Chemistry of Carbohydrates- classification and significance

 General Introduction

• Carbohydrates are the most abundant compounds found in nature (cellulose: 100 billion tons annually)
• They make up most of the organic matter on earth because of their extensive roles in all forms of life.
• One of the four major classes of biomolecules along with proteins, nucleic acids, and lipids.
• The term carbohydrate is derived from the French term: hydrate de Carbone, they are hydrates of carbon
• Compounds composed of C, H, and O
• Empirical formula (CH2O) n, for example, when n = 5 then C5H10O5
• Not all carbohydrates have this empirical formula: e.g. sugar derivatives deoxysugars, amino sugars, etc., do not follow this rule.

General characteristics

• They have a large number of hydroxyl groups (polyhydroxy)- Figure-1-a
• In addition, they may contain-an aldehyde group (polyhydroxy aldehydes) or a keto group (polyhydroxy ketones)- Figure-1-b
• Their derivatives may also contain nitrogen, phosphorus or sulfur- Figure-1-c

Figure-1-a)  Carbohydrates are derivatives of a polyhydroxy aldehyde or ketone compounds

b)  Aldehyde derivative  

1b) Aldehyde derivative

c) Ketone derivative

Figure-1-c- Keto derivative

Functions of Carbohydrates

1) Sources of energy, especially for brain and red blood cells.

2) Intermediates in the biosynthesis of other basic biomolecules (lipids and proteins)

3) Associated with other entities such as glycosides, vitamins, and antibiotics

4) Ribose and deoxyribose sugars form part of the structural framework of RNA and DNA.

4) Form structural tissues in plants and in microorganisms

5) Carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment.

Classification of carbohydrates

1) Monosaccharides (monoses or glycoses)
Trioses, Tetroses, Pentoses, Hexoses

2) Oligosaccharides
Di, tri, tetra, penta, up to 9 or 10
Most important are the disaccharides

3) Polysaccharides or glycans
a) Homo polysaccharides
b) Heteropolysaccharides

Monosaccharides

• Also known as simple sugars
• Cannot be hydrolyzed further
• Classified  either by the number of carbon atoms or by the nature of functional group-aldoses or ketoses
• Most  of the carbohydrates (99%) are straight-chain compounds
• D-glyceraldehyde is the simplest of the aldoses (aldotriose), all other sugars have the ending -ose (glucose, galactose, ribose, lactose, etc.)
• Keto group containing monosaccharides have the ending-ulose (Xylulose, ribulose, erythrulose), except dihydroxyacetone

MONOSACCHARIDES – Classification

1-According to number of carbons they contain in their backbone structures-
 It is a variable prefix followed by the suffix (-ose)

• Trioses=3 Carbon,
• Tetroses=4C,
• Pentoses=5C,
• Hexoses=6C,
• Heptoses=7C

 2- According to the nature of the reactive group – depending on the presence of an aldehyde or keto group.Aldose sugars e.g. glyceraldehyde (figure-2-a)

• Aldose sugars e.g. glyceraldehyde (figure-2-a)
• A ketose sugars e.g. dihydroxyacetone (figure-2-b)

Figure-2)-a) Glyceraldehyde-  An Aldotriose                          

b) Dihydroxyacetone- A ketotriose

Figure-2-b- Dihydroxyacetone

 Details of classification

Number of carbon atoms	Aldose	Ketose
3 (Trioses)	Glyceraldehyde	Dihydroxyacetone
4(Tetroses)	Erythrose	Erythrulose
5 (Pentoses)	Ribose, Arabinose, Xylose	Ribulose, Xylulose
6 (Hexoses)	Glucose, Galactose, Mannose	Fructose
7(Heptoses)	–	Sedoheptulose

The Biological significance of monosaccharides

• Monosaccharides are important fuel molecules as well as building blocks of nucleic acids.
• Trioses- Glyceraldehyde and dihydroxyacetone, in their phosphorylated forms are intermediates of glycolysis.
• Tetroses are intermediates of HMP (Hexose monophosphate) pathway, which is an intermediate pathway of glucose utilization
• Pentoses form the structural components of glycoproteins, nucleotides and nucleic acids. They also serve as intermediates in the HMP pathway.
• Hexoses
  • Glucose is an important fuel molecule, the preferred source of energy for the brain cells and the only source of energy for the red blood cells and the cells lacking mitochondria.
  • Fructose- component of table sugar(Sucrose), honey and source of energy for the spermatozoa
  • Galactose- An important component of milk sugar (Lactose)
  • Mannose- An important component of glycoproteins.
  • Heptoses- Sedoheptulose is an important intermediate of HMP pathway
  • Nonoses- Sialic acid is an important component of glycolipids.