Medical Biochemistry: Carbohydrates Chemistry

Medical Biochemistry and Molecular Biology

• Course taught by Dr. Mohamed Ahmed Abdelmoneim
• Arish University, Faculty of Medicine

Carbohydrates Chemistry

• Carbohydrates are organic molecules, polyhydroxy aldehydes, or ketones, derivatives.
• Serve several biochemical functions, major source of metabolic energy for plants and animals, making up 60% of our diet.
• Used in the biosynthesis of ATP, cell membranes, cell receptors, RNA, DNA, vitamins (B2 and C).
• Form structural elements like chitin in animals and cellulose in plants.
• Form fibers that lower blood glucose and cholesterol levels, preventing constipation.
• Dietary sources include fruits, vegetables, legumes, grains, milk, soft drinks, and corn.

Classification of Carbohydrates

• Monosaccharides: Single sugar unit (e.g., glucose, fructose, galactose).
• Disaccharides: Two sugar units (e.g., maltose, sucrose, lactose).
• Oligosaccharides: 3 to 10 sugar units.
• Polysaccharides: More than 10 sugar units (e.g., starch, glycogen, cellulose, inulin).

Monosaccharides

• Single sugar unit, simplest carbohydrates.
• Classified according to the presence of an aldehyde or ketone group:
  • Aldoses: Contain an aldehyde group (CHO).
  • Ketoses: Contain a ketone group (C=O), indicated with the suffix "ulose".
• Classified according to the number of carbon atoms:
  • Triose (3 carbons), Tetrose (4 carbons), Pentose (5 carbons), Hexose (6 carbons), Heptose (7 carbons).
• Important monosaccharides include ribose (RNA, ATP, GTP, NAD, FAD), deoxyribose (DNA), glucose (blood sugar), fructose (fruit sugar, semen), galactose (milk sugar), mannose.

Biological Importance of Monosaccharides

• Ribose is a component of RNA, ATP, GTP, NAD, and FAD.
• Deoxyribose is part of DNA.
• Glucose is a major blood sugar and important carbohydrate.
• Fructose is a main sugar in semen.
• Galactose is synthesized in the mammary gland to create lactose in milk.
• Fructose and galactose are converted to glucose in the liver.
• Mannose is a component of many glycoproteins.

Asymmetric Carbon (Chiral)

• Any carbon atom attached to four different atoms or groups.
• Example is the middle carbon of glyceraldehyde.

Isomerism

• Ability of a substance to exist in more than one form (isomers).

• Types of Isomers:

  • Structural isomers: Different order of atom attachment.
  • Stereoisomers: Different spatial arrangement.
    • Enantiomers: Non-superimposable mirror images (opposite configuration at each chiral carbon).
    • Diastereomers: Non-mirror image stereoisomers.
      • Epimers: Differ in configuration at only one chiral carbon.
      • Anomers: Differ in configuration at the anomeric carbon (the carbon that forms the ring).

• Optical activity: Ability of a substance to rotate the plane of polarized light.

D and L configurations

• Related to the position of the hydroxyl group (-OH) on the asymmetric carbon.
• D form: OH group is on the right side.
• L form: OH group is on the left side.
• Most monosaccharides in mammals are in the D configuration,

Monosaccharide Derivatives

• Aldoses oxidized to various acids (Aldonic, Uronic, Aldaric).
• Importance of Glucuronic acid: Often linked to xenobiotics, bile and steroid hormones.
• 2-Deoxy Sugars: Replacing an -OH group with a hydrogen atom (e.g., deoxyribose in DNA).
• 3-Amino Sugars: Replacing an -OH group with an amino group (e.g., glucosamine in heparin/hyaluronic acid, galactosamine in chondroitin sulfate).

Importance of Amino Sugars

• Formation of glycoproteins, proteoglycans, and gangliosides.
• Some antibiotics (e.g., erythromycin) contain amino sugars.

Glycosidic Bond

• Bond between a carbohydrate and another compound/molecule forming complex carbohydrates .
• The linkage occurs between the OH of anomeric carbon and another compound.
• Glycone: sugar unit
• Aglycone: non carbohydrate

Glycosides

• Compounds formed by condensation between monosaccharides and other compounds.
• Examples include disaccharides, sugar nucleotides, some antibiotics (e.g., streptomycin), and cardiac glycosides (e.g., digitalis).

Disaccharides

• Two monosaccharides linked by a glycosidic linkage.
• Examples include maltose (2 α-glucose), isomaltose (2 α-glucose), lactose (β-glucose + β-galactose), and sucrose (α-glucose + β-fructose).

Polysaccharides

• Composed of more than 10 monosaccharide units linked by glycosidic bonds.
  • Homopolysaccharides: Composed of repeated units of one type of monosaccharide (e.g., starch, glycogen, cellulose, inulin).
  • Heteropolysaccharides: Composed of repeated units of different types of monosaccharides (eg. Pectins, Glycosaminoglycans).

Glycoproteins and Proteoglycans

• Glycoproteins: Primarily protein with some carbohydrates (10-15%).
• Proteoglycans: Primarily carbohydrates with some proteins (50-60%).
• Examples include collagens, mucins, transferrin, immunoglobulins, other extracellular matrix molecules, plasma proteins, blood group antigens, cell receptors, hormones, enzymes, and immunoglobulin.

Complex Carbohydrates

• Carbohydrates attached to non-carbohydrate molecules via glycosidic bonds.
• Molecules included are Purines & pyrimidines (Nucleic acids), Proteins (in glycoproteins & proteoglycans), Lipids (Glycolipids), Aromatic rings (in steroids & bilirubin).