another
• Transaldolase, this enzyme transfers an aldehydic functional group between the sugar molecules

Metabolism

Ribose present in the human body can be processed in two ways. 

• It can be converted into glyceraldehyde-3-phosphate or fructose-6-phosphate by the process of reversible conversions. Both these molecules are then processed via glycolysis. 
• Ribose can be used in the process of nucleotide and nucleic acid synthesis.

The reversible conversions have already been discussed above. Here, we will only study how ribose is used in nucleotide synthesis. 

Nucleotide synthesis

The first step in the synthesis of nucleotides is the production of ‘activated pentose’. The activated pentose is the form of ribose that can undergo the further steps of nucleotide and nucleic acid synthesis.

Activated pentose is 5-phosphoribosyl-1-pyrophosphate molecule (also called PRPP). The synthesis of this molecule is the rate-limiting step in the synthesis of nucleotides. 

The activated pentose or PRPP is made from ribose-5-phosphate and ATP in an irreversible energy-consuming reaction. The process is catalyzed by PRPP synthetase enzyme. Magnesium ions are used as cofactors in this process. 

This process adds one phosphate group of ATP to the 5^th carbon of ribose, and the other two phosphate groups are added to the first carbon. 

The resulting compound is then used for the construction of the nitrogenous base on the first carbon so that a nucleotide is formed. 

Importance

Ribose is a pentose having prime importance in human beings. The important points regarding ribose are as follows. 

• It is a component of nucleic acids. 
• It is a component of nucleotides like NADH, NADPH, and FADH₂, that act as reducing agents and co-enzymes in various biochemical reactions. 
• It is a component of ATP, the main energy currency of living cells. 

Ribose also has important uses in the medical sector. For example, 

• D-ribose is used in the management of congestive heart failure.
• It is also suggested to be used in the management of myalgia encephalitis.
• It has been used to treat conditions like fibromyalgia, chronic fatigue syndrome and myocardial dysfunction. 
• The use of D-ribose is also suggested to reduce cramps, pain, and stiffness that occurs after exercise.
• It is used by athletes to improve their performance.

Deoxyribose

Deoxyribose is another important pentose present in living organisms. It is evident from the name that it is a deoxy sugar. 

Deoxyribose is originally derived from ribose by removal of an oxygen atom from the 2’ hydroxyl group. Thus, it is also called 2-deoxyribose. 

Like ribose, deoxyribose also has two optical isomers, a D-deoxyribose and an L-deoxyribose. 

Both these optical isomers can exist in either alpha form or beta form when dissolved in aqueous solution. 

Naturally, alpha-D-deoxyribose is present in living structure. L-deoxyribose is found only rarely. 

Synthesis

When the nucleotides required for DNA have been formed, the ribose present in their structure is then converted to deoxyribose by the process of deoxygenation.  This process is catalyzed by enzyme ribonucleotide reductase. 

This enzyme only acts on nucleotides having two phosphate groups i.e. ribonucleoside diphosphates.

One molecule of NADPH is used in this process. 

Importance

Deoxyribose has prime importance in biological molecules because it is a component of DNA. It is present in all living cells including viruses. It is believed that the absence of 2’ hydroxyl in deoxyribose provides more mechanical flexibility to DNA.

Summary

• Ribose is an important biological molecule present in all living organisms in one or another form. 
• It is a monosaccharide with 5 carbons having an aldehydic group.
• L-ribose and D-ribose are two optical isomers that can either form a pentagonal or hexagonal ring in aqueous solution.
• There are 8 isomers of ribose, each of which can have either an alpha or beta ring when dissolved in an aqueous solution. 
• Properties of ribose are the same as that of the other monosaccharides.
• It is made from glucose in a process known as pentose phosphate pathway.
• In living cells, it is either used to form nucleotides or converted into monosaccharides that can be channeled into the glycolysis. 
• Its importance is mainly because of being a component of nucleic acids and nucleotides.
• Deoxyribose is an important pentose sugar obtained after the deoxygenation of ribose. 
• The deoxygenation of ribose is carried out by ribonucleotide reductase enzyme. 
• The only difference between the deoxyribose and ribose is the presence or absence of a hydroxyl group at the second carbon of the molecule. 
• Deoxyribose is present in all forms of DNA. 
• It is structurally more flexible than ribose. 

Frequently Asked Questions

What is ribose?

Ribose is a poentose sugar. It is a simple carbohydrate that consists of five carbon atoms. It is an essential carbohydrate found in the genetic material of all living organisms.

What is the importance of ribose?

Ribose sugar is an essential component of nucleotides. These nucleotides join to form nucleic acids that contain the genetic material of a cell. Thus, ribose sugar is important for the storage and transfer of genetic material and the synthesis of proteins. It is essential for survival and continuity of life. 

What is the difference between ribose and deoxyribose?

Both of them are pentose sugars each having five carbon atoms. Deoxyribose lacks one oxygen at its carbon 2 as compared to ribose. Ribose is found in mRNA while deoxyribose is present in DNA.

Which pentose sugar is found in DNA?

DNA contains deoxyribose. It is a pentose sugar in which one carbon atom is in deoxygenated form. 

References

1. Bhutani, S. P. (2019). “Aldopentoses—The Sugars of Nucleic Acids”. Chemistry of Biomolecules (2nd ed.). CRC Press. pp. 63–65. ISBN 9781000650907.
2. Drew, Kenneth N.; Zajicek, Jaroslav; Bondo, Gail; Bose, Bidisha; Serianni, Anthony S. (February 1998). “¹³C-labeled aldopentoses: detection and quantitation of cyclic and acyclic forms by heteronuclear 1D and 2D NMR spectroscopy”. Carbohydrate Research. 307 (3–4): 199–209. doi:10.1016/S0008-6215(98)00040-8.
3. de Wulf, P.; Vandamme, E. J. (1997). “Microbial Synthesis of ᴅ-Ribose: Metabolic Deregulation and Fermentation Process”. Advances in Applied Microbiology. 44: 167–214. doi:10.1016/S0065-2164(08)70462-3. ISBN 9780120026449.
4. Steigerwald, Bill; Jones, Nancy; Furukawa, Yoshihiro (18 November 2019). “First Detection of Sugars in Meteorites Gives Clues to Origin of Life”
5. Bloomfield, Victor; Crothers, Donald; Tinoco, Ignacio (2000). Nucleic Acids: Structures, Properties, and Functions. University Science Books. pp. 19–25.
6. https://en.wikipedia.org/wiki/File:Ribose_Structure_2.svg
7. https://commons.wikimedia.org/wiki/File:Ribose_deoxyribose.png