Carbohydrates

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Carbohydrates are the most abundant organic in nature. The general formula is (CH2O)2. Carbohydrates provides wide range of functions such as energy providing, energy storage and form plasma membrane components.

carbohydrate, energy, work

 Carbohydrate structure classification

Carbohydrates are classified into four groups:

  1. Monosaccharides
  2. Disaccharides
  3. Oligosaccharides
  4. Polysaccharides

Monosaccharides

Monosaccharides are simple sugars, building blocks for all carbohydrates. They are classified into aldehyde and keto groups. Aldehyde sugars are called aldoses. Aldoses are named according the number of carbon atom such as aldotetroses, aldopentoses, aldohexoses, and aldoheptoses etc. Keto sugars are called ketoses. Ketoses are ketotetroses, ketopentoses, ketohexoses, and ketoheptoses.

Characteristics of monosaccharides

Monosaccharides have some common characteristics as:

1. Isomers and epimers

  • Isomers are compounds that have same chemical formula but differ in configuration. For example, glucose and fructose (Figure 1).
  • Epimers are compounds that differ in configuration around one specific carbon atom (except carbonyl bond). For example, glucose and mannose (Figure 2).
Monosaccharide, glucose, fructose

Figure 1: Examples of aldehyde and keto groups

 

epimer, glucose, mannose

Figure 2: Epimers

2. Enantiomer

  • A special type of isomerism in the pair of structures that are mirror image of each other (Figure 3).
  • The mirror images are called enantiomers. They are designed as D- and L- sugar.

enantiomer

Figure 3: Enantiomers
Image credit: Wikimedia commons

3. Cyclization of monosaccharides

  • Monosaccharides are normally present in ring forms. Aldehyde or ketone group has reacted with an alcohol group on the same sugar to form a hemiacetal or hemiketal ring.
  • Two types of ring: pyranose and furanose (Figure 4).
  • Pyranose has 6 members (5 carbons and 1 oxygen).
  • Furanose has 5 members (4 carbons and 1 oxygen).
Cyclic monosaccharides, carbohydrates

Figure 4: Cyclization of monosaccharides

  • The carbonyl carbon becomes a new chiral center, called the anomeric carbon. It appears in either of two different forms which are designated as ? and ? (Figure 5).
anomeric carbon

Figure 5: ? and ? of glucose

Disaccharides

Disaccharides are formed by two monosaccharides which bonded by glycosidic bond. Three forms of disaccharides are lactose, maltose and sucrose.

Lactose

Lactoses is found in milk. It is formed by galactose and glucose by ?(1 ->4) glycosidic linkage. Lactose is a reducing sugar because the group at the anomeric carbon of the glucose portion is not involved in a glycosidic linkage. Hence, it is free to react with oxidizing agents.

Maltose

Maltose is found in starch. It consists of two residues of D-glucose in an ?(1 -> 4) linkage.

Sucrose

Sucrose is common table sugar (Figure 6). It consists of glucose and fructose. Sucrose is not a reducing sugar because both anomeric groups are involved in the glycosidic linkage.

table sugar, sucrose, disaccharides

Figure 6: Table sugar

Oligosaccharides

Oligosaccharide is formed by a linkage of 3 to 10 monosaccharides units by glycosidic bond.

Polysaccharides

Polysaccharides are condensation products of more than ten monosaccharides by glycosidic bonds. They can be liner chains or branched polymers. Homopolysaccharide is formed when all monosaccharides are same. Heteropolysaccharide is formed on the other hand by different monosaccharides. Polysaccharides acts as as storage and structural functions.

The examples of polysaccharides is glycogen.

Glycogen

Glycogen (Figure 7) is carbohydrate storage in mammalian. Glycogen is branched chain polymer of ?-D-glucose. The linear chains is bonded by ?(1 -> 4) linkages and ?(1 -> 6) linkages at the branch points.

carbohydrate, glycosidic bonds, polysaccharides

Figure 7: Glycogen structure
Image credit: Wikimedia commons

Other complex polysaccharides

Polysaccharides are sometimes combine with protein and lipid to form glycoprotein and glycolipid respectively. Serum albumin is the example of glycoprotein. Glycoprotein and glycolipid play an important role is plasma membrane receptor.

References:

  1. Champe, PC, Harvey, RA. 2007. Biochemistry. Lippincott’s Illustrated Review. 4th Edition. Lippincott Williams & Wilkins.
  2. Murray, RK, Bender, DA, Botham, KM, Kennelly, PJ, Rodwell, VW, Weil, PA. 2006. Harper’s Illustrated Biochemistry, 28e. Lange, McGraw Hill.
  3. Campbell, MK, Farrell, SO. Biochemistry. 2009. 6th edition. Thomson Brooks/Cole.