Textbook Question
When a ketone is dissolved in 18O-labeled water, the 18O label is incorporated into the ketone. Suggest a mechanism that explains this observation.
21. Aldehydes and Ketones: Nucleophilic Addition
Nucleophilic Addition
Problem 39
Textbook Question
Carbonic anhydrase is an enzyme that catalyzes the conversion of carbon dioxide to bicarbonate ion. It is a metalloenzyme, with Zn2+ coordinated at the active site by three histidine side chains. Propose a mechanism for the reaction.
Verified step by step guidance1
Step 1: Recognize that carbonic anhydrase is a metalloenzyme with Zn²⁺ at its active site. The Zn²⁺ ion is coordinated by three histidine residues, which stabilize the enzyme's active site and facilitate the reaction.
Step 2: Understand the role of Zn²⁺ in activating water (H₂O). The Zn²⁺ polarizes the water molecule, making it more acidic and facilitating the loss of a proton (H⁺) to generate a hydroxide ion (OH⁻). This hydroxide ion is a strong nucleophile.
Step 3: The hydroxide ion attacks the carbon dioxide (CO₂) molecule, which is electrophilic due to the partial positive charge on the carbon atom. This nucleophilic attack forms a transient intermediate, likely a zinc-bound bicarbonate species.
Step 4: The intermediate rearranges to release the bicarbonate ion (HCO₃⁻) from the active site. The Zn²⁺ remains coordinated with water, ready to repeat the catalytic cycle.
Step 5: The proton (H⁺) released during the activation of water is transferred to the surrounding environment, completing the reaction and maintaining charge balance.
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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 refers to the process by which enzymes accelerate chemical reactions. Enzymes lower the activation energy required for a reaction to occur, allowing it to proceed more quickly and efficiently. In the case of carbonic anhydrase, it facilitates the conversion of carbon dioxide and water into bicarbonate and hydrogen ions, demonstrating how enzymes can enhance reaction rates in biological systems.
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Metalloenzymes
Metalloenzymes are enzymes that contain metal ions as essential components for their activity. These metal ions, such as zinc in carbonic anhydrase, play crucial roles in stabilizing enzyme structure and participating in the catalytic process. The coordination of zinc with histidine side chains at the active site is vital for the enzyme's function, influencing substrate binding and the overall reaction mechanism.
Acid-Base Chemistry
Acid-base chemistry involves the transfer of protons (H+) between molecules, which is fundamental to many biochemical reactions. In the reaction catalyzed by carbonic anhydrase, water acts as a base, accepting a proton to form hydroxide, while carbon dioxide is converted into bicarbonate. Understanding the acid-base dynamics is essential for grasping how the enzyme facilitates the conversion and maintains pH balance in biological systems.
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