Which one of the following binds calcium ions in a smooth muscle, causing contraction?

• A. actin
• B. myosin
• C. calmodulin
• D. tropomyosin
• E. troponin

Answer: (C)

Verified step by step guidance

1. Understand the role of calcium ions in muscle contraction: Calcium ions are crucial for muscle contraction as they interact with specific proteins to initiate the contraction process.
2. Identify the proteins involved in smooth muscle contraction: In smooth muscle, the key proteins involved are calmodulin, actin, myosin, tropomyosin, and troponin.
3. Recognize the function of calmodulin: Calmodulin is a calcium-binding messenger protein that plays a pivotal role in smooth muscle contraction by activating myosin light-chain kinase (MLCK) when bound to calcium ions.
4. Differentiate between skeletal and smooth muscle proteins: Unlike skeletal muscle, smooth muscle does not use troponin for calcium binding. Instead, calmodulin serves this function in smooth muscle.
5. Conclude that calmodulin is the protein that binds calcium ions in smooth muscle, leading to contraction by activating MLCK, which in turn phosphorylates myosin, allowing it to interact with actin.