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.
[3] The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons.
In the open form, there are eight aldopentoses and four 2-ketopentoses, stereoisomers that differ in the spatial position of the hydroxyl groups.
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.
The closed or cyclic form of a pentose forms when the carbonyl group reacts with a hydroxyl in another carbon, turning the carbonyl into a hydroxyl and creating an ether bridge –O– between the two carbons.
[3] The ring closure converts the carbonyl carbon into a chiral center, which may adopt either of two configurations, depending on the position of the new hydroxyl.