Question

During strenuous activity, the demand for ATP in muscle tissue is vastly increased. In rabbit leg muscle or turkey flight muscle, the ATP is produced almost exclusively by lactic acid fermentation. ATP is formed in the payoff phase of glycolysis by two reactions, promoted by phosphoglycerate kinase and pyruvate kinase. Suppose skeletal muscle were devoid of lactate dehydrogenase. Could it carry out strenuous physical activity; that is, could it generate ATP at a high rate by glycolysis? Explain.

Discuss the pros and cons of lactate production in active muscle. Why is lactate produced? What happens to the lactate?

The transformation of glucose to lactate in myocytes releases only about 7%
of the free energy released when glucose is completely oxidized to CO₂ and H₂O. Does this mean that anaerobic glycolysis in muscle is a wasteful use of glucose?

Answer 1

1. During strenous exercise, muscle cells carry out lactic acid fermentation due to lack of oxygen for aerobic respiration to occur. Lactate dehydrogenase is the main enzyme in lactic acid fermentation which converts pyruvate to lactate with simulatenous interconversion of NADH and NAD+.LDH deficiency is a condition in which body has to decide how to break sugar especially in skeletal muscle cells. If LDH is lacking in muscle cells, it could be either deficiency in lactate dehydrogenase-A or lactate dehydrogenase-B. Individuals having LDH-A deficiency may experience fatigue, muscle pain, and cramps during exercise. It sometimes also lead to the breakdown of muscle tissue which results in release of myoglobin. This protein is processed by the kidneys and eventually released in the urine. Individuals having LDH-B deficiency have no particular signs or symptoms. There is no difficulty with physical activity or any specific physical features. This deficiency can be diagnosed by routine blood test.Thus lack of LDH will not produce ATP at high rate and individual or organism cannot undergo strenous physical activity.

2. Pros and Cons of Lactate

Formation of lactate helps to perform physical work (intense exercise) even in absence of oxygen. During glycolysis, there is release of hydrogen ions (H+) into the muscle cells. In the absence of oxygen, H+ ions starts getting accumulated thereby increasing acidic conditions in muscle cell. This results in burning sensation. If acidic conditions increase, muscle cells may stops functioning as the glycolytic enzymes may become non-functional. Thus formation of lactate reduces acidic conditions in muscle cells. Thus lactate acts as a temporary buffering system to decrease acidosis with no further ATP synthesis. However if lactate does not get metabolized further, then it may cause acidic conditions in muscle cells.

Lactate which is produced in anaerobic glycolysis in muscle cells later gets transported to liver cells where it undergoes Cori cycle. In the liver cells,it gets converted back to glucose and is send back to muscle cells for production of lactate.

3. Anaerobic glycolysis involves breakdown of glucose in absence of oxygen. Although the amount of energy production is comparatively low as compared to aerobic respiration, it acts as a survival mechanism for the cells. If anaerobic glycolysis does not occur, it would result in breakdown of muscle tissue. Thus anaerobic glycolysis is not a wasteful use but life saving process.