23.13 Some Biological Carbonyl Condensation Reactions
3 min read•may 7, 2024
Carbonyl condensation reactions are vital in biological systems, driving key processes in carbohydrate metabolism and . These reactions involve the formation of carbon-carbon bonds, allowing cells to build complex molecules from simpler precursors.
Aldolases and enzymes like catalyze these reactions, using different mechanisms to stabilize reactive intermediates. Understanding these processes is crucial for grasping how cells manipulate carbon skeletons to create essential biomolecules.
Biological Carbonyl Condensation Reactions
Aldolases in carbohydrate metabolism
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- Catalyze reversible reactions in carbohydrate metabolism pathways
- Aldol addition of to aldehyde or ketone
- Stabilize intermediate through interactions with active site residues
- (FBP aldolase) catalyzes key step in glycolysis
- Cleaves fructose-1,6-bisphosphate into (DHAP) and (G3P)
- Allows splitting of six-carbon sugar (glucose) into two three-carbon molecules (triose phosphates)
- Participate in reverse reaction in
- Condense DHAP and G3P to form fructose-1,6-bisphosphate
- Enables synthesis of glucose from non-carbohydrate precursors (amino acids, lactate)
- Catalyze similar reactions in and
- Pentose phosphate pathway generates NADPH and pentose sugars (ribose for nucleotides)
- Calvin cycle fixes to produce glucose in photosynthesis
Type I vs type II aldolases
- found in animals and higher plants
- Utilize lysine residue in active site to form intermediate with substrate
- facilitates formation of enolate intermediate
- Require no for activity
- Examples: FBP aldolase, aldolase A in glycolysis
- found in bacteria and fungi
- Rely on divalent metal ion cofactor, typically , for catalytic activity
- Metal ion stabilizes enolate intermediate and activates substrate for nucleophilic attack
- Metal ion coordinated by conserved histidine and glutamate residues in active site
- Examples: FBP aldolase in E. coli, L-fuculose-1-phosphate aldolase
- Both types have barrel fold, also known as
- Named after enzyme triosephosphate isomerase (TIM)
- Consists of eight alternating -helices and -strands
- Active site located at C-terminal end of -strands
Claisen condensations for fatty acids
- Key reaction in elongation of fatty acid chains
- Involves condensation of two esters or thioesters to form -keto ester or thioester
- Driven by formation of stabilized enolate intermediate
- Enzyme -ketoacyl-ACP synthase (KS) catalyzes in fatty acid synthesis
- Condenses (malonyl-ACP) with growing acyl-ACP chain
- Releases carbon dioxide () and generates -ketoacyl-ACP product
- -ketoacyl-ACP then reduced, dehydrated, and further reduced to form saturated acyl-ACP
- Steps catalyzed by:
- -ketoacyl-ACP reductase
- -hydroxyacyl-ACP dehydrase
- Steps catalyzed by:
- Elongated acyl-ACP can undergo another round of Claisen condensation with malonyl-ACP
- Process repeats until desired fatty acid chain length achieved (typically 16 or 18 carbons)
- Allow efficient elongation of fatty acid chains by two carbon units per cycle
- Malonyl-ACP serves as two-carbon donor, with loss of driving reaction forward
Carbonyl Group and Related Concepts in Biological Reactions
- (C=O) is a key functional group in many biological molecules
- Plays a central role in aldol and Claisen condensation reactions
- Enolate formation is crucial for nucleophilic addition reactions in biological systems
- Stabilized by enzyme active sites or metal cofactors
- Fatty acid synthesis involves multiple carbonyl condensation reactions
- Utilizes enzyme-bound intermediates and specialized carrier proteins
Key Terms to Review (30)
Aldol Addition: Aldol addition is a fundamental organic reaction in which an enolate ion, formed from the deprotonation of an aldehyde or ketone, reacts with another carbonyl compound to form a new carbon-carbon bond. This reaction is a key step in various biological and synthetic processes, including the formation of complex organic molecules.
Calvin Cycle: The Calvin cycle, also known as the dark reactions or the light-independent reactions, is a series of biochemical reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is the second stage of photosynthesis, where the energy and chemical compounds produced during the light-dependent reactions (the first stage) are used to produce organic compounds, primarily glucose, from carbon dioxide.
