#metal ions

Which of the following metal ions is not commonly involved in enzyme catalysis?

Na⁺ is the correct choice because it does not accurately describe or belong to the category addressed in this question. In the context of Enzymes Basics, the other options (Cu²⁺, Fe²⁺, and Mn²⁺) are all valid and well-established concepts. Na⁺ is either unrelated to the topic, describes a different biological process, or represents a common misconception. Questions framed as 'which is NOT' require students to identify the exception among otherwise correct statements, demanding comprehensive knowledge of the topic rather than recognition of a single fact.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 6