19.12 Biological Reductions
2 min read•may 7, 2024
play a crucial role in cellular metabolism. These reactions involve the transfer of hydride ions to carbonyl compounds, converting them to alcohols. The process is essential for energy production and biosynthesis in living organisms.
Two key players in biological reductions are the and . While the Cannizzaro reaction occurs with non-enolizable aldehydes, NADH can reduce both aldehydes and ketones. Enzymes ensure these reductions are stereospecific, vital for proper cellular function.
Biological Reductions
Mechanism of Cannizzaro reaction
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- Base-induced disproportionation of non-enolizable aldehydes occurs in the presence of strong bases ( or )
- Nucleophilic addition of hydroxide ion to the carbonyl carbon of one aldehyde molecule forms a tetrahedral alkoxide intermediate
- from the tetrahedral intermediate to another aldehyde molecule
- Alkoxide acts as a hydride donor, reducing the second aldehyde
- Alkoxide is simultaneously oxidized to a carboxylate anion
- Proton transfer from water to the newly formed alkoxide yields the final products (an alcohol and a carboxylate salt)
NADH as biological reducing agent
- NADH (, reduced form) consists of a nicotinamide ring, an adenine nucleotide, and a phosphate group
- Reduces aldehydes and ketones to alcohols in the presence of enzymes ( or )
- Mechanism involves hydride transfer from NADH to the carbonyl carbon of the aldehyde or ketone, forming a tetrahedral alkoxide intermediate similar to the Cannizzaro reaction
- Proton transfer from the enzyme's active site to the alkoxide yields the final alcohol product
- NADH and other similar molecules act as , assisting enzymes in catalyzing
- Compared to the Cannizzaro reaction:
- Both involve hydride transfer to a carbonyl carbon
- NADH reduction is enzyme-catalyzed and stereospecific
- Cannizzaro reaction is base-induced and not stereospecific
- Cannizzaro reaction occurs with non-enolizable aldehydes, while NADH reduction can occur with both aldehydes and ketones
Stereochemistry in biological reductions
- Biological reductions are stereospecific due to the involvement of enzymes
- reduction by (LDH):
- NADH acts as the reducing agent, transferring a hydride to pyruvate
- Reduction occurs stereospecifically, producing
- Hydride is delivered to the re face of pyruvate, resulting in the L-configuration
- Other examples of stereospecific biological reductions:
- Reduction of acetoacetate to (R)-β-hydroxybutyrate by
- Reduction of 3-phosphoglyceraldehyde to (R)-glyceraldehyde-3-phosphate by
Enzymes in Biological Reductions
- are a class of enzymes that catalyze biological redox reactions
- provide a specific environment for substrate binding and catalysis
- allows enzymes to selectively catalyze reactions with particular molecules
Key Terms to Review (19)
Alcohol Dehydrogenases: Alcohol dehydrogenases (ADHs) are a group of enzymes that catalyze the oxidation of alcohols to aldehydes or ketones, playing a crucial role in the metabolism of alcohols in both plants and animals. These enzymes are particularly relevant in the context of the oxidation of alcohols and biological reductions.
Aldehyde Reductases: Aldehyde reductases are a class of enzymes that catalyze the reduction of aldehydes to alcohols, playing a crucial role in the biological reduction processes described in the chapter on 19.12 Biological Reductions. These enzymes help maintain cellular redox balance and detoxify harmful aldehyde compounds.
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.
Biological Reductions: Biological reductions refer to the process of reducing chemical compounds using enzymes or other biological catalysts, as opposed to traditional chemical reduction methods. This process is an important aspect of various metabolic pathways and plays a crucial role in the synthesis of important biomolecules.
Cannizzaro Reaction: The Cannizzaro reaction is a chemical reaction in which two molecules of an aldehyde, lacking an α-hydrogen atom, are disproportionated to produce an alcohol and a carboxylic acid. This reaction is particularly useful in the context of biological reductions, as it provides a means of converting aldehydes into more complex organic compounds without the need for additional reducing agents.
Cofactors: Cofactors are non-protein chemical compounds that are essential for the proper functioning of enzymes. They work in conjunction with enzymes to facilitate and enhance specific chemical reactions in biological systems, particularly in the context of metabolic processes.
Enzyme Active Sites: Enzyme active sites are the specific regions on the enzyme molecule where the substrate binds and the catalytic reaction takes place. These sites have a unique three-dimensional shape and chemical properties that allow them to facilitate the transformation of substrates into products during biological reductions.
Glyceraldehyde-3-Phosphate Dehydrogenase: Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme involved in the glycolytic pathway, catalyzing the oxidation and phosphorylation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. It plays a crucial role in both the processes of biological reductions and carbohydrate biosynthesis through gluconeogenesis.
Hydride Transfer: Hydride transfer is a fundamental reaction in organic chemistry and biochemistry where a hydride ion (H-) is transferred from one molecule to another, often as part of redox (reduction-oxidation) processes. This term is particularly relevant in the context of biological reductions, where hydride transfer reactions are crucial for energy production and other metabolic pathways.
L-lactate: L-lactate is the ionized form of lactic acid, a key metabolite produced during anaerobic glycolysis. It is an important intermediate in various biological reduction processes, particularly those related to energy metabolism.
Lactate Dehydrogenase: Lactate dehydrogenase (LDH) is an enzyme that catalyzes the interconversion of lactate and pyruvate, playing a crucial role in cellular energy production and anaerobic metabolism. It is found in various tissues throughout the body and is a key indicator of certain medical conditions.
NADH: NADH, or nicotinamide adenine dinucleotide, is a coenzyme that plays a crucial role in numerous metabolic processes within the body. It is the reduced form of NAD+, an important electron carrier that is involved in oxidation-reduction reactions throughout the cell's energy-producing pathways.
Nicotinamide Adenine Dinucleotide: Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells that plays a crucial role in various metabolic processes, including energy production, cellular signaling, and DNA repair. It is a key component in the electron transport chain and is involved in numerous redox reactions within the body.
Oxidoreductases: Oxidoreductases are a class of enzymes that catalyze oxidation-reduction (redox) reactions, where one substrate is oxidized while another is reduced. They are crucial in biological reductions and play a central role in the regulation of enzyme activity through coenzyme binding.
Pyruvate: Pyruvate is a key intermediate molecule in cellular metabolism, serving as a central hub that connects various metabolic pathways. It is the final product of glycolysis, the process of breaking down glucose to generate ATP, and plays a crucial role in energy production, biosynthesis, and other essential metabolic processes within the body.
Redox Reactions: Redox reactions, or reduction-oxidation reactions, are a class of chemical reactions where the oxidation state of atoms is changed. They involve the transfer of electrons between chemical species, with one substance losing electrons (being oxidized) and another gaining electrons (being reduced).
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.
Substrate Specificity: Substrate specificity refers to the ability of an enzyme to selectively bind and catalyze reactions with specific substrates, or reactant molecules, while ignoring or having limited activity towards other potential substrates. This property is a crucial feature of enzymes that allows them to efficiently and precisely carry out their biological functions within the complex environment of living organisms.
β-hydroxybutyrate dehydrogenase: β-hydroxybutyrate dehydrogenase is an enzyme that catalyzes the reversible oxidation of β-hydroxybutyrate to acetoacetate, a key step in the metabolism of ketone bodies. It plays a crucial role in the biological reduction processes outlined in the chapter on 19.12 Biological Reductions.