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Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 22 - Catalysis in Organic Reactions and in Enzymatic ReactionsProblem 39
Chapter 23, Problem 39

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.
Chemical reaction showing carbon dioxide and water converting to bicarbonate ion and hydrogen ion, catalyzed by carbonic anhydrase.

Verified step by step guidance
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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.

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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Nucleophilic Catalysis Concept 1

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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The Lewis definition of acids and bases.
Related Practice
Textbook Question

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

b. Both trans reactants form the same racemic mixture.

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Textbook Question

Co2+ catalyzes the hydrolysis of the lactam shown here. Propose a mechanism for the metal-ion catalyzed reaction.

Textbook Question

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

c. A trans reactant is more reactive than a cis reactant.

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Textbook Question

Propose a mechanism for the following reaction. (Hint: The rate of the reaction is much slower if the nitrogen atom is replaced by CH.)

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Textbook Question

The rate constant for the uncatalyzed reaction of two molecules of glycine ethyl ester to form glycylglycine ethyl ester is 0.6 M-1 s-1. In the presence of Co2+, the rate constant is 1.5 × 106 M-1 s-1. What rate enhancement does the catalyst provide?

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Textbook Question

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

a. Both cis reactants form an optically active trans product, but each cis reactant forms a different trans product.

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