In chemistry, a pentose is a monosaccharide (simple sugar) with five

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

s. The

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbol ...

of many pentoses is , and their molecular weight is 150.13 g/mol.-Ribose
. PubChem compound webpage, accessed on 2010-02-06. Pentoses are very important in

biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

. Ribose is a constituent of RNA, and the related molecule,

deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. D ...

, is a constituent of DNA.

Phosphorylated In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

pentoses are important products of the

pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-pho ...

, most importantly

ribose 5-phosphate Ribose 5-phosphate (R5P) is both a product and an intermediate of the pentose phosphate pathway. The last step of the oxidative reactions in the pentose phosphate pathway is the production of ribulose 5-phosphate. Depending on the body's state, r ...

(R5P), which is used in the synthesis of

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...

s and nucleic acids, and

erythrose 4-phosphate Erythrose 4-phosphate is a phosphate of the simple sugar erythrose. It is an intermediate in the pentose phosphate pathway and the Calvin cycle. In addition, it serves as a precursor in the biosynthesis of the aromatic amino acids tyrosine, pheny ...

(E4P), which is used in the synthesis of aromatic amino acids. Like some other monosaccharides, pentoses exist in two forms, open-chain (linear) or closed-chain (cyclic), that easily convert into each other in water solutions. The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons. Four of these carbons have one

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...

(–OH) each, connected by a single bond, and one has an oxygen atom connected by a double bond (=O), forming a

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

group (C=O). The remaining bonds of the carbon atoms are satisfied by six

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atoms. Thus the structure of the linear form is H–(CHOH)_''x''–C(=O)–(CHOH)_4-''x''–H, where ''x'' is 0, 1, or 2. The term "pentose" sometimes is assumed to include deoxypentoses, such as

: compounds with general formula that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.

Classification

The aldopentoses are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1, forming an

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

derivative with structure H–C(=O)–(CHOH)₄–H. The most important example is ribose. The

ketopentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.ketone derivative with structure H–CHOH–C(=O)–(CHOH)₃–H (2-ketopentose) or H–(CHOH)₂–C(=O)–(CHOH)₂–H (3-ketopentose). The latter is not known to occur in nature and are difficult to synthesize. In the open form, there are eight aldopentoses and four 2-ketopentoses,

stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

s that differ in the spatial position of the hydroxyl groups. These forms occur in pairs of

optical isomer In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are d ...

s, generally labelled "" or "" by conventional rules (independently of their optical activity).

Aldopentoses

The aldopentoses have three chiral centers; therefore, eight (2³) different

s are possible. Ribose is a constituent of RNA, and the related molecule,

, is a constituent of DNA. Phosphorylated pentoses are important products of the

, most importantly

(R5P), which is used in the synthesis of

s and nucleic acids, and

(E4P), which is used in the synthesis of

aromatic amino acid An aromatic amino acid is an amino acid that includes an aromatic ring. Among the 20 standard amino acids, the following are classically considered aromatic: phenylalanine, tryptophan and tyrosine. Although histidine contains an aromatic ring, ...

Ketopentoses

The 2-ketopentoses have two chiral centers; therefore, four (2²) different stereoisomers are possible. The 3-ketopentoses are rare.

Cyclic form

The closed or cyclic form of a pentose is created when the carbonyl group interacts with a hydroxyl in another carbon, turning the carbonyl into a hydroxyl and creating an ether bridge –O– between the two carbons. This intramolecular reaction yields a cyclic molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then called

furanose A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle furan, bu ...

s, for having the same rings as the

cyclic ether Cycle, cycles, or cyclic may refer to: Anthropology and social sciences * Cyclic history, a theory of history * Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr. * Social cycle, various cycles in so ...

tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

. Library of Congress catalog 66-25695 The closure turns the carboxyl carbon into a chiral center, which may have any of two configurations, depending on the position of the new hydroxyl. Therefore, each linear form can produce two distinct closed forms, identified by prefixes "α" and "β".

Deoxypentoses

The one deoxypentose has two total stereoisomers.

Properties

In the cell, pentoses have a higher metabolic stability than

hexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert ...

s. A

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

composed of pentose sugars is called a pentosan.

Tests for pentoses

The most important tests for pentoses rely on converting the pentose to furfural, which then reacts with a chromophore. In Tollens’ test for pentoses (not to be confused with Tollens' silver-mirror test for

reducing sugar A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...

s), the furfural ring reacts with

phloroglucinol Phloroglucinol is an organic compound with the formula C6H3(OH)3. It is a colorless solid. It is used in the synthesis of pharmaceuticals and explosives. Phloroglucinol is one of three isomeric benzenetriols. The other two isomers are hydroxyqu ...

to produce a colored compound; in the

aniline acetate test The aniline acetate test is a chemical test for the presence of certain carbohydrates, in which they are converted to furfural with hydrochloric acid, which reacts with aniline acetate to produce a bright pink color. Pentoses give a strong reactio ...

with aniline acetate; and in

Bial's test Bial's test is a chemical test for the presence of pentoses. It is named after Manfred Bial, a German physician. The components include orcinol, hydrochloric acid, and ferric chloride. A pentose, if present, will be dehydrated to form furfural ...

, with

orcinol Orcinol is an organic compound with the formula CH3C6H3(OH)2. It occurs in many species of lichens including '' Roccella tinctoria'' and ''Lecanora''. Orcinol has been detected in the "toxic glue" of the ant species ''Camponotus saundersi''. It is ...

. In each of these tests, pentoses react much more strongly and quickly than hexoses.

References

{{carbohydrates