Muscle contraction primarily depends on which two proteins?

Muscle contraction is primarily facilitated by the interaction between actin and myosin proteins. Actin, a thin filament, and myosin, a thick filament, work together in the muscle fibers to enable contraction through a mechanism known as the sliding filament model.

During muscle contraction, the myosin heads attach to binding sites on the actin filaments, pulling them closer together and shortening the muscle. This process is powered by ATP (adenosine triphosphate), which is hydrolyzed by myosin to release energy, allowing the myosin heads to repeatedly attach, pivot, and detach from the actin filaments. The coordinated interaction between these two proteins is crucial for all forms of muscle movement, whether it be skeletal, cardiac, or smooth muscle contraction.

In contrast, other proteins mentioned, such as elastin, keratin, collagen, and fibrin, play different structural or functional roles in connective tissues or cellular structure but are not directly involved in the process of muscle contraction. Therefore, the correct answer highlights the essential roles of actin and myosin in facilitating the contraction of muscles.