Carbohydrates

Carbohydrates Overview

  • Carbohydrates are major biomolecules crucial for various biological functions.

Definition of Carbohydrates

  • Most carbohydrates are polyhydroxy aldehydes or ketones.

  • General empirical formula: [CH2O]n

    • Derived from combining carbon and water (carbo- for carbon; hydrate for water).

Classification of Carbohydrates

  • Monosaccharides: Basic building blocks; includes aldoses and ketoses.

    • Aldose: Monosaccharide with an aldehyde functional group.

    • Ketose: Monosaccharide with a ketone group.

  • Classification based on the number of carbon atoms:

    • Trioses: 3 carbons

    • Tetroses: 4 carbons

    • Pentoses: 5 carbons

    • Hexoses: 6 carbons

    • Heptoses: 7 carbons

  • Example: Glucose is classified as an aldohexose.

Types of Carbohydrates

  • Monosaccharides: 1 unit.

  • Oligosaccharides: 2-10 units, commonly include:

    • Disaccharides: 2 monosaccharide units

    • Trisaccharides: 3 monosaccharide units

    • Tetrasaccharides: 4 monosaccharide units

  • Polysaccharides: Can have hundreds or thousands of units.

Enantiomers

  • Many carbohydrates feature chiral centers (asymmetrical centers).

  • Enantiomers: Isomers that are mirror images; labeled as D- and L-.

  • Example: D- and L-glyceraldehyde are the simplest carbohydrate enantiomers.

  • Natural carbohydrates predominantly exist as D- isomers.

  • Chiral centers affect the rotation of polarized light, critical for characterization.

  • Equal mix of two enantiomers results in racemic mixture (no net rotation).

Perspective Formulas

  • Perspective formulas: Show D- and L-isomers; groups in bold come out of the screen while regular type groups go into the screen.

  • Fischer projection: Assumes similar orientation for carbon atoms.

Configuration vs. Conformation

  • Configuration: The 3D arrangement of substituent groups around a chiral center.

  • Conformation: Different spatial arrangements due to free rotation around C-C bonds.

Epimers

  • Epimers: Monosaccharides differing in stereochemistry at one chiral carbon.

  • Generally, they are not mirror images or chemically equivalent.

Number of Isomers

  • A monosaccharide with N chiral centers has 2^N isomers:

    • Aldotrioses (1 chiral center) have 2 isomers.

    • Aldotetroses (2 chiral centers) have 4 isomers.

    • Aldopentoses (3 chiral centers) have 8 isomers.

    • Aldohexoses (4 chiral centers) have 16 isomers (8 D-L pairs).

Aldotrioses and Aldotetroses

  • Chiral centers defined by D- or L- based on the lowest chiral center's -OH group position in Fischer projection.

D-Aldohexoses

  • There are eight D-isomers of aldohexoses, including glucose, galactose, and mannose.

Mutarotation of D-Glucose

  • D-Glucose can react in water to form two cyclic products (α- and β-anomers).

  • This interconversion is known as mutarotation.

  • Fischer to Haworth formula conversion involves the arrangement of -OH groups in cyclic form.

Anomers

  • Anomers: α- and β-anomers differ at the new asymmetric center formed upon cyclization.

  • Configuration of the anomeric -OH determines the classification (α or β).

Hemiacetal and Hemiketal Formation

  • Hemiacetal: Formed by aldose cyclization (aldehyde + alcohol in the same molecule).

  • Hemiketal: Formed by ketose cyclization (ketone + alcohol in the same molecule).

Acetal Formation

  • Formation of an acetal from a hemiacetal with an alcohol creates a more stable compound.

Reducing and Non-Reducing Sugars

  • Reducing sugars can mutarotate to form aldehydes; identified by Fehling’s reagent.

  • Non-reducing sugars: If the anomeric hydroxyl is methylated or locked in, they cannot mutarotate.

Ketoses

  • Ketoses have the carbonyl at the #2 carbon and include fructose, which is important to learn.

Mutarotation of Fructose

  • Can cyclize to form α-D-fructofuranose and can also form a six-membered pyranose ring.

Cyclic Forms of Fructose

  • Like aldoses, ketoses can form α- and β-anomers based on hydroxyl group position.

Conformational Structure

  • Carbohydrates can exist in various conformations due to rotation about C-C bonds.

Steric Hindrance

  • Negative interactions between bulky functional groups can affect conformations. Conformations minimizing steric hindrance are preferred.

Chair and Boat Forms of Glucose

  • Chair and Boat forms: Represent common carbohydrate conformations; stability favors equatorial positions.

Methylated Anomers of Glucose

  • Methylation locks cyclic forms and prevents mutarotation, making them non-reducing.

Deoxysugars

  • Lacking a hydroxyl group; ribose and deoxy-ribose are key components of RNA and DNA.

Sugar Phosphates

  • Have covalently bound phosphates and function in glycolysis pathways.

Sugar Acids

  • Contain carboxyl groups; examples include galacturonic acid.

Sugar Alcohols

  • Formed by replacing aldehyde or ketone with hydroxy group; sorbitol and inositol are examples.

N-Glycosidic Bonds

  • Covalent linkage between nitrogen of an organic base and anomeric group of ribose or deoxyribose.

Glycosidic Bonds

  • Monosaccharides linked by glycosidic bonds in disaccharides, oligosaccharides, and polysaccharides.

  • Example: In maltose, two glucose units connected by an α-1,4-glycosidic bond.

Common Disaccharides

  • Sucrose: Non-reducing sugar from glucose and fructose.

  • Lactose: Reducing sugar made of galactose and glucose.

  • Maltose: Reducing sugar made from two glucose units.

Glycogen Structure

  • Primarily contains α-1,4 linkages, with branching from α-1,6 linkages.

Cellulose vs. Glycogen

  • Cellulose: Consists of β-1,4 linkages (indigestible by humans).

  • Glycogen: Contains α-1,4 and α-1,6 linkages (digestible).

Amino Sugars

  • Substitution of amino group for hydroxyl; components of glycoproteins and proteoglycans.

Acidic Disaccharides

  • Composed of sulfate and carboxylate groups.

Nucleotide Sugars

  • Synthesis of carbohydrate polymers through enzymes called glycotransferases.