Question

The regulatory protein which controls muscle contraction and relaxation through its interaction with calcium is

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

Hint:

Muscle Protein Roles:
Regulatory $\text{Ca}^{2+}$-binding Protein = TROPONIN (Troponin C).
Main Contractile Proteins = Myosin & Actin.
Connective Stroma = Collagen.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Biochemistry of muscle contraction in fish muscle: the regulatory troponin-tropomyosin heterotrimer complex binds calcium ions via troponin C to expose actin-myosin cross-bridge binding sites.
Key Formula or Approach:
\[ \text{Ca}^{2+} + \text{Troponin C} \longrightarrow \text{Conformational Shift in Tropomyosin} \implies \text{Exposes Myosin Binding Sites on Actin} \implies \text{Crossbridge Cycling} \]

Step 2: Detailed Explanation:
In muscle biochemistry and post-mortem fish rigor mortis:
1. Contractile Structural Proteins: Actin (thin filaments) and Myosin (thick filaments, major myofibrillar protein).
2. Regulatory Proteins (Troponin & Tropomyosin):
- Troponin (A): A regulatory heterotrimer protein complex bound along actin filaments composed of three subunits:
- Troponin C (Tn-C): Specifically binds $\text{Ca^{2+}$ ions} released from the sarcoplasmic reticulum. Binding of $\text{Ca}^{2+}$ induces a stereochemical conformational shift that pulls tropomyosin off actin binding sites, initiating actomyosin cross-bridge cycling (contraction); removal of $\text{Ca}^{2+}$ induces relaxation.
- Troponin I (Tn-I): Inhibitory subunit.
- Troponin T (Tn-T): Tropomyosin-binding subunit.

Step 3: Final Answer:
Therefore, the regulatory calcium-binding protein is Troponin, matching option (A).