Carbohydrate Chemistry Overview

Carbohydrate Chemistry

• Carbohydrates are polyhydroxyaldehydes or ketones
• They are classified into monosaccharides, oligosaccharides, and polysaccharides
• They are held together by glycosidic bonds
• They provide energy and serve as structural components

Carbohydrate Properties

• Chemically, carbohydrates contain carbon, hydrogen, and oxygen
• The general formula is [CH₂O]n (mostly hydrate of carbon as the ratio of hydrogen to oxygen 2:1) where n is the number of carbon atoms, typically 3 to 8
• Carbohydrates are found in grains, vegetables, fruits, milk, and other dairy products
• The term "sugar" is used for carbohydrates that are soluble in water and sweet to taste
• Some carbohydrates are not digestible
• Carbohydrates are a primary energy source (4 kcal/g)
• Storage forms include glycogen in animals and starch in plants
• Structural components include glycosaminoglycans and cellulose

Classification of Carbohydrates

• Monosaccharides: one sugar unit
• Disaccharides: two sugar units
• Oligosaccharides: 3-10 sugar units
• Polysaccharides: more than 10 sugar units

Monosaccharides

• Cannot be further hydrolyzed
• Contain one sugar unit
• Building blocks of all carbohydrates
• Classified according to the number of carbon atoms: Trioses, Tetroses, Pentoses, Hexoses, Heptoses, Nanoses
• Further classified by functional groups: Aldoses (aldehyde group) and Ketoses (ketone group)
• Examples: Glucose, Fructose, Galactose, Ribose, etc.

Monosaccharides: Types

• Aldoses:

  • Glyceraldehyde
  • Erythrose
  • Ribose
  • Xylose
  • Arabinose
  • Glucose
  • Galactose
  • Mannose

• Ketoses:

  • Dihydroxyacetone
  • Erythrulose
  • Ribulose
  • Xylulose
  • Fructose

Biochemical and Biological Importance of Some Hexoses

• Glucose: Sugar of grape; main blood sugar; principle sugar used by tissues
• Fructose: Found in fruits, honey; sugar of fetal blood; principle sugar of semen
• Galactose: Found in combination with glucose in milk sugar (Lactose); present in tissues as galactolipid and glycoprotein

Isomers

• Compounds with the same molecular formula but different structures
• Constitutional isomers: Differ in functional group arrangement
• Stereoisomers: Differ in spatial arrangement of atoms
  • Diastereomers: Non-mirror image isomers
    • Epimers: Differ in configuration around only one carbon atom (excluding the carbonyl carbon)
    • Anomers: Differ in configuration around the carbonyl carbon after cyclization
  • Enantiomers: Mirror image isomers
    • Optical isomers: Different spatial arrangements affect the rotation of polarized light

Optical Isomerism

• D isomers: OH group on the right side of the penultimate carbon (majority of sugars in humans)
• L isomers: OH group on the left side of the penultimate carbon

Diastereomers of Monosaccharides

• C2-Epimer of Glucose: Mannose
• C4-Epimer of Glucose: Galactose

Cyclic Structure of Monosaccharides

• Monosaccharides in solution exist primarily as cyclic structures
• Cyclization creates a new asymmetric carbon atom (anomeric carbon)
• Glucose forms a six-membered ring; Fructose forms a five-membered ring

Mutarotation

• Spontaneous change in the specific rotation of freshly prepared sugar solutions
• Results from interconversion of α and β anomers
• Equilibrium mixture typically has a specific rotation of +52.5° for glucose

Chemical Properties of Monosaccharides

• Reactions with concentrated acids (e.g., H₂SO₄), used for general carbohydrate tests
• Esterification: Formation of esters, important example is phosphorylation (e.g., glucose-6-phosphate)
• Oxidation: Oxidation of the last primary alcoholic group gives uronic acids (e.g., glucuronic acid).
• Reduction: Reduction produces corresponding alcohols (e.g., sorbitol).
• Reducing properties: Reduction of Fehling or Benedict reagent (used to detect glucose in urine)

Disaccharides

• Formed by condensation of two monosaccharides
• Crystalline, water-soluble, and sweet
• Classified by presence/absence of free reducing groups (aldehyde or ketone)
• Examples: Sucrose, Lactose, and Maltose

Sucrose

• Cane/beet sugar
• Formed by condensation of a-D-glucose and β-D-fructose
• Does not contain free reducing groups
• Does not reduce Fehling or Benedict solutions
• Does not undergo mutarotation

Lactose

• Milk sugar
• Formed by condensation of β-D-galactose and a-D-glucose
• Contains free aldehyde group (from glucose) and reduces Fehling/Benedict reagents
• Non-fermentable by yeast

Maltose

• Formed by condensation of two a-D-glucose units
• Contains free aldehyde group (from glucose) and reduces Fehling/Benedict reagents

Isomaltose

• Formed by hydrolysis of starch and glycogen
• a-1, 6-glucosidic linkage

Derived Sugars

• Monosaccharides with unusual structures or not represented by the general formula
• Acid sugars: (by oxidation of sugars)
• Sugar alcohols: (by reduction of sugars)
• Deoxy sugars: (e.g., deoxyribose in DNA)
• Amino sugars: (e.g., glucosamine, galactosamine)
• Glycosides