Question

where is the ATPase enzyme required for muscle contraction located?
A) Actin
B) Myosin
C) Troponin
D) Actin

Answer 1

Each muscle fiber is lined by the plasma membrane called sarcolemma which encloses the sarcoplasm. A characteristic feature of the muscle fiber is that it contains a large number of parallelly organized filaments in sarcoplasm known as myofilaments or myofibrils.

Complete answer: Each myofibril has alternatives light and dark bands on it which is due to the presence of Actin and Myosin protein. Each actin filament is made up of two 'F' actions helically wound to each other. Each 'F' actin is a polymer of monomeric G (globular) actins. Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length. A complex protein troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments. Each myosin (thick) filaments is also a polymerized protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM), and the light meromyosin (LMM). The globular head is an active ATPase enzyme and has a binding site for ATP and an active site for actin.
Thus, option B is the correct answer.

Note: The function of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fiber takes place by the sliding of the thin filament over the thick filaments. Muscle contraction is initiated by a signal sent by the central nervous system by a motor neuron.