CARBOHYDRATES

CARBOHYDRATES

  • Alenna A. Dela Cruz, RPh, MD

TOPIC OUTLINE

  1. Overview of Carbohydrates

  2. Monosaccharides

  3. Disaccharides

  4. Polysaccharides

OVERVIEW OF CARBOHYDRATES

  • Most abundant organic molecules in nature.

  • Defined as polyhydroxy aldehydes or ketones, or compounds yielding them upon hydrolysis.

  • Empirical formula: (CH2O)n

  • Also known as Saccharides.

FUNCTIONS OF CARBOHYDRATES

  • Main energy source in the body (glucose).

  • Precursors for organic molecules (fats, amino acids).

  • Serve as storage form of energy (glycogen).

  • Sugars ribose and deoxyribose are essential for nucleic acids.

  • Participate in the structure of cell membranes (as glycoproteins and glycolipids).

  • Structural components of many organisms; example: cellulose in plants, microbial cell walls.

CLASSIFICATION OF CARBOHYDRATES

  • Three main groups:

  • Monosaccharides: One sugar unit (building blocks of carbohydrates).

    • Examples: Glucose, Fructose, Ribose.

  • Oligosaccharides: 2 to 10 sugar units.

    • Examples: Sucrose, Lactose, Maltose.

  • Polysaccharides: More than 10 sugar units.

    • Examples: Starch, Glycogen.

  • Sugar units are linked by glycosidic bonds.

MONOSACCHARIDES

  • Simplest carbohydrates; referred to as simple sugars.

  • Cannot be further hydrolyzed into simpler sugars.

  • **Sub-classification based on: **

  • Number of carbons.

  • Functional groups.

MONOSACCHARIDES - Carbon Number Classification

  • Generic Name:

    • Trioses (3 Carbons)

    • Tetroses (4 Carbons)

    • Pentoses (5 Carbons)

    • Hexoses (6 Carbons)

    • Heptoses (7 Carbons)

    • Nonoses (9 Carbons)

MONOSACCHARIDES - Functional Group Classification

  • Aldoses: with an aldehyde group.

  • Ketoses: with a keto group.

MONOSACCHARIDES - Biochemical Importance

  • Glyceraldehyde (3C): Intermediate in glycolysis.

  • D-Ribose (5C): Structure of RNA, nucleotides, co-enzyme (ATP).

  • D-Glucose (6C): Predominant sugar in the body, excreted in diabetes.

  • D-Galactose (6C): Constituent of lactose (Galactose + Glucose).

  • D-Fructose (6C): Fruit sugar.

ISOMERS

  • Compounds with the same chemical formula but different structures are isomers.

  • Examples: Fructose, Glucose, Mannose, Galactose (all with formula C6H12O6).

EPIMERS

  • Isomers differing in configuration around one carbon atom.

  • Examples: Glucose and Galactose (C4-epimers), Glucose and Mannose (C2-epimers).

STEREOISOMERS

  • Compounds with same formula but different spatial configurations.

  • Two types: Enantiomers and Diastereomers.

  • Glucose is chiral due to different substituents on carbon atoms.

ENANTIOMERS

  • Pairs of structures that are mirror images.

  • Examples: D and L sugars (humans predominantly have D-sugars).

CYCLIZATION OF MONOSACCHARIDES

  • Over 99% of monosaccharides in solution exist in cyclic form.

  • Reaction of the carbonyl group with alcohol groups forms an asymmetric carbon (anomeric carbon).

ANOMERS

  • Differ in configuration around the anomeric carbon.

  • α-anomer: Hydroxyl group points down.

  • β-anomer: Hydroxyl group points up.

REACTIONS OF MONOSACCHARIDES

  • Oxidation: Forms gluconic acids and glucuronic acid.

  • Reduction: Yields corresponding hydroxy alcohols.

    • Glucose → Sorbitol;

    • Galactose → Dulcitol;

    • Mannose → Mannitol;

    • Ribose → Ribitol.

  • Dehydration: Forms furfural in acid conditions.

  • Esterification: Common in metabolism; forms phosphoric acid.

  • Enediol formation/Tautomerization: Shifts hydrogen atoms, forms enediols, can lead to fructose formation.

  • Reducing properties: All monosaccharides are reducing sugars.

  • Osazone Formation: Reaction with phenylhydrazine gives osazones.

  • Glycosides: Formed through reaction of carbohydrate hydroxide with another hydroxyl.

  • Amino Sugars: Hydroxyl groups replaced by amino groups (e.g., Glucosamine).

  • Deoxy Sugars: Hydroxyl oxygen removed to create deoxy sugars (e.g., Deoxyribose).

DISACCHARIDES

  • Formed from two monosaccharides linked by glycosidic bonds.

  • Classified into reducing (Maltose, Lactose) and non-reducing (Sucrose).

MALTOSE

  • Composed of 2 α-D-glucose units.

  • Reducing disaccharide formed during starch digestion.

  • Hydrolyzed by maltase or dilute HCl to yield glucose.

LACTOSE

  • Milk sugar; composed of β-D-galactose and β-D-glucose.

  • Hydrolyzed by lactase into glucose and galactose.

SUCROSE

  • Composed of glucose and fructose; non-reducing sugar.

  • Hydrolyzed mixture known as invert sugar.

POLYSACCHARIDES

  • Composed of repeated monosaccharide units linked by glycosidic bonds.

  • Divided into homopolysaccharides and heteropolysaccharides.

HOMOPOLYSACCHARIDES

  • Composed of one type of monosaccharide.

  • Examples: Starch, Glycogen, Cellulose.

STARCH

  • Reserve carbohydrate in plants; abundant in potatoes and grains.

  • Consists of amylose (unbranched, α-1,4 linkage) and amylopectin (branched, α-1,6 and α-1,4 linkages).

GLYCOGEN

  • Animal starch stored in liver and muscles; has more branches than starch.

CELLULOSE

  • Chief carbohydrate in plants; forms the plant cell wall.

  • Composed of β-D-glucose units linked by β-1,4 bond; not digestible by humans.

HETEROPOLYSACCHARIDES

  • Composed of two or more different monosaccharide units.

  • Mucopolysaccharides: Negatively charged, important in cell structure.

  • Examples of GAGs: Hyaluronic acid, Heparin, Chondroitin sulfate, Dermatan sulfate.

GLYCOPROTEINS

  • Proteins with carbohydrates attached covalently.

  • N-linked: Oligosaccharides linked to Asparagine.

  • O-linked: Oligosaccharides linked to Serine or Threonine.

PROTEOGLYCANS

  • Proteins with carbohydrates that differ chemically from glycoproteins.

  • Often involved in structural roles (e.g., collagen).

TABLES OF SUGARS

  • Table 15-2: Pentoses of Physiological Importance.

  • Table 15-3: Hexoses of Physiological Importance.

  • Table 15-4: Disaccharides of Physiological Importance.

SUCCESS QUOTE

  • "Failure is not the opposite of success; it's part of success."