Question

Explain how ATP levels regulate glycolysis in resting muscle

biochemistry

Answer 1

Our muscles comprise a large amount of our body mass, and they require enormous amounts of energy to contract.  Even at rest, our muscles require a lot of energy. Ultimately, energy comes from the food we eat. Muscle cells, however, don't use sugar, fats or proteins to contract. Rather, our cells convert the energy stored in those nutrient molecules into energy stored within ATP (adenosine triphosphate). That's the universal energy molecule for living cells. ATP, in turn, provides the energy needed for muscular contraction. Exercising muscle gobbles up billions of ATP molecules every second.

ATP and Creatine Phosphate

While resting, skeletal muscle makes more ATP than it needs. As ATP is not very stable, the excess ATP transfers energy to creatine. That's a molecule made by our muscles from amino acids. This is the reaction: ATP + Creatine -> ADP + Creatine Phosphate (CP).

So, you see, the phosphate is transferred from ATP to creatine to make creatine phosphate. As creatine phosphate, or phosphocreatine, is more stable than ATP, it provides an effective way to store energy. During contraction, the contractile protein myosin breaks down ATP producing ADP and phosphate. The energy stored in creatine phosphate is then used to recharge the ADP as follows: CP + ADP -> Creatine + ATP.

So, you see, the phosphate is now transferred back to ADP to make ATP, and the ATP can be used for contraction. These reactions are catalyzed by the enzyme we call creatine phosphokinase (or CPK), and it's located in the muscle cell. CPK leaks into the bloodstream when muscles are damaged. For example, this happens with a myocardial infarction, or a heart attack, that results in heart muscle damage. Clinical tests are used to measure circulating levels of CPK and thus, assess the level of muscle damage.

It is important to note that ATP and CP reserves are exhausted within about 15 seconds of exercise. That's not very long. Therefore, the cell must be able to generate or synthesize, ATP if it is to continue working.

Muscle contractions are fueled by adenosine triphosphate (ATP), an energy-storing molecule. Four potential sources of ATP power muscle contractions.

Free ATP

Low levels of ATP exist within the muscle fibres and can immediately provide energy for contraction. However, the pool is very small and after a few muscle twitches will be exhausted.

Phosphocreatine

Phosphocreatine, also known as creatine phosphate, can rapidly donate a phosphate group to ADP to form ATP and creatine under anaerobic conditions. Enough phosphocreatine is present in the muscle to provide ATP for up to 15 seconds of contraction.

The reaction of phosphocreatine + ADP to ATP + creatine is reversible. During periods of rest, the store of phosphocreatine is regenerated from ATP.

Glycolysis

Glycolysis is the metabolic reaction which produces two molecules of ATP through the conversion of glucose into pyruvate, water, and NADH in the absence of oxygen.

The glucose for glycolysis can be provided by the blood supply but is more often converted from glycogen in the muscle fibres. If glycogen stores in the muscle fibres are expended, glucose can be created from fats and proteins. However, this conversion is not as efficient.

Pyruvate is continually processed into lactic acid. With pyruvate accumulation, the amount of lactic acid produced is also increased. This lactic acid accumulation in the muscle tissue reduces the pH, making it more acidic and producing the stinging feeling in muscles when exercising. This inhibits further anaerobic respiration, inducing fatigue.

Glycolysis alone can provide energy to the muscle for approximately 30 seconds, although this interval can be increased with muscle conditioning.

Cellular Respiration

While the pyruvate generated through glycolysis can accumulate to form lactic acid, it can also be used to generate further molecules of ATP. Mitochondria in the muscle fibres can convert pyruvate into ATP in the presence of oxygen via the Krebs Cycle, generating an additional 30 molecules of ATP.

Cellular respiration is not as rapid as the above mechanisms; however, it is required for exercise periods longer than 30 seconds. Cellular respiration is limited by oxygen availability, so lactic acid can still build up if pyruvate in the Krebs Cycle is insufficient.

Cellular respiration plays a key role in returning the muscles to normal after exercise, converting the excess pyruvate into ATP and regenerating the stores of ATP, phosphocreatine, and glycogen in the muscle that are required for more rapid contractions.