Lecture Notes: Carbohydrates

Introduction

• Carbohydrates are the second class of biomolecules studied in the course.
• They are the most abundant biological molecules, also known as saccharides.
• Divided into:
  • Monosaccharides: Simple sugars (e.g., fructose, glucose).
  • Polysaccharides: Consist of multiple monosaccharides (e.g., glycogen).

Roles of Carbohydrates

• Energy sources: Crucial in metabolism.
• Structural materials: Provide structure to cells.
• Facilitate protein interactions: Play a role in specific cellular functions.

Monosaccharides

Classification

• Based on carbonyl group and number of carbon atoms.
• Aldoses: Carbonyl group as an aldehyde.
• Ketoses: Carbonyl group as part of a ketone.
• Number of carbons: triose (3), tetrose (4), pentose (5), etc.

Structure

• Composed of carbon, oxygen, and hydrogen.
• Formula: CnH2nOn, where n ≥ 3.

Isomerism

• Chiral carbons: Presence of chiral centers leads to isomeric forms.
• D and L forms: Based on the position of the OH group (right for D, left for L).
• Enantiomers: Mirror image isomers.
• Epimers: Differ in stereochemistry at one carbon (e.g., D-galactose vs. D-glucose).

Important Monosaccharides

• D-glucose (aldohexose) and D-fructose (ketohexose).
• Structures of interest for biochemistry and entrance exams.

Fischer Projections

• Used to represent 3D structures in 2D.
• Tetrahedral stereocenters are a focus, with D-monosaccharides showing OH on the right.

Aldoses and Ketoses

• Easily interconverted via isomerases like triose phosphate isomerase (TPI).
• TPI catalyzes these reactions through an endial intermediate.

Cyclic Structures

Formation

• Result from the nucleophilic attack of an OH group on a carbonyl group.
• Hemiacetal: From aldehydes; Hemiketal: From ketones.

Cyclic Forms

• Pyranose: Six-membered ring.
• Furanose: Five-membered ring.
• Can exist in alpha (OH opposite C6) or beta (OH same as C6) forms.

Derivatives of Monosaccharides

• Sugar acids: From oxidation.
• Sugar alcohols: From reduction.
• Amines: From replacing OH with NH2.
• Gluconic and glucuronic acids: Important in metabolism.

Enzyme Reaction Example

• Triose phosphate isomerase: Catalytically perfect, demonstrating interconversion of aldoses and ketoses.

Conclusion

• The lecture sets the foundation for understanding the structure and functional implications of carbohydrates.
• Further discussions will explore polysaccharides and their biochemical significance.