Question

Calmodulin a calcium dependent regulatory protein and of 17-kDa is homologous to which muscle protein?

• A. Tropomyosin
• B. Actin
• C. Myosin
• D. Troponin C

Hint:

The "EF-hand" calcium-binding motif consists of a helix-loop-helix structure.
It is the highly conserved structural feature shared by both Calmodulin and Troponin C.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Calmodulin (CaM) is an intracellular calcium receptor protein found in all eukaryotic cells.
It mediates a vast array of calcium-dependent cellular processes, including inflammation, metabolism, muscle contraction, and memory.
Proteins that respond to intracellular calcium levels often utilize a conserved structural motif known as the EF-hand to bind calcium ions ($\text{Ca}^{2+}$).

Step 2: Detailed Explanation:
Calmodulin is a relatively small, acidic protein with a molecular weight of approximately $17\text{ kDa}$.
Structurally, it consists of two symmetrical globular domains separated by a highly flexible, central $\alpha$-helical linker.
Each of the globular domains contains two EF-hand motifs, enabling a single calmodulin molecule to bind up to four $\text{Ca}^{2+}$ ions.
This structural configuration is homologous to Troponin C, the specialized calcium-binding subunit of the troponin complex located in striated muscle.
Troponin C also consists of two globular domains containing EF-hand motifs that coordinate $\text{Ca}^{2+}$ ions.
In skeletal muscle, the binding of calcium to Troponin C initiates a conformational change that moves tropomyosin away from actin filaments, allowing myosin cross-bridge formation and muscle contraction.
Due to their evolutionary relationship, Calmodulin and Troponin C share more than $70\%$ sequence similarity and have nearly identical secondary and tertiary folds.
While Troponin C is specialized for skeletal and cardiac muscle, Calmodulin functions globally across all cells and tissue types, including smooth muscle.

Step 3: Final Answer:
Calmodulin is homologous to the muscle-specific protein Troponin C.