[3] Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.
Hexoses are extremely important in biochemistry, both as isolated molecules (such as glucose and fructose) and as building blocks of other compounds such as starch, cellulose, and glycosides.
When the carbonyl is in position 1, forming an formyl group (−CH=O), the sugar is called an aldohexose, a special case of aldose.
Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a ketone, and is called a ketohexose, a special case of ketose; specifically, an n-ketohexose.
Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "D-" or "L-", depending on whether the hydroxyl in position 5, in the Fischer projection of the molecule, is to the right or to the left of the axis, respectively.
In general, only one of the two enantiomers occurs naturally (for example, D-glucose) and can be metabolized by animals or fermented by yeasts.
The linear forms of the eight D-aldohexoses, in the Fischer projection, are Of these D-isomers, all except D-altrose occur in living organisms, but only three are common: D-glucose, D-galactose, and D-mannose.
The L-isomers are generally absent in living organisms; however, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.
The chemist Emil Fischer is said[citation needed] to have devised the following mnemonic device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings: referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose.
The Fischer diagrams of the eight L-aldohexoses are the mirror images of the corresponding D-isomers; with all hydroxyls reversed, including the one on carbon 5.
The closure turns the carboxyl carbon into a chiral center, which may have either of two configurations, depending on the position of the new hydroxyl.
[11] D-Fructose is responsible for the sweet taste of many fruits, and is a building block of sucrose, the common sugar.
It is named as the parent hexose, with the prefix "x-deoxy-", the x indicating the carbon with the affected hydroxyl.