Biochemistry: Cyclic Forms of Sugars

Carbohydrates: Structure and Function

• Carbohydrates are the most abundant organic molecules in nature, serving various functions, including providing energy and forming cell membrane components.
• They also play a crucial role as structural components in many organisms, such as bacterial cell walls (peptidoglycan), insect exoskeletons (chitin), and plant cell walls (cellulose).

Classification of Carbohydrates

• Monosaccharides: Classified by the number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses).
  • Further categorized as aldoses (aldehyde group) or ketoses (ketone group) based on the functional group.
• Disaccharides: Formed by the linkage of two monosaccharides via a glycosidic bond (e.g., sucrose, maltose, lactose).
• Oligosaccharides: Consist of 3-10 monosaccharide units. Raffinose is an example.
• Polysaccharides: Long chains of monosaccharides, serving diverse functions.
  • Storage polysaccharides (e.g., starch, glycogen): store glucose for later use.
  • Structural polysaccharides (e.g., cellulose, chitin): provide structural support.

Isomers of Carbohydrates

• Isomers are compounds with the same chemical formula but different structures.
• Carbohydrate isomers can be epimers (differ in configuration around only one specific carbon atom) or enantiomers (mirror images).
  • Examples: glucose and mannose, glucose and galactose.
• D-sugars are the most common type found in living organisms.

Cyclization of Monosaccharides

• Monosaccharides exist in linear and cyclic forms.
• Ring closure often involves the -OH group on carbon 5 reacting with the carbonyl group (C=O), creating a hemiacetal or hemiketal.
• Cyclic forms are designated as alpha (α) or beta (β) anomers depending on the position of the hydroxyl group on the anomeric carbon (carbon 1).

Glycosidic Bonds

• Glycosidic bonds are formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another.
• Alpha (α) or beta (β) linkages, depending on the orientation of the hydroxyl group.
• Different glycosidic linkages lead to different structures and properties of the resulting disaccharides or polysaccharides.

Reducing and Non-reducing Sugars

• Reducing sugars have a free aldehyde or ketone group, allowing them to be oxidized.
• Sucrose is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond, preventing oxidation.

Polysaccharides: Homopolysaccharides and Heteropolysaccharides

• Homopolysaccharides contain only one type of monosaccharide (e.g., starch, glycogen, cellulose, chitin).
• Heteropolysaccharides contain more than one type of monosaccharide (e.g., glycosaminoglycans like hyaluronic acid and chondroitin sulfate).

Glycoproteins and Proteoglycans

• Glycoproteins are proteins with covalently attached oligosaccharides. Function in various cellular processes, including cell surface recognition and signaling.
• Proteoglycans are a specialized type of glycoprotein characterized by the high concentration of carbohydrate.

Carbohydrate Classification Summary

• Monosaccharides (single sugars)
• Disaccharides (two monosaccharides linked)
• Oligosaccharides (3–10 monosaccharides linked)
• Polysaccharides (many monosaccharides linked)
• Homopolysaccharides (all the same sugar)
• Heteropolysaccharides (more than one type of sugar)