USA Biology Olympiad (USABO) Practice Exam

In skeletal muscle relaxation, which enzyme is responsible for breaking down acetylcholine?

1. Choline acetyltransferase

2. Acetylcholine esterase

3. Succinyl CoA synthetase

4. ATPase

The correct answer is: Acetylcholine esterase

In the process of skeletal muscle relaxation, the breakdown of acetylcholine is crucial for stopping muscle contraction. Acetylcholine, a neurotransmitter released at the neuromuscular junction, binds to receptors on the muscle cell membrane, leading to muscle contraction. To ensure that the muscle can relax properly, acetylcholine must be quickly and effectively removed from the synaptic cleft to prevent prolonged stimulation of the muscle fibers. The enzyme responsible for this breakdown is acetylcholine esterase. This enzyme hydrolyzes acetylcholine into acetic acid and choline, thus terminating its action on the receptors. This activity is vital for muscle relaxation because it allows the muscle fibers to stop contracting when the signal from the motor neuron ceases. In contrast, choline acetyltransferase is involved in the synthesis of acetylcholine rather than its breakdown, while succinyl CoA synthetase is related to energy metabolism rather than neurotransmitter activity. ATPase is an enzyme that catalyzes the hydrolysis of ATP, which plays a role in energy release for various cellular processes but does not directly influence acetylcholine levels in the synapse.