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Carbohydrates

All carbohydrates follow the general formula: Cn(H2O)n. Carbohydrates have a variety of different functions, which include acting as metabolic intermediates and storage forms of carbon and energy. Other carbohydrates constitute structural materials, while still others are indicators and antigens used for cell recognition.

Stereochemistry

Carbohydrates derive their vast diversity from stereochemistry, in other words, L forms and their mirror D forms. The following structures are the D forms of some common monosaccharides.

D-glucose.png D-galactose.png D-ribose.png D-fructose.png
glucose galactose ribose fructose

Glucose, galactose, and ribose are called aldoses because they have an aldehyde group attached to their carbon-1. Fructose is called a ketose because it has a keto group attached to its carbon-2.

Cyclization

Cyclization of an aldose, such as glucose, yields a pyranose ring with an anomeric carbon-1, which gives rise to α and β forms of the cyclic carbohydrate. A mixture of the linear, α, and β forms exist in solution at all times.

linear glucose.png Picture 2.png alpha.png beta.png
      D-glucose   cyclization α-D-glucopyranose β-D-glucopyranose

The following are the cyclic structures of the other aldoses mentioned above:

 D-galactose:      D-ribose:  
alpha galactose.png beta galactose.png alpha ribose.png beta ribose.png
α-D-galactopyranose β-D-galactopyranose α-D-ribopyranose β-D-ribopyranose

Cyclization of a ketose yields a furanose ring with an anomeric carbon-2:

furanose.png

  D-fructose            α-D-fructofuranose             β-D-fructofuranose

Oligosaccharides

"Oligo-" literally means few, so oligosaccharides are chains of a few monosaccharides linked together by glycosidic bonds. The following are a few common oligosaccharides, each serving a different function. The red "HOH"s represent the possibility of anomeric forms of the specific oligosaccharides. They are attached to the free anomeric carbon and are the reducing ends of the sugars. Note that sucrose and trehalose do not have free anomeric carbons, and therefore are not reducing sugars.

lactose.png maltose.png cellobiose.png
lactose maltose cellobiose
(galactose-β-1,4-glucose) (glucose-α-1,4-glucose) (glucose-β-1,4-glucose)
sucrose.png trehalose.png
sucrose trehalose
(glucose-α-1,2-fructose) (glucose-α α 1,1-glucose)

 

A reducing sugar is a mono- or oligosaccharide that contains a hemiacetal or a hemiketal group. All monosaccharides above are reducing sugars, and all polysaccharides are non-reducing.

Polysaccharides

The prefix "poly-" means many, so polysaccharides are branches of many monosaccharides linked together through glycosidic bonds. Polysaccharides vary greatly in their structures, and this variety leads to differentiation of function.

For example, amylose and amylopectin (poly-α1,4(α1,6)-glucose) have disticntly different structures from other polysaccharides like cellulose and chitin. Amylose is able to form helices and can be tested for using iodine. On the other hand, cellulose and chitin (poly-β1,4-N-acetyl-glucoseamine), form straight chains and associate to make cables, and thus provide structural support for organisms.

amylose.png cellulose.png
amylose (poly-α1,4-glucose) cellulose (poly-β1,4-glucose)

Still other polysaccharides decorate and act as indicators for proteins, especially the ones that are secreted. For example, erythropoietin is a secreted protein that is marked by branches of many differents polysaccharides.