Aerobic Metabolic Pathways

Aerobic metabolic pathways are the means we have for obtaining energy from fuels (carbohydrate, protein, and fat) in the presence of oxygen. The controlled release of energy during aerobic metabolism allows for a large amount of the energy in glucose to be stored as energy in ATP. The chemical reaction for the full oxidation of glucose produces energy, carbon dioxide, and water:

Glucose + 6 O2 + 38 ADP + 39 Phosphate = >6 CO2 + 6 H2O + 38 ATP

[Glucose = C6H12O6]

In anaerobic metabolism, pyruvate is converted to lactate. However, in the presence of sufficient oxygen, pyruvate can be oxidized for energy in the mitochondria (often referred to as the energy factories of the cells). Glucose, a six-carbon molecule, is converted to two molecules of pyruvate, a three-carbon molecule. When pyruvate enters the mitochondria, it undergoes further conversion to a two-carbon molecule to form acetyl-coenzyme A (commonly abbreviated to acetyl-CoA).² Acetyl-CoA can also be created from the beta-oxidation of fatty acids that reside in the mitochondria. During beta-oxidation, carbon is cleaved from the long carbon chains of fatty acids in two-carbon units. These two-carbon molecules form acetyl-CoA. The newly created acetyl-CoA from pyruvate or beta-oxidation of fats can be oxidized to carbon dioxide (CO2) in the tricarboxylic acid cycle (TCA).³ The critical aspect of the TCA cycle is producing hydrogen atoms for transport to the electron transport chain. It is in the electron transport chain that oxidative phosphorylation occurs to create ATP from ADP. With sufficient hydrogen to feed into the electron transport chain and enough oxygen for oxidative phosphorylation, the electron transport chain can continuously produce energy in the form of ATP.

If there is excess production of acetyl-CoA (i.e., inadequate oxidative enzymes to process the acetyl-CoA for energy or inadequate oxygen delivery), the excess can be converted to either fat for storage or to the amino acid alanine. Alanine can be converted by the liver to glucose or can be made part of larger protein structures.

Anaerobic metabolic processes have the capacity to provide ATP energy immediately but only for a short duration, while aerobic metabolic processes begin providing ATP energy more slowly but for long durations, provided there is sufficient substrate and oxygen available to the cells. We have large stores of energy that we can call upon to create ATP energy for muscular work .

Of the energy stores available to us, fat is the most efficiently stored and provides the greatest mass from which we can derive ATP energy. Glycogen requires approximately 3 grams of water for storage, while fat storage is essentially anhydrous, making fat a more efficient form of energy storage. Muscle and liver glycogen stores represent a small fraction of the energy in fat stores but have the advantage of being able to be metabolized either anaerobically or aerobically, while fat can only be metabolized aerobically. Protein stores are from functional tissues that, under ideal conditions, would never be catabolized as a source of energy. Nevertheless, a small amount of protein (approximately 5 percent of total energy needs) does appear to be metabolized to meet energy requirements in most activities. In the absence of carbohydrate, protein stores are catabolized at a faster rate to provide a source of glucose (the amino acid alanine can be converted to glucose by the liver) and a source of acetyl-CoA and oxidative metabolism. However, this protein catabolism is not desirable and can be avoided with a regular supply of carbohydrates and adequate total energy consumption.

At the initiation of exercise, the majority of ATP is derived anaerobically. For highly intense, maximal-effort activities, the requirement for a high volume of energy mandates a continuous dependence on anaerobic processes. However, for lower-intensity activities the majority of ATP is initially provided anaerobically, but then the activity switches to aerobic metabolism to meet most ATP needs. Anaerobic and aerobic metabolic processes should be thought of as proceeding simultaneously, with the intensity of the activity determining the predominant metabolic pathway for the supply of ATP. High-intensity, maximal-effort activities rely more on anaerobic metabolism, while lower-intensity activities rely more on aerobic metabolism. Because far more energy is available to us aerobically (fat can only be metabolized aerobically), the high energy needs of endurance athletes force them to train muscles to be more aerobically competent. Cells of well-trained aerobic athletes have more mitochondria and more aerobic enzymes in the mitochondria, resulting in a higher capacity to derive energy aerobically.