Carbonyl group: A carbonyl group is a functional group characterized by a carbon atom double-bonded to an oxygen atom, represented as C=O. This group is pivotal in organic chemistry as it forms the backbone of various important classes of compounds, influencing their chemical properties and reactivity.
Claisen Condensation: The Claisen condensation is a carbon-carbon bond-forming reaction that occurs between the α-carbon of one carbonyl compound and the carbonyl carbon of another carbonyl compound, resulting in the formation of a β-keto ester or β-diketone. This reaction is a key step in many organic synthesis pathways and is closely related to the concepts of functional groups, enolate ion formation, and biological carbonyl condensation reactions.
Claisen condensation reaction: A Claisen condensation reaction is an organic chemical reaction where two esters or one ester and another carbonyl compound react in the presence of a strong base, leading to the formation of a β-keto ester or a β-diketone. It's a key method for forming carbon-carbon bonds in organic synthesis.
Dihydroxyacetone Phosphate: Dihydroxyacetone phosphate is a key intermediate in several important metabolic pathways, including the catabolism of triacylglycerols, the catabolism of carbohydrates through glycolysis, and certain biological carbonyl condensation reactions. It serves as a critical link between these diverse metabolic processes.
Enolate: An enolate is a negatively charged oxygen-containing species that arises from the removal of a proton from the α-carbon of a carbonyl compound. Enolates are important reactive intermediates in various organic reactions, including aldol condensations, Claisen condensations, and α-substitution reactions.
Enolate ion: An enolate ion is a negatively charged intermediate formed from the deprotonation of an alpha carbon adjacent to a carbonyl group in aldehydes and ketones. It plays a crucial role in various organic reactions, including nucleophilic addition and substitution reactions.
Enolate Ion: An enolate ion is a type of conjugate base formed when the alpha hydrogen of a carbonyl compound is removed, resulting in a negatively charged oxygen atom adjacent to a carbon-carbon double bond. This reactive intermediate is a key player in various organic reactions, including conjugate nucleophilic additions, reactions of carboxylic acids, and carbonyl condensation reactions.
Enoyl-ACP reductase: Enoyl-ACP reductase is an enzyme that plays a crucial role in the fatty acid biosynthesis pathway. It catalyzes the reduction of the enoyl intermediate, a key step in the iterative elongation of fatty acid chains.
Enzyme Cofactors: Enzyme cofactors are non-protein chemical compounds that are required for the proper functioning of enzymes, which are biological catalysts that accelerate chemical reactions in living organisms. These cofactors can be essential for the structural integrity of enzymes or can directly participate in the catalytic process.
Fatty Acid Synthesis: Fatty acid synthesis is the metabolic pathway that converts acetyl-CoA into long-chain fatty acids, which are essential components of cell membranes, energy storage, and various biological processes. This term is particularly relevant in the context of understanding the chemistry of thioesters and acyl phosphates, as well as some biological carbonyl condensation reactions.
Fructose-1,6-bisphosphate aldolase: Fructose-1,6-bisphosphate aldolase is a key enzyme involved in glycolysis, the metabolic pathway that converts glucose into energy. It catalyzes the reversible cleavage of fructose-1,6-bisphosphate into two three-carbon sugar molecules, dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, which can then be further processed to produce ATP.
Gluconeogenesis: Gluconeogenesis is the metabolic process by which the body synthesizes glucose from non-carbohydrate precursors, such as amino acids, lactate, and glycerol. It is an important pathway that helps maintain blood glucose levels, especially during periods of fasting or starvation when glucose availability is limited.
Glyceraldehyde-3-Phosphate: Glyceraldehyde-3-phosphate, also known as G3P or GAP, is a crucial intermediate in several metabolic pathways, including glycolysis, the Calvin cycle, and the catabolism of triacylglycerols. It is a three-carbon sugar phosphate that serves as a key branch point in cellular energy production and biosynthesis.
Malonyl-Acyl Carrier Protein: Malonyl-acyl carrier protein (malonyl-ACP) is a crucial intermediate in the biosynthesis of fatty acids, serving as the donor of the two-carbon malonyl group for the iterative elongation of the fatty acid chain. It is an essential component in the biological carbonyl condensation reactions that drive fatty acid synthesis.
