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Carbohydrates Overview

  • Composition: Carbohydrates are classified into the general formula (CH₂O)n. Typically, they play diverse roles in biological systems:

    • Energy storage

    • Structural roles (e.g. fibers in plants)

    • Precursors for other biomolecules (e.g. amino acids, nucleotides)

    • Components of nucleic acids (ribose in RNA & deoxyribose in DNA)

    • Cell-to-cell interactions and signaling

Monosaccharides

  • Definition: Monosaccharides are the simplest form of carbohydrates, also known as simple sugars. They are characterized as polyhydroxy aldehydes or ketones, typically with 3 to 7 carbon atoms.

  • Classification:

    • Aldoses: Sugars with an aldehyde functional group.

    • Ketoses: Sugars with a ketone functional group.

  • Examples of Monosaccharides:

    • Trioses: glyceraldehyde

    • Tetroses: erythrose

    • Pentoses: ribose

    • Hexoses: glucose, fructose, galactose

Isomerism in Monosaccharides

  • Concept of Isomers:

    • Isomers: Compounds with the same molecular formula but different structures.

      • Constitutional Isomers: Different connectivity of atoms.

      • Stereoisomers: Same connectivity but different spatial arrangement.

        • Enantiomers: Non-superimposable mirror images (e.g., L-glucose vs. D-glucose).

        • Diastereomers: Not mirror images (e.g., glucose & galactose).

        • Epimers: A type of diastereomer that differs at only one chiral center.

Cyclic Structures of Monosaccharides

  • Cyclic Formation:

    • Monosaccharides with 5 or more carbons primarily exist in cyclic forms due to the reaction of aldehyde or ketone groups with hydroxyl groups, forming hemiacetals and hemiketals.

    • Anomers: Resulting from cyclization, differ in the position of the hydroxyl group on the anomeric carbon (C-1 for aldoses and C-2 for ketoses).

    • Haworth vs Fischer: Haworth structures depict more accurate bond angles and lengths in ring forms compared to Fischer projections.

Reactions of Monosaccharides

  • Key Reactions:

    1. Oxidation:

      • Aldehyde group forms aldonic acid (e.g., gluconic acid).

      • Terminal -CH2OH can form uronic acid (e.g., glucuronic acid).

    2. Reduction: Produces sugar alcohols (e.g., sorbitol).

    3. Isomerization: Can generate different isomers (e.g., D-glucose to D-mannose or D-fructose).

    4. Esterification: Formation of esters from OH groups (e.g., phosphate esters).

    5. Glycoside Formation: Acetal formation from sugars and alcohols leads to glycosides.

    6. Glycosylation: Attachment to proteins or lipids, crucial in cell signaling.

    7. Glycation: Non-enzymatic reaction of sugar with amino groups (e.g., Maillard reaction in cooking, forms flavor and color).

Glycation and Health Effects

  • Importance of Glycation:

    • The reaction can lead to Advanced Glycation End Products (AGEs) which contribute to various health issues including diabetes complications, vascular damage, and possibly aging.

    • Hemoglobin A1c is a vital marker used to assess long-term glucose control in diabetic patients.

Summary of Carbohydrate Functions

  • Carbohydrates are not only sources of energy but also essential structural components, intermediates in metabolism, and key players in cellular recognition processes. Understanding their structure and reactivity is pivotal to biochemistry and metabolic pathways.