Question

The protein whose removal enables myosin to bind actin in smooth muscle is (a) tropomyosin (b) caldesmon (c) myosin light chain kinase (d) calmodulin.

Short answer

The protein whose removal enables myosin to bind actin in smooth muscle is (b) caldesmon.

Step-by-step solution

Understand the biological context

Knowing that the question is about smooth muscle contraction, one should recall that this process is regulated differently than in skeletal muscle. In particular, the protein that prevents myosin from binding to actin must be identified.

Recognize the role of each protein

Each protein listed has a different function in muscle contraction. Tropomyosin and caldesmon inhibit actin-myosin interaction, myosin light chain kinase phosphorylates myosin to initiate contraction, and calmodulin activates myosin light chain kinase in the presence of calcium.

Identify the correct protein

In smooth muscle, caldesmon plays a similar role to that of tropomyosin in skeletal muscle, blocking actin sites that myosin would bind to. It is only when caldesmon is removed that myosin can bind to actin.

Key concepts

Caldesmon

Caldesmon is a regulatory protein prominently involved in smooth muscle contraction. Its primary function is to bind to actin filaments and regulate the interaction between actin and myosin, the two main proteins involved in muscle movement. When caldesmon is bound to actin, it prevents myosin from attaching which inhibits contraction.

This control is vital for smooth muscle, as this type of muscle must maintain gentle, sustained contractions for functions such as peristalsis in the digestive tract or controlling vascular resistance. Only with the removal or inactivation of caldesmon can myosin access the binding sites on actin, allowing contraction to occur. This inactivation can happen via several mechanisms, such as changes in calcium ion concentrations or the phosphorylation of caldesmon itself.

Actin-Myosin Interaction

Understanding actin-myosin interaction is crucial for comprehending muscle contractions. Actin and myosin are the core proteins that slide past each other to generate movement. In the simplest terms, myosin heads attach to specific sites on the actin filaments, stroke along them, and then release, a sequence known as the cross-bridge cycle.

This process is powered by ATP, which myosin hydrolyzes to gain energy. Regulation of their interaction is essential for muscle function, as this determines the strength and duration of muscle contractions. In smooth muscle, this regulation is influenced by proteins like caldesmon, which modulate when and how myosin binds to actin.

Muscle Contraction Regulation

Muscle contraction regulation is a complex process involving several biochemical pathways. In smooth muscle, contraction is tightly controlled by the interaction of several proteins, calcium ions, and phosphorylation events. One of the key regulators is calmodulin, which, when activated by calcium ions, activates myosin light chain kinase (MLCK).

MLCK then phosphorylates the myosin light chains, promoting the interaction of myosin with actin, leading to muscle contraction. Caldesmon also plays a role here; its removal or deactivation is necessary for myosin to bind to actin. These regulatory mechanisms ensure that muscle contractions are precisely modulated according to the body's physiological demands.