Ribose

File:Ribose.png

Ribose in acyclic form

A conventional skeletal formula for ribofuranose

Ribose, primarily seen as D-ribose, is a water-soluable, pentose sugar (monosaccharide with five carbon atoms) that is an important component of nucleic acids, nucleotides, the vitamin riboflavin, and various co-enzymes. Ribose has the chemical formula C₅H₁₀O₅.

Ribonucleic acid (RNA) is a nucleic acid based on the sugar ribose. Deoxyribonucleic acid (DNA) is a nucleic acid based on the closely related sugar deoxyribose. The bases in these nucleic acids (adenine, uracil, guanine, and cytosine in RNA, and thymine instead of uracil in DNA) represents the genetic information in living cells. As a component of RNA, which is used for genetic transcription, ribose is critical to living creatures.

Ribose is also a component of ATP, NADH, and several other chemicals that are critical to metabolism.

Structure

Ribose is an aldopentose, which means a pentose sugar with an aldehyde functional group in position 1. An aldehyde group consists of a carbon atom that is bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula O=CH-).

Ribose forms a five-member ring composed of four carbon atoms and one oxygen. Hydroxyl (-OH) groups are attached to three of the carbons. The fourth carbon in the ring (one of the carbon atoms adjacent to the oxygen) has attached to it the fifth carbon atom and a hydroxyl group.

Deoxyribose

Deoxyribose, also known as 2-deoxyribose, is also an aldopentose. It is derived from ribose by the replacement of the hydroxyl group at the 2 position (the carbon furthest from the attached carbon) with hydrogen, leading to the net loss of an oxygen atom. DNA has the chemical formula C₅H₁₀O₄.

Ribose was discovered in 1909 by Phoebus Levene, who also discovered DNA (1929) and found that DNA contained adenine, guanine, thymine, cytosine, deoxyribose, and a phosphate group.

Biological importance of ribose

Ribose and derivatives have an important role in biology.

Among the most important derivatives are those with phosphate groups attached at the 5 position. Mono-, di-, and triphosphate forms are important, as well as 3-5 cyclic monophosphates. There are also important diphosphate dimers called coenzymes that purines and pyrimidines form an important class of compounds with ribose and deoxyribose. When these purine and pyrimidine derivatives are coupled to a ribose sugar, they are called nucleosides. In these compounds, the convention is to put a ′ (pronounced "prime") after the carbon numbers of the sugar, so that in nucleoside derivatives a name might include, for instance, the term "5′-monophosphate", meaning that the phosphate group is attached to the fifth carbon of the sugar, and not to the base. The bases are attached to the 1′ ribose carbon in the common nucleosides. Phosphorylated nucleosides are called nucleotides.

One of the common bases is adenine (a purine derivative); coupled to ribose it is called adenosine; coupled to deoxyribose it is called deoxyadenosine. The 5′-triphosphate derivative of adenosine, commonly called ATP, for adenosine triphosphate, is an important energy transport molecule in cells.

2-Deoxyribose and ribose nucleotides are often found in unbranched 5′-3′ polymers. In these structures, the 3′carbon of one monomer unit is linked to a phosphate that is attached to the 5′carbon of the next unit, and so on. These polymer chains often contain many millions of monomer units. Since long polymers have physical properties distinctly different from those of small molecules, they are called macromolecules. The sugar-phosphate-sugar chain is called the backbone of the polymer. One end of the backbone has a free 5′phosphate, and the other end has a free 3′OH group. The backbone structure is independent of which particular bases are attached to the individual sugars.

Genetic material in earthly life often contains poly 5′-3′, 2′-deoxyribose nucleotides, in structures called chromosomes, where each monomer is one of the nucleotides deoxy- adenine, thymine, guanine or cytosine. This material is commonly called deoxyribonucleic acid, or simply DNA for short.

DNA in chromosomes forms very long helical structures containing two molecules with the backbones running in opposite directions on the outside of the helix and held together by hydrogen bonds between complementary nucleotide bases lying between the helical backbones. The lack of the 2′ hydroxyl group in DNA appears to allow the backbone the flexibility to assume the full conformation of the long double-helix, which involves not only the basic helix, but additional coiling necessary to fit these very long molecules into the very small volume of a cell nucleus.

In contrast, very similar molecules, containing ribose instead of deoxyribose, and known generically as RNA, are known to form only relatively short double-helical complementary base paired structures. These are well known, for instance, in ribosomal RNA molecules and in transfer RNA (tRNA), where so-called hairpin structures form from palindromic sequences within one molecule.

External links

• D-Ribose

Template:ChemicalSources

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