11.11 Biological Elimination Reactions
3 min read•may 7, 2024
Elimination reactions are key players in biological systems, shaping how our bodies create and break down molecules. These reactions, often following the , 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
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- 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:
- such as and
- like and
- E1cB mechanism involves the following steps:
- Deprotonation of the α-carbon by a base, forming a stabilized
- Subsequent loss of the (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 of the product is influenced by the orientation of the during the elimination process
Conversion of 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:
- Deprotonation of the α-carbon by a base (often an enzyme) forms a carbanion intermediate
- The carbanion intermediate is stabilized by resonance with the adjacent carbonyl group, which helps to drive the elimination forward
- Loss of the hydroxyl group from the β-carbon yields the unsaturated carbonyl product
- Examples of this conversion in biological pathways include:
- of to in fatty acid oxidation ()
- Dehydration of (HMG-CoA) to 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 , 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:
- Deprotonation of the α-carbon by the active site base forms the carbanion intermediate
- Stabilization of the carbanion intermediate by resonance with the thioester carbonyl group
- 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
- 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
Key Terms to Review (22)
3-Hydroxy Carbonyl Compounds: 3-Hydroxy carbonyl compounds are a class of organic molecules that contain a hydroxyl group (-OH) attached to the third carbon of a carbonyl group (C=O). These compounds are important intermediates in various biological elimination reactions, particularly in the context of metabolic pathways.
3-hydroxy-3-methylglutaryl-CoA: 3-hydroxy-3-methylglutaryl-CoA, also known as HMG-CoA, is a key intermediate in the mevalonate pathway, which is a series of enzymatic reactions involved in the biosynthesis of cholesterol and other isoprenoid compounds. It is an important molecule in the context of biological elimination reactions.
3-hydroxybutyryl-CoA: 3-hydroxybutyryl-CoA is an important metabolic intermediate that is formed during the breakdown of fatty acids, particularly in the process of beta-oxidation. It represents a key step in the conversion of fatty acids into energy-rich acetyl-CoA molecules that can then enter the citric acid cycle.
3-methylglutaconyl-CoA: 3-methylglutaconyl-CoA is an important intermediate in the metabolism of certain amino acids, specifically leucine and isoleucine. It is a key compound involved in biological elimination reactions, where it undergoes a series of enzymatic transformations as part of the catabolic pathways for these branched-chain amino acids.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Carbanion Intermediate: A carbanion intermediate is a negatively charged carbon species that serves as a key reactive intermediate in various organic reactions, particularly in the context of elimination reactions like the E1 and E1cB mechanisms, as well as in certain biological elimination processes.
Crotonyl-CoA: Crotonyl-CoA is a key intermediate in the beta-oxidation pathway, which is the process of breaking down fatty acids to generate energy for the body. It is formed when the fatty acid is dehydrogenated and the resulting double bond is shifted during the cyclic degradation of the fatty acid chain.
Dehydration: Dehydration is a chemical process in which water is removed from a compound, typically resulting in the formation of a new compound with fewer hydrogen and oxygen atoms. This term is particularly relevant in the context of various organic reactions and transformations, where dehydration plays a crucial role in the preparation and interconversion of different functional groups.
E1cB Mechanism: The E1cB mechanism, or the Elimination Unimolecular Conjugate Base mechanism, is a type of elimination reaction that occurs in organic chemistry. It involves the removal of a proton and a leaving group from adjacent atoms, resulting in the formation of a carbon-carbon double bond.
Enoyl-CoA hydratase: Enoyl-CoA hydratase is an enzyme that catalyzes a key step in the beta-oxidation of fatty acids, where it adds water to an enoyl-CoA intermediate to form a 3-hydroxyacyl-CoA product. This enzyme plays a crucial role in the catabolic breakdown of triacylglycerols and the generation of energy from fatty acid sources.
Enzyme Catalysis: Enzyme catalysis is the process by which enzymes, which are biological catalysts, accelerate the rate of chemical reactions in living organisms. Enzymes achieve this by lowering the activation energy required for a reaction to occur, allowing it to proceed more rapidly under physiological conditions.
Leaving group: A leaving group in organic chemistry is an atom or group that detaches from the parent molecule during a nucleophilic substitution (SN2) reaction, forming a lone pair or negative ion. The ease with which a leaving group departs affects the rate and success of the reaction.
Leaving Group: A leaving group is a functional group or atom that is displaced or removed from a molecule during a chemical reaction. It is a key component in many organic reactions, particularly substitution and elimination reactions, as it facilitates the formation of a new bond or the creation of a new product.
Metabolic Pathways: Metabolic pathways are the series of interconnected chemical reactions that occur within cells to sustain life. These pathways are responsible for the breakdown, synthesis, and transformation of molecules, providing the energy and building blocks necessary for cellular function and organismal survival.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
β-Amino Acids: β-Amino acids are a class of organic compounds that contain an amino group (-NH2) attached to the β-carbon (the second carbon atom) of a carboxylic acid group. These amino acids are important in biological elimination reactions, which are a type of organic reaction that involves the removal of a small molecule from a larger molecule.
β-Amino carbonyl compounds: β-Amino carbonyl compounds are organic molecules that contain both an amino group and a carbonyl group separated by two carbon atoms. These compounds are important intermediates in various biological reactions, including elimination reactions.
β-Amino Ketones: β-Amino ketones are organic compounds that contain a ketone functional group and an amino group separated by two carbon atoms. These compounds are important intermediates in various biological elimination reactions, particularly in the context of 11.11 Biological Elimination Reactions.
β-Hydroxy carbonyl compounds: β-Hydroxy carbonyl compounds are organic molecules that contain a hydroxyl group (-OH) on the carbon atom adjacent to the carbonyl group (C=O). These compounds are important intermediates in various biological elimination reactions.
β-Hydroxy ketones: β-Hydroxy ketones are a class of organic compounds that contain a hydroxyl group (-OH) located on the β-carbon (the second carbon from the carbonyl group) of a ketone functional group. These compounds are important intermediates in various biological elimination reactions.
β-Hydroxy thioesters: β-Hydroxy thioesters are organic compounds that contain a thioester functional group with a hydroxyl group located on the β-carbon. These compounds are intermediates in various biological elimination reactions, particularly those involved in fatty acid metabolism.
β-Oxidation Pathway: The β-oxidation pathway is a series of enzymatic reactions that break down fatty acids to generate acetyl-CoA, which can then enter the citric acid cycle to produce energy in the form of ATP. It is a crucial metabolic process that occurs in the mitochondria of cells to provide energy from stored fat reserves.