Question 64

Troponin and tropomyosin are regulatory proteins in skeletal muscle cells.

Troponin and tropomyosin play crucial roles in regulating muscle contraction in skeletal muscle cells by controlling the interaction between actin and myosin.

A) Actin and myosin

Actin and myosin are not regulatory proteins; they are the primary contractile proteins in muscle cells. Myosin is the motor protein that interacts with actin to produce muscle contraction, while actin serves as the filament along which myosin moves. Therefore, this option is incorrect.

B) Actin and troponin

While troponin is a regulatory protein, actin is a contractile protein. This combination does not represent a complete set of regulatory proteins since actin does not regulate muscle contraction but rather participates in the contraction process itself. Thus, this option is also incorrect.

C) Myosin and tropomyosin

Myosin is primarily a contractile protein, not a regulatory one. Tropomyosin, on the other hand, is a regulatory protein that works with troponin to control muscle contraction. However, since myosin is included in this option, it does not correctly answer the question about regulatory proteins. Hence, this option is incorrect.

D) Troponin and tropomyosin

Troponin and tropomyosin work together to regulate the contraction of skeletal muscle. Troponin binds calcium ions and, in response, changes shape to move tropomyosin away from actin's binding sites, allowing myosin to attach and initiate contraction. This makes this option the correct answer.

Conclusion

Troponin and tropomyosin are the only pair listed that function as regulatory proteins in skeletal muscle cells, directly influencing the contraction mechanism. The other options either include contractile proteins or fail to provide the complete regulatory function needed for muscle contraction, confirming that D is the correct choice.