Ketopentose
A pentose is a monosaccharide with five carbon atoms. Pentoses are organized into two groups: Aldopentoses have an aldehyde functional group at position 1. Ketopentoses have a ketone functional group at position 2 or 3. In the cell, pentoses have a higher metabolic stability than hexoses.
The aldopentoses have three chiral centers; therefore, eight (23) different stereoisomers are possible.
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.
The 2-ketopentoses have two chiral centers; therefore, four (22) different stereoisomers are possible. The 3-ketopentoses are rare.
The one deoxypentose has two steroisomers, for two total steroisomers.
The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl functional groups to form intramolecular hemiacetals and hemiketals, respectively. The resulting ring structure is related to furan, and is termed a furanose. The ring spontaneously opens and closes, allowing rotation to occur about the bond between the carbonyl group and the neighbouring carbon atom — yielding two distinct configurations (α and β). This process is termed mutarotation.
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