Carbohydrate Notes

Carbohydrates

Learning Objective

• Describe the structure, functions, and clinical correlations of carbohydrates.

General Structure

• Carbohydrates consist of carbon (C), hydrogen (H), and oxygen (O) atoms.

• Their molecular formula is (CH2O)n, where n \geq 3.

Classification

• Monosaccharides: 1 sugar unit.

• Disaccharides: 2 sugar units.

• Oligosaccharides: 3-10 sugar units.

• Polysaccharides: >10 sugar units.

Simplest Sugars

• Glyceraldehyde (aldose) and dihydroxyacetone (ketose) are 3-carbon sugars.

Structures

• Glyceraldehyde:

  
  \begin{aligned}
  &\text{O} \
  &|| \
  &\text{H-C} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

• Dihydroxyacetone:

  
  \begin{aligned}
  &\text{O} \
  &|| \
  &\text{C=O} \
  &| \
  &\text{CH}2\text{OH} &| &\text{CH}2\text{OH}
  \end{aligned}
  

Monosaccharides

• Classified by carbon atoms:

  • Trioses (3C)

  • Tetroses (4C)

  • Pentoses (5C), etc.

Glucose

• Aldohexose (6C, aldehyde).

• Preferred metabolic fuel; universal fetal fuel.

• Most abundant sugar in the body.

Fructose

• Ketohexose (6C, ketone).

• From fruit juices; metabolic fuel.

Structures of Glucose and Fructose

• Glucose:

  
  \begin{aligned}
  &\text{1} \quad \text{CHO} \
  &\text{2} \quad | \
  &\text{HCOH} \
  &\text{3} \quad | \
  &\text{HOCH} \
  &\text{4} \quad | \
  &\text{HCOH} \
  &\text{5} \quad | \
  &\text{HCOH} \
  &\text{6} \quad | \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

• Fructose:

  
  \begin{aligned}
  &\text{1} \quad \text{CH}2\text{OH} &\text{2} \quad | &\text{C=O} &\text{3} \quad | &\text{HOCH} &\text{4} \quad | &\text{HCOH} &\text{5} \quad | &\text{HCOH} &\text{6} \quad | &\text{CH}2\text{OH}
  \end{aligned}
  

Chirality and Stereoisomers

• Most carbohydrates are chiral, with multiple carbons having 4 different substituents.

• Dihydroxyacetone is an exception.

Enantiomers: D and L Forms

• Enantiomers are nonsuperimposable mirror images.

• D-sugar: OH on the right in Fischer projections.

• L-sugar: OH on the left.

• Most natural sugars are D-configuration.

Examples

• L-Glyceraldehyde:

  
  \begin{aligned}
  &\text{CHO} \
  &| \
  &\text{HO-C-H} \
  &| \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

• D-Glyceraldehyde:

  
  \begin{aligned}
  &\text{CHO} \
  &| \
  &\text{H-C-OH} \
  &| \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

Epimers

• Differ in configuration at one chiral carbon. Galactose is a glucose epimer at C4.

Example: D-Glucose and D-Galactose

• D-Glucose:

  
  \begin{aligned}
  &\text{CHO} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{HOCH} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

• D-Galactose:

  
  \begin{aligned}
  &\text{CHO} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{HOCH} \
  &| \
  &\text{HOCH} \
  &| \
  &\text{HCOH} \
  &| \
  &\text{CH}_2\text{OH}
  \end{aligned}
  

Anomers

• Cyclization forms α and β anomers.

Cyclic Sugars: Haworth Projections

• Darker lines = groups toward you; lighter lines = groups away.

• Right in Fischer = down in Haworth; left in Fischer = up in Haworth.

Anomeric Carbon

• C1 becomes chiral; new OH group points either down (α) or up (β).

  • α form: OH down, opposite to CH_2OH.

  • β form: OH up, same side as CH_2OH.

• These interconvert in water.

Example Illustration using Fischer and Haworth projections

• Illustrative representations of converting Fischer projections to Haworth projections for α and β anomers are referenced but not explicitly transcribed.

Anomeric Carbon Equilibrium

• In glucose solution: ~64% β, ~36% α, trace amounts of linear form.

• Anomeric carbon:

  • Straight-chain: carbonyl carbon.

  • Cyclic: carbon bonded to ring oxygen and hydroxyl group.

Chair Conformation

• Sugar rings are not flat;