Furanose refers to the five‑membered cyclic form of a monosaccharide in which the ring consists of four carbon atoms and one oxygen atom, analogous in size to the heterocyclic compound furan. The term is derived from “furan,” a planar aromatic heterocycle, combined with the suffix “‑ose,” denoting a sugar. In biochemistry, furanoses are commonly encountered as the cyclic structures adopted by aldoses and ketoses that contain five carbon atoms (pentoses) or, in the case of certain six‑carbon sugars, as the result of intramolecular hemiacetal or hemiketal formation that yields a five‑membered ring rather than the more typical six‑membered pyranose form.
Structural features
- The furanose ring is formed by a nucleophilic attack of a hydroxyl group on the carbonyl carbon of the linear sugar, resulting in a hemiacetal (for aldoses) or hemiketal (for ketoses).
- The ring contains an anomeric carbon (C‑1 in aldoses, C‑2 in ketoses) that can adopt either the α‑ or β‑configuration, distinguished by the relative orientation of the substituent at the anomeric centre with respect to the ring oxygen.
- Substituents on the remaining carbons may be oriented above (axial) or below (equatorial) the plane of the ring, influencing the stereochemistry and physical properties of the sugar.
Common examples
- Ribose in its β‑D‑furanose form constitutes the backbone of ribonucleic acid (RNA).
- Deoxyribose (2‑deoxy‑D‑ribose) also adopts a β‑D‑furanose configuration in deoxyribonucleic acid (DNA).
- Arabinose and xylose can exist in both pyranose and furanose forms, with the furanose conformers playing roles in certain bacterial polysaccharides and nucleoside analogues.
Biological significance
Furanose rings are integral to the structure of nucleic acids, where the furanose sugar links to a phosphate group via phosphodiester bonds, forming the nucleic acid backbone. The geometry of the furanose ring influences the overall conformation of RNA and DNA, affecting processes such as replication, transcription, and enzymatic recognition. Additionally, furanose derivatives are employed in pharmaceutical chemistry as building blocks for nucleoside analogues with antiviral or anticancer activity.
Physical properties
- Furanose sugars are typically more flexible than their pyranose counterparts due to the smaller ring size, which can lead to increased conformational mobility.
- In solution, many monosaccharides interconvert between open‑chain, pyranose, and furanose forms; the equilibrium distribution depends on factors such as solvent, temperature, and the presence of catalysts (e.g., acids or enzymes).
Synthesis and interconversion
Enzymatic and chemical reactions can convert between pyranose and furanose forms. For example, aldolase enzymes catalyze the formation of furanose rings during the metabolism of pentose sugars, while mutarotation processes allow interconversion of α‑ and β‑anomers within the furanose series.
References
- Voet, D.; Voet, J. G. Biochemistry, 5th ed.; Wiley: New York, 2016.
- Berg, J. M.; Tymoczko, J. L.; Gatto, G. J. Stryer’s Biochemistry, 9th ed.; W. H. Freeman: New York, 2019.
- Alberts, B.; Johnson, A.; Lewis, J.; et al. Molecular Biology of the Cell, 7th ed.; Garland Science: New York, 2020.