Manganese-dependent enzymes — selecting the exception Manganese serves as an essential metal cofactor for several enzymes. Which enzyme listed is NOT a manganese protein and therefore represents the exception?

Introduction / Context:
Transition metals act as essential cofactors in many enzymes. Manganese is required for antioxidant defense, nitrogen metabolism, and carbohydrate biosynthesis, among others. Distinguishing true Mn enzymes from proteins that use other metals is crucial in trace element biochemistry and clinical nutrition.

Given Data / Assumptions:

• Mn-superoxide dismutase (SOD2) in mitochondria contains manganese at its active site.
• Arginase is a metalloenzyme that can use Mn for activity in many species.
• Several glycosyltransferases depend on Mn for optimal function during matrix biosynthesis.
• Ceruloplasmin is a copper-containing ferroxidase, not Mn-dependent.

Concept / Approach:

Identify the protein known to be copper-based rather than manganese-based. Ceruloplasmin binds multiple copper ions and catalyzes oxidation of Fe2+ to Fe3+, facilitating iron transport on transferrin. Therefore, it is the clear exception among the listed choices.

Step-by-Step Solution:

Verification / Alternative check:

Biochemical characterization shows ceruloplasmin’s active copper centers and loss of function in copper deficiency (e.g., Menkes/Wilson disease contexts), confirming non-Mn dependence.

Why Other Options Are Wrong:

Arginase and Mn-SOD are classical Mn enzymes. Many glycosyltransferases require Mn for activity. Pyruvate carboxylase can use Mn2+ as a divalent cofactor even though Mg2+ may suffice, so it is not the best exception.

Common Pitfalls:

Confusing magnesium-dependent enzymes with manganese-dependent ones; remembering that “Mn-SOD” explicitly names manganese.

Final Answer:

Ceruloplasmin