11.11 Biological Elimination Reactions

Elimination reactions are key players in biological systems, shaping how our bodies create and break down molecules. These reactions, often following the E1cB mechanism, are crucial in processes like fat breakdown and cholesterol production.

Enzymes are the unsung heroes of biological eliminations. They create the perfect environment for these reactions, making them faster and more precise. Understanding these processes helps us grasp how our bodies function at a molecular level.

Biological Elimination Reactions

E1cB mechanism in biological pathways

Top images from around the web for E1cB mechanism in biological pathways

• 

  Protein Metabolism · Anatomy and Physiology View original

  Is this image relevant?

• 

  Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type ... View original

  Is this image relevant?

• 

  Using Light Energy to Make Organic Molecules | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Protein Metabolism · Anatomy and Physiology View original

  Is this image relevant?

• 

  Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type ... View original

  Is this image relevant?

1 of 3

Top images from around the web for E1cB mechanism in biological pathways

• 

  Protein Metabolism · Anatomy and Physiology View original

  Is this image relevant?

• 

  Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type ... View original

  Is this image relevant?

• 

  Using Light Energy to Make Organic Molecules | OpenStax Biology 2e View original

  Is this image relevant?

• 

  Protein Metabolism · Anatomy and Physiology View original

  Is this image relevant?

• 

  Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type ... View original

  Is this image relevant?

1 of 3

• E1cB (Elimination Unimolecular conjugate Base) commonly occurs in biological systems plays a significant role in the biosynthesis and degradation of various molecules
• Typical substrates for E1cB in biological pathways include:
  • β-Hydroxy carbonyl compounds such as β-hydroxy thioesters and β-hydroxy ketones
  • β-Amino carbonyl compounds like β-amino acids and β-amino ketones
• E1cB mechanism involves the following steps:
  1. Deprotonation of the α-carbon by a base, forming a stabilized carbanion intermediate
  2. Subsequent loss of the leaving group (water, thiol) from the β-carbon, yielding an unsaturated product
• The carbanion intermediate is stabilized by resonance with the adjacent carbonyl group, which helps to lower the activation energy and facilitate the elimination process
• E1cB reactions in biological systems are often enzyme-catalyzed, which enhances reaction rates and specificity by providing a favorable microenvironment for the reaction to occur (active site)
• The stereochemistry of the product is influenced by the orientation of the leaving group during the elimination process

Conversion of 3-hydroxy carbonyl compounds

• 3-Hydroxy carbonyl compounds undergo elimination reactions to form unsaturated carbonyl compounds, with the hydroxyl group at the β-position acting as a leaving group
• The reaction proceeds via an E1cB mechanism with the following steps:
  1. Deprotonation of the α-carbon by a base (often an enzyme) forms a carbanion intermediate
  2. The carbanion intermediate is stabilized by resonance with the adjacent carbonyl group, which helps to drive the elimination forward
  3. Loss of the hydroxyl group from the β-carbon yields the unsaturated carbonyl product
• Examples of this conversion in biological pathways include:
  • Dehydration of 3-hydroxybutyryl-CoA to crotonyl-CoA in fatty acid oxidation (β-oxidation pathway)
  • Dehydration of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to 3-methylglutaconyl-CoA in the mevalonate pathway (cholesterol biosynthesis)
• The resulting unsaturated carbonyl compounds often serve as intermediates in further metabolic processes, such as energy production or the synthesis of complex molecules

Enzymes in biological elimination reactions

• Enzymes play a crucial role in catalyzing biological elimination reactions by lowering the activation energy and increasing reaction rates
• Enzymes provide a specific environment for the reaction, enhancing selectivity and ensuring that the desired product is formed
• Dehydration of 3-hydroxybutyryl thioester (3-hydroxybutyryl-CoA) is catalyzed by the enzyme enoyl-CoA hydratase, which is involved in the β-oxidation pathway of fatty acid metabolism
• The enzyme's active site contains:
  • A base (often a glutamate residue) that deprotonates the α-carbon of 3-hydroxybutyryl-CoA
  • A hydrophobic pocket that accommodates the substrate and stabilizes the carbanion intermediate
• The enzyme facilitates the E1cB mechanism through the following steps:
  1. Deprotonation of the α-carbon by the active site base forms the carbanion intermediate
  2. Stabilization of the carbanion intermediate by resonance with the thioester carbonyl group
  3. Elimination of the hydroxyl group from the β-carbon, yielding the unsaturated product (crotonyl-CoA)
• The enzyme's specific structure and catalytic properties ensure efficient and selective dehydration of 3-hydroxybutyryl-CoA, which is essential for the proper functioning of the β-oxidation pathway

Metabolic Pathways and Elimination Reactions

• Elimination reactions play crucial roles in various metabolic pathways
• Enzyme catalysis is essential for these reactions to occur efficiently in biological systems
• Dehydration reactions are common elimination reactions in metabolism, often involving the removal of water molecules
• The nature of the leaving group can affect the rate and specificity of elimination reactions in metabolic processes