Carbohydrate Chemistry Overview

Study Notes

Carbohydrates are essential organic compounds, characterized by three features
They provide a primary source of energy for tissues such as the brain, muscles, and erythrocytes
They are widely distributed in plants and animals
They are important for energy production, forming structural elements in animal and plant cells, forming glycolipids and glycoproteins, both of which are involved in cell membrane structure and the matrix between tissues

Divided into monosaccharides, disaccharides, oligosaccharides, and polysaccharides

The simplest type of sugar
Formed of a single sugar unit
Cannot be hydrolyzed further
Classified by the presence of an aldehyde or ketone group and the number of carbon atoms.

Aldoses: Possess an aldehyde group (-CHO)
Ketoses: Possess a ketone group (-C=O)
Trioses: Contain 3 carbon atoms
- Aldotrioses: Examples include glyceraldehyde ("glycerose")
- Ketotrioses: Examples include dihydroxyacetone
Tetroses: Contain 4 carbon atoms
- Aldotetroses: Examples include erythrose
- Ketotetroses: Examples include erythulose
Pentoses: Contain 5 carbon atoms
- Aldopentoses: Examples include ribose, arabinose, xylose, and lyxose
- Ketopentoses: Examples include ribulose and xylulose
Hexoses: Contain 6 carbon atoms
- Aldohexoses: Examples include glucose, mannose, and galactose
- Ketohexoses: Examples include fructose

Ribose and deoxyribose form the structural basis of nucleic acids RNA and DNA
Ribose forms part of ATP, GTP, and other high energy phosphate compounds
Ribose forms part of coenzymes NAD, NADP, and flavo-proteins
Ribose phosphate and ribulose phosphate are intermediates in the pentose phosphate pathway, a minor pathway for glucose oxidation
Arabinose and xylose are constituents of glycoproteins in plants and animals
Lyxose is a constituent of lyxoflavin, isolated from human heart muscle
Xylulose is an intermediate in the uronic acid pathway, a minor pathway for glucose oxidation

Glucose is the most important sugar in the body
- Main sugar in the blood
- One of the major sources of energy in the body
- It is converted to other carbohydrates in the liver and other tissues such as glycogen, galactose, ribose, and fructose
Fructose (fruit sugar)
- Can be converted into glucose in the liver
- Main sugar in semen
Galactose
- Converted to glucose in the liver
- Synthesized in mammary glands to make lactose (milk sugar)
Mannose
- Constituent of many glycoproteins

The simple open chain formula for sugars doesn’t explain some reactions, for example, glucose with an aldehyde group, does not give all the reactions of an aldehyde. This suggests that the -CHO group is masked or combined in some way

Sugars with an aldehyde group undergo hydration to form an aldenol group (alcohol)
Intra-molecular reactions occur between the -OH of the aldenol group and the -OH group of C4 or C5 to form a ring structure (hemiacetal structure)
- The carbonyl group becomes an asymmetric carbon atom
- If the remaining -OH is on the right side, it is an α-sugar
- If the remaining -OH is on the left side, it is a β-sugar

1-5 ring form is called pyranose, resembling an organic compound called pyran, e.g., α and β glucopyranose
1 -4 rind form is called furanose, resembling an organic compound called furan, e.g., α and β glucofuranose

Cyclic structures of sugars can exist in Haworth or chair forms
Haworth formula:
- All -OH groups on the right side of the old ring structure are written downwards
- All -OH groups on the left side of the old ring structure are written upwards
- These rules are reversed at CH2-OH groups, e.g., the last carbon atom of glucose that is attached to oxygen (C4 in furanose, C5 in pyranose)