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.