Fructose is a monosaccharide, a simple sugar, and is classified as a ketohexose because it has six carbon atoms and contains a ketone functional group. The detailed structure of fructose can be explained in the context of its cyclic form, specifically as a five-membered ring known as a furanose ring.

Structural Formula

In its Haworth projection, which provides a three-dimensional view of the cyclic structure, people often depict the structural formula for fructose.

In this representation:

– The horizontal lines represent bonds from the plane towards you (wedges).

– The vertical lines represent bonds going into the plane away from you (dashes).

– The “C” represents carbon atoms.

– The “H” represents hydrogen atoms.

– The “OH” represents hydroxyl groups.

Cyclic Structure

Fructose exists in two predominant forms: alpha fructose and beta fructose. The structures differ in the orientation of the hydroxyl group attached to the anomeric carbon (C2 in fructose).

– Alpha Fructose:

  In the alpha configuration, the hydroxyl group at the anomeric carbon is below the plane of the ring.

– Beta Fructose:

  In the beta configuration, the hydroxyl group at the anomeric carbon is above the plane of the ring.

A reaction between the ketone group on carbon two and the hydroxyl group on carbon 5 forms the five-carbon ring, resulting in a five-membered ring.

Fischer Projection

Fructose can also be represented using Fischer projection:

Function

Fructose, commonly found in fruits and honey, constitutes a component of the disaccharide sucrose (table sugar), which comprises one molecule of glucose and one molecule of fructose.It is an important energy source in the human diet and, like glucose, participates in various metabolic pathways within cells.