Question

During exercise we activate large numbers of striated muscles in our body, in coordinated patterns that give rise to running, swimming, kicking (or whatever activity we are taking part in). The generation of force or shortening (or both) by our muscles is mediated, at the molecular level, by the process of “cross-bridge cycling”.

a) Describe what cross-bridges are, which proteins are involved in their formation (2 marks)

b) Describe the sequence of events involved in one cross-bridge cycle (4 marks)

c) Describe the sources of ATP that provide the energy for striated muscles to contract during prolonged bouts of exercise (4 marks)

Answer 1

a. Skeletal muscle are type of skeletal muscle. Cross bridging is the binding of myosin thick filaments to the actin thin filaments at their binding site and pulling of the actin filaments to the center of muscle sarcomere. This results in contraction of the muscle fiber. Calcium is required to bind to troponin in tropomyosin. This causes the actin binding site on myosin to be exposed. As a result, the myosin can now bind actin on thin filaments. ATP is associated with myosin head. It hydrolyzes to ADP and Pi. ADP + Pi on myosin head will cause myosin to bind actin. As a result, actin is shifted to center of sarcomeres causing shortening of sarcomeres and leads to muscle contraction. The two proteins involved in cross bridging are actin and myosin.

b. Cross bridging will involved these steps.

1) Once actin binding sites on myosin are exposed by binding of calcium to troponin, there is hydrolysis of ATP, releasing energy. ATP bound to myosin is hydrolyzed. ADP and Pi will remain bound to myosin S1 head.

2) Myosin head will now reach forward, and bind actin.

3) The actin filaments are shifted to center of the sarcomeres causing shortening of the sarcomeres. This is known as cross bridging. Contraction of myosin S1 segment is called power stroke.

4) Flexing of the myosin heads exposes the ATP binding sites. ATP will now bind to myosin.

5) Myosin will now detach from the actin binding sites. Energy from ATP causes myosin to return to its cocked forward position. The sarcomere is lengthened causing relaxation. Contraction and relaxation cycles are repeated.

c ATP synthesis for muscle contraction occurs in three ways:

a) Creatine phosphate binds to ADP to form ATP and creatine and energy is released. This ATP synthesis takes place after 15 seconds of muscle contraction and is the fastest method to generate ATP.

b) Anaerobic respiration: Glucose is broken down by glycolysis two pyruvate. Each Pyruvate is broken down to lactic acid by lactate dehydrogenase. This results in generation of 2 ATP per glucose molecule. This respiration occurs in absence of oxygen.

Glucose +Pi + 2ADP-> 2 lactate+2 ATP+2 H20

3) Aerobic respiration: Glycolysis involves breakdown of glucose to 2 pyruvate. Pyruvate is converted to acetyl coA by pyruvate dehydrogenase which then enters Krebs cycle and electron transport chain to generate ATP. 38 ATP are produced from one glucose molecule. This process generates highest amount of ATP and is used when muscles are contraction for long time.