Ch 10 - Carbs - Abridged - Copy

Chapter 10: Carbohydrates

10.1 Monosaccharides Are the Simplest Carbohydrates

  • Learning Objective: Differentiate between monosaccharides and polysaccharides regarding structure and function.

  • Definition: Monosaccharides are aldehydes or ketones with two or more alcohol groups.

  • General Structure: (CH2O)n – referred to as hydrated carbon.

Classification of Sugars

  • Sugars categorized by the number of carbons and spatial arrangement:

    • Triose Sugars: Simplest monosaccharides have 3 carbons (C3H6O3).

    • Constitutional Isomers: Aldoses and ketoses differ in atom arrangement.

    • Enantiomers: D- and L-Glyceraldehyde have the same order of attachment but differ in spatial arrangement.

    • Epimers: Glucose and mannose differ at one chiral center.

10.2 Many Common Sugars Exist in Cyclic Forms

  • Sugars often form cyclic structures in aqueous solution.

  • Haworth Projections: Flat planar representations of ring structures.

Ring Structures Classification

  • Pyranose: Six-membered ring formed during cyclization (similar to pyran).

  • Furanose: Five-membered ring formed during cyclization (similar to furan).

Cyclization Mechanisms

  • Glucose Cyclization:

    • Nucleophilic attack by oxygen forms an ether bond.

    • Creates a new chiral center at C-1, forming an anomer.

    • α Anomer: Hydroxyl at C-1 below the plane of the ring.

    • β Anomer: Hydroxyl at C-1 above the plane of the ring.

  • Fructose Cyclization:

    • Nucleophilic attack occurs at C-2, forming different cyclic forms like furanose and pyranose.

Sugars Form Multiple Cyclic Structures

  • Fructose Furanose: Can exist in α and β forms concerning the hydroxyl group at C-2.

  • Fructopyranose: Formed when the hydroxyl on C-6 attacks the keto group at C-2.

  • Both glucose and fructose can cycle into different structures.

10.3 Laboratories Insight: Glucose Is a Reducing Sugar

  • Reducing Sugars: Can be oxidized to carboxylic acids. Glucose is a reducing sugar because its open-chain form can be oxidized.

  • 99% of glucose is in cyclic form, interconverting between α and β anomers via the open-chain form.

  • Useful in assays like Benedict’s test due to the aldehyde group’s reactivity.

10.4 Monosaccharides Are Linked to Form Complex Carbohydrates

  • Oligosaccharides: Contain two or more monosaccharides linked by O-glycosidic bonds (a type of ether bond).

    • Disaccharides: Formed by the linkage of two monosaccharides.

    • Polysaccharides: Composed of many monosaccharides linked together.

Common Disaccharides

  • Sucrose: α-glucose and β-fructose joined by 1,2 linkage - non-reducing sugar.

  • Lactose: Galactose and glucose in β-1,4 linkage - reducing sugar.

  • Maltose: Two glucoses linked by α-1,4 linkage - reducing sugar.

Enzyme Cleavage of Disaccharides

  • Enzymes that cleave common disaccharides:

    • Sucrase: Cleaves sucrose (table sugar).

    • Lactase: Cleaves lactose (milk sugar).

    • Maltase: Cleaves maltose.