Carbohydrates

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Carbohydrates are a major source of energy for life and are important structural molecules in many organisms. They are molecules that are made of oxygen, carbon, and hydrogen, with the empirical formula for most carbohydrates being (CH2O)n. They may also contain Sulphur and/ or Nitrogen. All carbohydrates have at least one hydroxyl group (OH) and an aldehyde or ketone group. The two simplest carbohydrates that are present naturally are Dihydroxyacetone and Glyceraldehyde. Carbohydrates contribute to structure, Protein targeting, Energy source,Metabolism and Cell recognition.

The simplest types of carbohydrates are monosaccharides. These are aldehydes or ketones with one or more hydroxyl groups. The smallest monosaccharide is composed of three carbon atoms. These are called trioses. Other simple monosaccharides are tetroses (4 carbon), pentoses (5 carbon) - [e.g the well-known ribose sugar in DNA], hexoses (6 carbon), and heptoses (7 carbon).

Many common sugars exist in cyclic forms. This occurs when the aldehyde or ketone group of the carbohydrate reacts with one of its own hydroxyl group to form either a hemiacetal or hemiketal. A 6-membered (6 carbon) cyclic hemiacetal/hemiketal is called a pyranose. A 5-membered ring is called a furanose[1].

The most frequently occurring of these is glucose and is important in energy metabolism. When glucose is not immediately required it is synthesized into glycogen. This is known as the Glycogen Synthase Reaction and it involves glucose donating a glucosyl residue to the non-reducing end of a glycogen branch[2].

Disaccharides are carbohydrates made up of two monosaccharides which are joined by an O-glycosidic bond. Polymers of carbohydrates can be created by the further addition of monosaccharides, complex carbohydrates containing more than one molecule are called oligosachharides[3]. and polymers of carbohydrates with more than ten monomers are called polysaccharides.

There is a vast range of isomeric forms in which carbohydrates can exist. Carbohydrates with the same chemical formula can differ as a result of the arrangement of bonding of the atoms, these isomers are called constitutional isomers. They can differ as a result of the spatial arrangement of the atoms, and are referred to as stereoisomers. The stereoisomer can exist in either D or L configuration, which is determined by the orientation of the asymmetric carbon that is farthest away from the keto or aldehyde group of the carbohydrate. The term Levo (L) applies to enantiomers which rotate plane polarised light in the same direction as L-glyceraldehyde[4]. The term Dextro (D) refers to enantiomers which rotate plane polarised light in the same direction as D-glyceraldehyde. Enantiomers are stereoisomers that are mirror images of each other, whereas diastereoisomers are stereoisomers that are not mirror images of each other[5].

References

  1. Berg J., Tymoczko J and Stryer L. (2011) Biochemistry, 7th edition, New York: WH Freeman. pg 332
  2. Matthews C, Van holde K, Ahern K, (2000) BIOCHEMISTRY, third edition, San Francisco, Addison-Wesley publishing company
  3. Jeremy M. Berg, John l. Tymoczko, Lubert Stryer with Gregory J gatto, Jr, (2012), Biochemistry, International 7th edition, W.H Freeman and company
  4. https://www.masterorganicchemistry.com/2017/05/24/d-and-l-sugars/
  5. Berg J., Tymoczko J and Stryer L. (2011) Biochemistry, 7th edition, New York: WH Freeman. pg 331
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