Q.28 Zinc is essential for the function of
(A) carboxypeptidase A.
(B) chlorophyll a.
(C) myoglobin.
(D) vitamin B12.

Zinc serves as an essential cofactor for carboxypeptidase A, a key metalloenzyme involved in peptide bond hydrolysis. This makes option (A) the correct answer in this CSIR NET-style question on bioinorganic chemistry and enzyme function.

Option Analysis

Carboxypeptidase A: This pancreatic enzyme requires a zinc ion (Zn²⁺) at its active site in tetrahedral coordination with residues like His, Glu-72, and others to activate water for nucleophilic attack on C-terminal peptide bonds. Loss of zinc abolishes activity, confirming its essential role.

Chlorophyll a: Zinc acts as a micronutrient for chlorophyll biosynthesis and chloroplast function in plants, but it is not a structural component of the chlorophyll a molecule, which contains magnesium at its core.

Myoglobin: This oxygen-binding heme protein in muscle relies on iron in the protoporphyrin ring; zinc supports muscle zinc homeostasis and may influence expression indirectly, but it is not essential for myoglobin’s core function.

Vitamin B12: Known as cobalamin, this coenzyme contains cobalt as its central metal ion in a corrin ring, not zinc.

Zinc plays a critical role as a cofactor in metalloenzymes, making “zinc essential for carboxypeptidase A function” a key concept for CSIR NET Life Sciences aspirants preparing for bioinorganic chemistry topics.

Carboxypeptidase A Mechanism

Carboxypeptidase A hydrolyzes C-terminal peptide bonds using Zn²⁺ coordinated to His-69, His-196, Glu-72, and a water molecule, polarizing the carbonyl for attack. This zinc-dependent catalysis is vital for protein digestion and processing.

Why Not Other Options?

• Chlorophyll a synthesis benefits from zinc nutrition, but Mg²⁺ forms its porphyrin core.

• Myoglobin uses Fe²⁺ for O₂ binding; zinc aids differentiation but is non-essential.

• Vitamin B12’s cobalt enables methyl transfers, unrelated to zinc.

Exam Relevance

This MCQ tests metal cofactors in enzymes, a high-yield area linking biochemistry and biotechnology for CSIR NET.