Carbohydrate structure

As their name implies, carbohydrates are molecules made up of ‘hydrated carbon’, containing many hydroxyl groups, plus an aldehyde or a keto group, and can be represented by the simple empirical formula CH₂O_n. Simple carbohydrates (or monosaccharides) consist of a single carbohydrate unit, the smallest of which is glyceraldehyde. This has three carbons and is therefore classed as a triose. Typically, carbohydrates in humans range from three to nine (nonose) carbon atoms, with many abundant and important carbohydrates containing six carbons (hexoses). Monosaccharides with an aldehyde group are classed as aldoses (e.g. glyceraldehyde), while those with keto groups are ketoses (e.g. dihydroxyacetone).

Glyceraldehyde (Figure 6.1) has an asymmetric carbon atom at position 2 (the four groups attached to it are different) and there­­fore can exist in two conformations that are mirror images of each other (enantiomers). These are known as the D and L forms. The majority of carbohydrates in humans are in the D-form. Enzymes are stereospecific and will only act on the correct enantiomer of a carbohydrate.

Glucose

The most abundant carbohydrate in the human body is glucose, with the empirical formula C₆H₁₂O₆, which can be represented as a linear chain of carbon atoms with an aldehyde group at one end (designated carbon atom 1) and hydroxyl groups attached to the other carbons (Figure 6.2). Other hexoses share the same empirical formula as glucose but differ in the arrangement of the various hydroxyl and aldehyde groups and are therefore isomers. Epimers differ in the arrangement of groups around a single carbon, e.g. glucose and mannose. In solution, monosaccharides rapidly cyclise by reaction of the aldehyde (e.g. C6 in an aldohexose) or keto (e.g. C5 in ketohexose) to form ring structures resembling a chair. A pyranose is a six-membered ring and a furanose a five-membered ring. Formation of a ring creates an anomeric carbon (which carried the aldehyde or keto group), and two isomers (diastereomers) are possible, α and β. Glucose exists in both forms in solution, but these rapidly interconvert; however, enzymes are specific for a particular diastereomer. Glucose also exists as enantiomers, but it is the D form that is metabolically active in humans.