Furanose is a simple sugar, or monosaccharide, that forms a five-membered ring structure. The name “furanose” is derived from furan, a chemically similar organic compound that also has a five-membered ring with an oxygen atom.
The Structure of Furanose
The furanose ring is composed of four carbon atoms and one oxygen atom, creating a nearly planar five-membered structure. This arrangement is visualized using a Haworth projection, a chemical drawing method that represents the three-dimensional shape of the cyclic sugar. The Haworth projection depicts the ring as if viewed from the side, with some bonds thickened to show they are closer to the observer, providing a sense of depth.
A feature of the furanose structure is the anomeric carbon, which was part of the carbonyl group in the sugar’s linear form. In the ring structure, this carbon becomes a new chiral center, meaning it can exist in two different spatial arrangements. These two distinct forms are known as anomers, labeled as alpha (α) and beta (β), based on the orientation of the hydroxyl (-OH) group attached to the anomeric carbon. In a D-configuration sugar, the alpha anomer has the hydroxyl group pointing down, while the beta anomer has it pointing up.
For example, in ribofuranose, the ring consists of carbons numbered 1′ through 4′ and an oxygen atom. The anomeric carbon is at the 1′ position. The orientation of the -OH group at this 1′ position determines whether the molecule is α-ribofuranose or β-ribofuranose.
Formation from Linear Sugars
The transformation of a linear sugar into a furanose ring is a spontaneous and reversible chemical reaction in aqueous solutions. This process, known as intramolecular cyclization, does not require enzymes. The process involves a reaction where a hydroxyl group attacks the carbonyl carbon of the sugar’s open-chain form.
This internal attack forms a more stable cyclic structure called a hemiacetal or a hemiketal. For a furanose ring to form, the hydroxyl group on the fourth carbon of an aldopentose or the fifth carbon of a ketohexose initiates the attack. The oxygen from this attacking hydroxyl group becomes the oxygen atom within the newly formed five-membered ring.
For instance, when the ketohexose fructose cyclizes, the hydroxyl group on its C-5 attacks the ketone group at the C-2 position. This reaction creates a covalent bond between the C-5 oxygen and the C-2 carbon, forming a five-membered fructofuranose ring. In solution, this cyclic form exists in equilibrium with the open-chain form, though the ring forms are more prevalent.
Furanose Versus Pyranose
Sugars in solution exist in two main cyclic forms: furanose and pyranose. The difference between them is the ring size; a furanose is a five-membered ring, while a pyranose is a six-membered ring composed of five carbons and one oxygen.
The stability of these forms differs, influencing which structure a sugar adopts. Pyranose rings are more thermodynamically stable than furanose rings because their six-membered structure allows for a “chair” conformation. This conformation minimizes strain between atoms, making it an energetically favorable arrangement. For this reason, aldohexoses like glucose exist almost exclusively in the pyranose form in solution.
However, the formation of a furanose ring is often kinetically preferred, meaning it can form more quickly. While the pyranose form is dominant for many sugars, the furanose structure is adopted in specific biological contexts where its unique shape and properties are advantageous. Fructose, for example, can exist as both a pyranose and a furanose, but the furanose form is incorporated into certain biological molecules.
Biological Significance of Furanose
The furanose ring forms the structural backbone of nucleic acids. The sugars ribose and deoxyribose are the components of RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), respectively, and exist in their furanose forms within these molecules. The geometry of the furanose ring is suited to create the helical polymer structure of DNA and RNA for storing and transmitting genetic information.
The specific conformation of the furanose sugar within the nucleic acid backbone influences the overall shape and stability of the DNA or RNA duplex. Changes in the sugar’s structure, such as the inversion of a hydroxyl group at a single carbon, can alter the helical structure, demonstrating the importance of the furanose form.
Beyond nucleic acids, the furanose structure is present in other molecules. The common table sugar, sucrose, is a disaccharide formed by linking a glucose molecule with a fructose molecule. Specifically, the fructofuranose form of fructose bonds with glucose to create sucrose. This demonstrates that even for sugars that can exist in more stable forms, the furanose structure is selected for specific biochemical roles.