​Localized increases in the cytosolic level of free Ca²⁺ are critical to its function as second messenger. Calmodulin, a small cytosolic protein, mediates many cellular effects of Ca²⁺. Which of the following is NOT CORRECT for Ca²⁺-calmodulin interaction?​

Correct option is A

Explanation-

Option a: "Each calmodulin molecule binds six Ca²⁺ ions in a cooperative fashion."  - INCORRECT
Calmodulin binds four Ca²⁺ ions, not six. It has four EF-hand motifs, each capable of binding one Ca²⁺ ion, and the binding is indeed cooperative, but the number stated here is wrong.
Option b : "Binding of Ca²⁺ causes calmodulin to undergo a conformational change leading to active calmodulin."  -  CORRECT
This is true. Ca²⁺ binding induces a structural change in calmodulin, activating it to interact with target enzymes and proteins.
Option c : "Since binding of Ca²⁺ is cooperative, a small change in the level of cytosolic Ca²⁺ leads to a large change in the level of active calmodulin."  - CORRECT
This is a true characteristic of cooperative binding—small increases in Ca²⁺ levels sharply increase calmodulin activation.
Option d : "One of the many enzymes activated by Ca²⁺-calmodulin is cAMP phosphodiesterase, which degrades cAMP and links Ca²⁺ and cAMP signaling."  - CORRECT
This is also true. Calmodulin activates cAMP phosphodiesterase, creating a connection between Ca²⁺ signaling and the cAMP pathway.

Since option a  is the only incorrect statement that makes it a correct answer.