Nucleophilic Addition: Nucleophilic addition is a fundamental organic reaction in which a nucleophile, a species that donates electrons, adds to an electrophilic carbon center, typically a carbonyl carbon, to form a new product. This reaction is central to understanding many important topics in organic chemistry, including functional groups, polar reactions, carbocation stability, reaction stereochemistry, and the chemistry of aldehydes, ketones, alcohols, and other carbonyl-containing compounds.
Nucleophilic addition reaction: A nucleophilic addition reaction is a chemical process where a nucleophile forms a bond with an electrophilic carbon atom of a compound, typically found in aldehydes and ketones. This reaction results in the conversion of the carbonyl group into a more complex, often larger, molecule.
Pentose Phosphate Pathway: The pentose phosphate pathway, also known as the hexose monophosphate shunt, is an alternative metabolic pathway to glycolysis that generates NADPH and pentose sugars. It plays a crucial role in the context of 23.13 Some Biological Carbonyl Condensation Reactions and 25.6 Reactions of Monosaccharides by providing reducing power and precursors for biosynthetic processes.
Schiff base: A Schiff base is a compound featuring a nitrogen atom double-bonded to a carbon atom, which is also bonded to an aryl or alkyl group but not a hydrogen atom, created by the condensation of an amine with an aldehyde or ketone. In the context of glycolysis, it plays a role in the enzymatic conversion of intermediates.
Schiff Base: A Schiff base is a functional group that consists of a carbon-nitrogen double bond, where the nitrogen atom is connected to an aryl or alkyl group rather than hydrogen. Schiff bases are formed through the condensation reaction between a primary amine and an aldehyde or ketone, and they play important roles in various biological processes.
TIM Barrel: The TIM barrel, also known as the (β/α)8 barrel, is a common protein fold found in many enzymes. It consists of eight parallel β-strands that are surrounded by eight α-helices, forming a barrel-like structure. This unique arrangement allows for efficient catalysis of a variety of biochemical reactions.
Type I Aldolases: Type I aldolases are a class of enzymes that catalyze the reversible aldol condensation reaction, a key step in many biological carbonyl condensation reactions. These enzymes are crucial in various metabolic pathways, facilitating the formation and cleavage of carbon-carbon bonds in a stereospecific manner.
Type II Aldolases: Type II aldolases are a class of enzymes that catalyze the reversible aldol condensation reaction, a key step in many biological carbonyl condensation reactions. These enzymes are involved in the formation and cleavage of carbon-carbon bonds, making them essential for various metabolic pathways in living organisms.
β-hydroxyacyl-ACP dehydrase: β-hydroxyacyl-ACP dehydrase is an enzyme involved in the biosynthesis of fatty acids. It catalyzes the dehydration of β-hydroxyacyl-ACP intermediates, a key step in the elongation of fatty acid chains.
β-Keto ester: A β-keto ester is an organic compound containing a ketone functional group (carbonyl) and an ester group, where the carbonyl carbon is positioned two carbons away from the ester oxygen. This structure makes the hydrogen atoms on the carbon between the carbonyl and ester groups (alpha hydrogens) particularly acidic, facilitating their removal and formation of enolate ions.
β-keto ester: A β-keto ester is a type of organic compound that contains a ketone group (C=O) at the β-carbon position relative to an ester functional group. These compounds are important intermediates in various organic reactions, particularly in the context of enolate ion reactivity, the Claisen condensation reaction, and certain biological carbonyl condensation reactions.
β-ketoacyl-ACP: β-ketoacyl-ACP is an intermediate compound involved in the biosynthesis of fatty acids, which is a key process in the metabolism of many organisms. It is a critical component in the series of reactions that elongate fatty acid chains.
β-ketoacyl-ACP reductase: β-ketoacyl-ACP reductase is an enzyme involved in the biosynthesis of fatty acids. It catalyzes the reduction of a β-ketoacyl intermediate to a β-hydroxyacyl intermediate, an important step in the fatty acid synthesis pathway.
β-ketoacyl-ACP synthase: β-ketoacyl-ACP synthase is an enzyme that plays a crucial role in the biosynthesis of fatty acids within the context of biological carbonyl condensation reactions. This enzyme catalyzes the condensation of a growing fatty acid chain with malonyl-CoA, a key step in the iterative process of fatty acid elongation.