Biological Systems MCAT Practice Exam 2025 – Complete Study Resource

Question: 1 / 400

In smooth muscle, what molecule binds calcium to activate myosin light chain kinase (MLCK)?

Troponin

Calmodulin

In smooth muscle, the molecule that binds calcium to activate myosin light chain kinase (MLCK) is calmodulin. When calcium ions enter the smooth muscle cells, they bind to calmodulin, a calcium-binding messenger protein. The calcium-calmodulin complex then activates MLCK, which is responsible for phosphorylating the myosin light chains. This phosphorylation is a critical step that allows myosin to interact with actin, leading to muscle contraction.

This mechanism differs significantly from the process in striated muscle, where troponin plays a key role instead. In smooth muscle, the absence of troponin means that the calcium-calmodulin interaction is essential for triggering contraction. Therefore, calmodulin is crucial for initiating the contraction process in smooth muscle by facilitating the action of MLCK in response to calcium signaling.

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Myoglobin

Actin

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