Pentose

In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is C
₅H
₁₀O
₅, and their molecular weight is 150.13 g/mol.

Pentoses are very important in biochemistry. Ribose is a constituent of RNA, and the related molecule, deoxyribose, is a constituent of DNA. Phosphorylated pentoses are important products of the pentose phosphate pathway, most importantly ribose 5-phosphate (R5P), which is used in the synthesis of nucleotides and nucleic acids, and erythrose 4-phosphate (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 functional group (–OH) each, connected by a single bond, and one has an oxygen atom connected by a double bond (=O), forming a carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by six hydrogen 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 deoxyribose: compounds with general formula C
₅H
₁₀O
_5-y that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.