Metalloenzymes: Which of the following enzymes contains a Zinc (Zn) ion as a catalytic cofactor?

Difficulty: Easy

Correct Answer: Carboxypeptidase A

Explanation:


Introduction:
Many enzymes require metal ions for catalytic activity or structural stability. Recognizing which enzymes are Zn-dependent helps in understanding catalytic mechanisms and inhibitor design.


Given Data / Assumptions:

  • Classical biochemistry of metalloenzymes.
  • Focus on presence of Zn at the catalytic center.


Concept / Approach:

Zinc commonly functions as a Lewis acid to polarize water or substrate functional groups. Proteases such as carboxypeptidase A and matrix metalloproteinases are well-known Zn enzymes. Zn coordinates with histidine, glutamate, or aspartate side chains in the active site, activating water for nucleophilic attack on peptide bonds.


Step-by-Step Solution:

1) Identify known Zn proteases: carboxypeptidase A is archetypal.2) Evaluate alternatives: phosphorylase b kinase is regulated by Ca^2+ and phosphorylation, not Zn at its catalytic core.3) Tyrosine hydroxylase uses a non-heme Fe^2+ cofactor.4) Many PDEs are Mg^2+/Mn^2+ dependent rather than Zn-centered at catalysis.5) Pyruvate dehydrogenase complex uses thiamine pyrophosphate, lipoamide, FAD, NAD^+, and Mg^2+; Zn is not central.


Verification / Alternative check:

Structural studies of carboxypeptidase A show tetrahedral Zn coordination essential for peptide bond hydrolysis.


Why Other Options Are Wrong:

B, D: These rely primarily on other metal ions (Ca^2+, Mg^2+/Mn^2+). C: Requires Fe^2+. E: PDH uses Mg^2+ with TPP; no Zn at the catalytic core.


Common Pitfalls:

Assuming all hydrolases are Zn-dependent; many are serine-, cysteine-, or metallo-enzymes with diverse cofactors.


Final Answer:

Carboxypeptidase A

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