Introduction to Carbohydrates

Introduction to Carbohydrates

• Carbohydrates are the most abundant organic molecules in nature.
• They are also known as "hydrates of carbon".
• Their empirical formulas are (CH₂O)ₙ, where n ≥ 3.

Functions of Carbohydrates

• Provide dietary calories.
• Store energy in the body.
• Form components of cell membranes, involved in intercellular communication.
• Compose carbohydrate polymers – structural and protective elements in connective tissue (animals).
• Lubricate joints.
• Contribute to cell-cell recognition.
• Part of complex CHO polymers covalently attached to lipids or proteins (glycoconjugates). These polymers direct the intracellular destination or metabolic fate of hybrid molecules.
• Are structural components in organisms other than humans (e.g., cell walls of bacteria, exoskeletons of insects, fibrous cellulose of plants).

Classification by Number of Carbon Atoms

• Monosaccharides: one monomeric unit.
• Disaccharides: two monomeric units.
• Oligosaccharides: approximately 3-10 monosaccharides.
• Polysaccharides: more than 10 monosaccharides (can be hundreds).
• Glycoconjugates: linked to proteins or lipids.

Classification by Carbonyl Group

• Aldoses have an aldehyde group as their carbonyl group.
• Ketoses have a ketone group as their carbonyl group.

Isomers and Epimers

• Isomers have the same chemical formula but different structures.
• C₆H₁₂O₆: glucose, fructose, mannose, and galactose are examples.
• Epimers differ in the configuration around one carbon atom (other than the carbonyl carbon).

Monosaccharides

• Trioses (3 carbons): glyceraldehyde
• Tetroses (4 carbons): erythrose
• Pentoses (5 carbons): ribose, deoxyribose
• Hexoses (6 carbons): glucose, fructose
• Heptoses (7 carbons): sedoheptulose
• Nonoses (9 carbons): neuraminic acid

Important Disaccharides

• Sucrose
• Lactose
• Maltose

Monosaccharides to Polysaccharides

• Glycogen (branched) from animal sources.
• Starch from plants.
• Cellulose (unbranched) from plants.
• All are polymers of glucose linked by glycosidic bonds.

Glycosidic Bonds

• Bonds that join sugars together.
• Catalyzed by glycosyltransferases.
• Use nucleotide sugars (e.g., UDP-glucose) as substrates.
• Named according to the numbers of connected carbons and the position of the anomeric hydroxyl group. They are either α or β.

Complex Carbohydrates (Glycosides)

• Carbohydrates bonded to noncarbohydrates via glycosidic bonds.
• Examples include: purine and pyrimidine bases (nucleic acids), aromatic rings (steroids, bilirubin), proteins (glycoproteins, proteoglycans), and lipids (glycolipids).

N- and O-glycosides

• N-glycosides: the noncarbohydrate molecule has an -NH₂ group.
• O-glycosides: the noncarbohydrate molecule has an -OH group

The Sugar Code

• Monosaccharides form oligosaccharides (glycans).
• Glycans are dense and more informative than nucleic acids or proteins.
• Lectins (proteins) have highly specific carbohydrate-binding domains (glycan codes).
• Lectins initiate interactions between cells and govern the rate of peptide hormone and protein degradation; involved in cellular recognition and disease.

Thalidomide

• A drug used in the 1950s for morning sickness.
• Caused congenital deformities (phocomelia) in some babies.
• Exists as a mixture of optical isomers (R and S).
• R-form had sedative properties, S-form was teratogenic.

Reducing Sugars

• Sugars where the oxygen on the anomeric carbon is not attached to any other structure.
• Act as reducing agents.
• Examples: glucose, maltose, lactose.

Importance of Diastereomers

• Enzymes distinguish between different forms (e.g., α- and β-glucose) and preferentially work on one or the other.

This quiz covers the basics of carbohydrates, their functions in the body, and their classification based on the number of carbon atoms. Understand their importance in nutrition and cellular processes, as well as their various forms such as monosaccharides and disaccharides.