Carbohydrates Lecture Notes

Carbohydrates

• Carbohydrates are widely distributed in plants and animals, fulfilling both structural and metabolic roles.
• Plants synthesize glucose from CO2 and H2O through photosynthesis, storing it as starch or cellulose.
• Animals synthesize some carbohydrates from fat and protein, but most carbohydrates come from plants.

Definition and Importance

• Carbohydrates are defined as polyhydroxy aldehydes or ketones, or substances that yield them upon hydrolysis.
• They are the major source of energy in most animal tissues, including the brain and RBCs.
• Glucose is the most important sugar in the blood and the universal fuel for fetuses.
• Carbohydrates have highly specific functions, such as glycogen for storage, ribose in nucleic acids, galactose in lactose, and combination with protein in glycoproteins and proteoglycans.

Classification

• Carbohydrates are classified into four categories:
  • Monosaccharides (simple carbohydrates): cannot be hydrolyzed into simpler carbohydrates.
  • Disaccharides: yield two molecules of monosaccharide upon hydrolysis (e.g., maltose, sucrose, and lactose).
  • Oligosaccharides: yield three to ten monosaccharide units upon hydrolysis (e.g., maltotriose).
  • Polysaccharides: yield more than ten molecules of monosaccharides upon hydrolysis (e.g., starch and dextrins).

Monosaccharides

• Simple carbohydrates found naturally in foods, such as fruits, vegetables, milk, and milk products, and added during food processing and refining.
• May be subdivided based on the number of carbon atoms (e.g., trioses, tetroses, pentoses, hexoses, heptoses, or octoses).
• Classified as aldoses or ketoses depending on the presence of an aldehyde or ketone group.
• Glucose is the most important monosaccharide, with three ways to represent its structure: straight chain, Haworth projection (cyclic structure), and chair form.

Isomerism

• Stereoisomers: compounds with the same structural formula but differing in spatial configuration.
• Asymmetric carbon atoms allow the formation of isomers, with the number of possible isomers depending on the number of asymmetric carbon atoms.
• Glucose, with 4 asymmetric carbon atoms, has 16 isomers.
• Optical activity: the presence of asymmetric carbon atoms confers optical activity, causing rotation of plane-polarized light.
• Types of isomerism:
  • D and L isomerism: depends on the orientation of —H and —OH groups around the carbon atom adjacent to the terminal primary alcohol carbon.
  • Pyranose and furanose ring structures: similar to the ring structures of pyran or furan.
  • Alpha and beta anomers: isomerism about position 1 (the anomeric carbon atom).
  • Epimers: isomers differing in configuration of —OH and —H on carbon atoms 2, 3, and 4 of glucose.
  • Aldose-ketose isomerism: differs in the structural formula due to the presence of an aldehyde or ketone group.

Deoxy Sugars and Amino Sugars

• Deoxy sugars: lack an oxygen atom, replacing it with hydrogen (e.g., deoxyribose in DNA).
• Amino sugars: contain amino groups, found in glycoproteins, gangliosides, and glycosaminoglycans (e.g., D-glucosamine, D-galactosamine, and D-mannosamine).

Glycosides

• Formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide and a second compound.
• O-glycosidic bond: formed with a hydroxyl group.
• N-glycosidic bond: formed with an amine group (e.g., between adenine and ribose in nucleotides like ATP).