Molecule of the Day: Other Sugars

Carbohydrates can be designated with a variety of special terms that you need to be familiar with.

Pentose – a carbohydrate with five carbon atoms
Hexose – a carbohydrate with six carbon atoms
Aldose – a carbohydrate that has an aldehyde carbonyl group
Ketose – a carbohydrate that has a ketone carbonyl group
Pyranose – a six-membered ring form of a carbohydrate
Furanose – a five-membered ring form of a carbohydrate
D-Carbohydrate – the stereocenter farthest from the carbonyl group is the same as is found in D-glyceraldehyde (-OH group is to the right on a Fischer projection)
L-Carbohydrate – the stereocenter farthest from the carbonyl group is the same as is found in L-glyceraldehyde (-OH group is to the left on a Fischer projection)
Alpha – the –OH group on the anomeric carbon is on the opposite side of the ring as the terminal –CH2OH group. For glucose in the ring (pyranose) form this is axial.
Beta - the –OH group on the anomeric carbon is on the same side of the ring as the terminal –CH2OH group. For glucose in the ring form (pyranose) this is equatorial.
These terms are combined to give full descriptions of individual carbohydrates. For example, glucose is an aldohexose. When it cyclizes, it forms a pyranose ring. The full description of the alpha anomer of cyclized glucose is “alpha-D-glucopyranose”, also called simply “alpha-D-glucose”. Similarly the beta anomer is referred to as “beta-D-glucopyranose”, also called simply “beta-D-glucose.”

Five-membered rings are the most stable form of some carbohydrates. For example D-fructose, a ketohexose, forms a stable five-membered ring. Ribose, a common aldopentose, also forms a five-membered ring. Ribose is important in biochemistry because it is the “sugar” part of the RNA bases such as cytidine. Some sugars are missing –OH groups, and these are indicated by the number corresponding to the location of the missing –OH group, followed by the term “deoxy” meaning “without oxygen”. 2-Deoxy-D-ribose is the “sugar” on DNA bases, for example the base 2’-deoxycytidine.