Carbohydrate Chemistry Notes

Carbohydrate Chemistry

Introduction

• Presented by: Dr. B.K. Manjunatha Goud

• Associate Professor and In-charge Chairperson of Biochemistry

• Date: 13/09/2022

• Time: 10.45am-11.45am

Learning Outcomes

• Classification of carbohydrates with examples.

• Physiological functions of each carbohydrate.

• Clinical importance of various carbohydrates.

Case Studies

• Case 1:

  • A 50-year-old diabetic patient with decreased vision.

  • High blood glucose levels and lens changes were observed.

  • Suspected condition: Diabetic cataract.

  • Abnormally increased substance in lens: Sorbitol may have led to this condition.

• Case 2:

  • A 25-year-old boy brought to the emergency department with loss of consciousness and altered sensorium.

  • Increased intracranial tension.

  • Substance used for treatment: Mannitol.

Carbohydrates Overview

• Most abundant organic molecules.

• Composed of carbon, hydrogen, and oxygen.

• General formula: $C<em>n(H</em>2O)_n$

• Primary energy source as glucose.

• Formed from carbon and water (hydrates).

Definition

• Polyhydroxy aldehydes or ketones.

• Substances that yield such compounds on hydrolysis.

• Include aldehyde and ketone groups.

Functions of Carbohydrates

• Energy source: 4 Kcal/gram.

• Storage form of energy.

• Constituent of nucleotides and nucleic acids.

• Important component of milk (lactose).

• Structural component of cells.

• Cardiac glycosides.

• Antibiotics.

• Dietary fiber.

• Intravascular anticoagulant (Heparin).

Classification of Carbohydrates

1. Monosaccharides:

   • 3-9 carbon residues.

2. Disaccharides:

   • 2 monosaccharide units.

3. Oligosaccharides:

   • 3-10 monosaccharide units.

4. Polysaccharides:

   • More than 10 monosaccharide units.

Monosaccharides

• Simplest carbohydrates that cannot be further hydrolyzed.

• Mono means one, saccharides means sugar.

• Aldoses:

  • Contain an aldehyde functional group (e.g., glucose).

• Ketoses:

  • Contain a ketone functional group (e.g., fructose).

Examples of Aldoses and Ketoses

• Trioses:

  • Glyceraldehyde.

  • Dihydroxyacetone.

• Tetroses:

  • Erythrose.

  • Erythrulose.

• Pentoses:

  • Arabinose.

  • Ribulose.

• Hexoses:

  • Glucose.

  • Fructose

Monosaccharide Categories and Examples

Structural Aspects of Monosaccharides

• Configurations represented by Fischer or Haworth projection formulae.

• Haworth Projections:

  • Six-membered ring resembling pyran (pyranoses).

  • Five-membered ring resembling furan (furanoses).

Pyranose and Furanose Forms

• Cyclic forms of pyranose sugars adopt "chair" or "boat" configurations due to the tetrahedral nature of carbon bonds.

Fischer and Haworth Projections

• Illustrations provided for D-Glucose in aldehyde form, α-D-Glucopyranose, and β-D-Glucopyranose.

Isomerism in Carbohydrates

1. Functional Isomerism

2. Stereoisomerism

   • Enantiomers

   • Optical isomers

   • Pyranose and furanose isomers

   • Anomers

   • Epimers

Asymmetric Carbon Atom

• Carbon atom attached to 4 different groups or atoms is called asymmetric (chiral center).

• Glyceraldehyde (a triose) is the simplest monosaccharide with one asymmetric carbon atom.

• Glucose has 4 asymmetric carbons.

Stereoisomers

• Compounds with the same chemical formula but different positions of hydroxyl groups on their asymmetric carbons.

• Example: glucose, mannose, galactose, and fructose (formula $C<em>6H</em>{12}O_6$).

Enantiomers

• Carbohydrates exist in two conformations based on the hydroxyl group orientation about the asymmetric carbon adjacent to the terminal primary alcohol carbon.

• Physiologically significant carbohydrates exist in the D-conformation.

Epimers

• Carbohydrates differing in configuration around a single specific carbon atom (other than the anomeric carbon C1) in the position of the –OH group.

• Example: Glucose and Mannose are epimers at C-2. Glucose and Galactose are epimers at C-4.

Anomers

• Cyclization creates an anomeric carbon, generating α and β configurations called anomers.

• Example: α-Glucose and β-Glucose differ only around C1 (anomeric carbon).

• Haworth projection:

  • α-configuration: OH is trans to the $CH_2OH$ group.

  • β-configuration: OH is cis to the $CH_2OH$ group.

Properties of Sugars

• Muta rotation

• Reducing property

• Osazone formation

• Oxidation of sugars

• Reduction to form alcohols

• Glycosides

• Formation of esters

Crystal Shapes of Sugars

• Glucose: Needle-shaped crystals.

• Fructose: Needle-shaped crystals.

• Mannose: Needle-shaped crystals.

• Galactose: Balls with thorny edge-shaped crystals.

• Xylose: Fine-long needle-shaped crystals.

• Maltose: Sunflower-shaped crystals.

Optical Isomers

• Isomers that differ in the direction they rotate plane-polarized light.

• Dextrorotatory (+): Dextrose.

• Levorotatory (-): Levulose.

Chemical Properties of Sugars

• Reducing Sugars:

  • Sugars with a free aldehyde or keto group at the anomeric carbon atom.

• Benedict’s test:

  • Clinical significance in detecting:

    • Diabetes Mellitus

    • Lactosuria

    • Galactosuria

    • Fructosuria

Reactions of Monosaccharides

1. Oxidation of $CH_2OH$ group

   • Glucose → Glucuronic acid (Detoxification of bilirubin)

   • Galactose → Galacturonic acid (Components of Glycosaminoglycans)

2. Oxidation of CHO group

   • Converted to aldonic acids

     • Glucose → Gluconic acid

     • Mannose → Mannonic acid

     • Galactose → Galactonic acid

3. Oxidation of both CHO and $CH_2OH$ groups

   • Sugars converted to aldaric acids

     • Glucose → Glucaric acid

     • Galactose → Mucic acid

Reduction Reactions

• Reduction of aldehyde or keto group produces a new alcohol group.

  • Glucose → Sorbitol

  • Galactose → Dulcitol

  • Mannose → Mannitol

Deoxy Sugars

• Sugars containing one oxygen less than the parent molecule.

  • Ribose → 2-Deoxy ribose

Glycosides

• Compounds formed by condensation of the –OH group at the anomeric carbon of a monosaccharide with another group.

• O- and N-Glycosides.

• Examples:

  • Digitonin: Cardiac stimulant.

  • Digoxin: Treatment of congestive cardiac failure (CCF).

  • Streptomycin, Doxorubicin

Amino Sugars

• Hydroxyl group replaced with an amino group.

  • Glucosamine

  • Galactosamine

  • Mannosamine

• Important compounds found in connective tissues.