Bioinorganic Chemistry

The question asks us to identify which enzymes contain zinc. Let's evaluate each of the given options based on their known composition and structure:

1. Carboxypeptidase: This is an enzyme that contains zinc. It is a metalloprotease, meaning it requires a metal ion, in this case, zinc, for its enzymatic activity. The zinc ion in carboxypeptidase plays a crucial role in its mechanism by stabilizing a water molecule, which is involved in the hydrolysis of peptide bonds.
2. Hydrogenase: This enzyme does not typically contain zinc. Hydrogenases are enzymes that catalyze the reversible oxidation of hydrogen gas. They often contain metals such as nickel and iron in their active sites.
3. Carbonic Anhydrase: Like carboxypeptidase, carbonic anhydrase is another zinc-containing enzyme. It catalyzes the conversion of carbon dioxide and water to bicarbonate and protons and is essential in maintaining acid-base balance in the blood and tissues. Zinc is vital for its catalytic function.
4. Urease: This enzyme does not contain zinc; it actually contains nickel in its active site. Urease catalyzes the hydrolysis of urea into carbon dioxide and ammonia.

Thus, the correct answers are carboxypeptidase and carbonic anhydrase, both of which are known as zinc metalloenzymes.

The question asks about the metal ion present in human carbonic anhydrase, an enzyme that catalyzes the reversible hydration of carbon dioxide. Let's explore the options:

• Option A: Fe³⁺ - Iron ions are found in several enzymes but not commonly associated with carbonic anhydrase.
• Option B: Cu²⁺ - Copper ions are typically found in enzymes involved in electron transfer processes and are not relevant for carbonic anhydrase.
• Option C: Zn²⁺ - Zinc ions play a crucial role in the structure and function of carbonic anhydrase by acting as a cofactor that stabilizes the enzyme’s active site.
• Option D: Ni²⁺ - Though nickel can be a catalytic metal ion in some enzymes, it is not the metal ion used by carbonic anhydrase.

Among these choices, the correct answer is Zn²⁺. Human carbonic anhydrase specifically relies on zinc ions for its enzymatic activity. The presence of Zn²⁺ facilitates the conversion of carbon dioxide and water to bicarbonate and protons, a reaction essential for many physiological processes.

The structure of carbonic anhydrase includes a zinc ion bound at its active site, coordinated to three histidine residues and a water molecule/hydroxide ion. This specific metal coordination is critical for its catalytic function.

The answer is Zn²⁺, which is essential for the enzyme's activity by stabilizing the transition state and facilitating the conversion of CO₂ into bicarbonate.