Textbook Question
How many asymmetric carbons and stereoisomers are there for an aldohexose? For a ketohexose?
28. Carbohydrates
Monosaccharide
6:08 minutesProblem 24
Textbook Question
Draw the mechanism for the hydroxide ion–catalyzed cleavage of fructose-1,6-bisphosphate.
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Identify the structure of fructose-1,6-bisphosphate. It is a six-carbon sugar with phosphate groups attached to the first and sixth carbons. The molecule contains a ketone functional group at the second carbon and hydroxyl groups on the remaining carbons.
Recognize that the reaction is catalyzed by hydroxide ions (OH⁻), which act as a nucleophile. Hydroxide ions will attack the electrophilic carbonyl carbon of the ketone group in fructose-1,6-bisphosphate.
Draw the nucleophilic attack of the hydroxide ion on the carbonyl carbon of the ketone group. This forms a tetrahedral intermediate, where the carbonyl oxygen becomes negatively charged.
Illustrate the breakdown of the tetrahedral intermediate. The negatively charged oxygen reforms the carbonyl group, leading to the cleavage of the C3-C4 bond. This results in the formation of two products: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
Show the stabilization of the products. Both DHAP and G3P are stabilized by resonance and the presence of phosphate groups, which help to delocalize the negative charge. Ensure the final structures of the products are clearly drawn with proper stereochemistry and functional groups.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enzyme Catalysis
Enzyme catalysis involves the acceleration of chemical reactions by enzymes, which are biological catalysts. In the context of fructose-1,6-bisphosphate cleavage, enzymes like aldolase facilitate the reaction by lowering the activation energy, allowing the reaction to proceed more efficiently. Understanding how enzymes interact with substrates is crucial for grasping the mechanism of this reaction.
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Nucleophilic Catalysis Concept 1
Mechanism of Hydrolysis
The mechanism of hydrolysis refers to the chemical process where water is used to break down a compound. In the case of fructose-1,6-bisphosphate, hydroxide ions act as nucleophiles, attacking the carbonyl carbon of the phosphate group, leading to the cleavage of the molecule. Familiarity with nucleophilic attack and the role of water in hydrolysis is essential for understanding this reaction.
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Phosphate Group Cleavage
Phosphate group cleavage is a critical reaction in biochemistry, where a phosphate group is removed from a molecule, often releasing energy. In the cleavage of fructose-1,6-bisphosphate, the breaking of the phosphate bond is a key step that leads to the formation of two three-carbon molecules. Recognizing the significance of phosphate groups in metabolic pathways is vital for comprehending the overall reaction.
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