The three dominant systems utilized in the production of ATP are the phosphagen system, Glycolysis, and the oxidative system (Baechle & Earle, 2008). The energy substrates for bioenergetics reactions include phoshpagens (both ATP and phosphocreatine), glucose, glycogen, lactate, free fatty acids and amino acids. All of these substrates can be depleted during highly intense activities or those of long duration which reduces the amount of energy that can be produced through the bioenergetics pathways (Baechle & Earle, 2008). When phosphocreatine is the starting substrate, it donates a phosphate to ADP and has an immediate production of ATP, but is only a 1:1 ATP producer (Powers & Howley, 2015). Glucose is used during glycolosis (anaerobic) to net yield 2 ATP (Powers & Howley, 2015). The oxidative system produces the most ATP, 32 molecules, by breakdown of carbohydrates and fats in the presence of oxygen (Powers & Howley, 2015).
The anaerobic biochemical pathways to generating ATP are the phosphocreatine system and Glycolysis systems. The phosphocreatine system is the most rapid system due to only requiring one enzymatic break reaction. Phosphocreatine donates a high energy phosphate to ADP, phosphorylating it to ATP, but its overall relative yield of ATP is extremely limited. The reaction is listed below:
Through rapid glycolysis and glucose being utilized as the primary chemical fuel, ATP can also be produced, but once again in a relatively limited yield. Unlike the phosphocreatine system, rapid glycolysis via degradation of glucose to pyruvate or lactate requires a series of enzymatic steps to yield ATP. Pyruvate can readily participate in aerobic production of ATP once oxygen becomes available in the cell, thus glycolysis is considered the initial phase in the aerobic degradation of carbohydrates. (Ehrman, 2010)
The aerobic biochemical pathway for producing ATP is oxidative phosphorylation which combines the metabolic processes of the Krebs Cycle and Electron Transport Chain, both of which occur within the mitochondria of a cell (Ehrman, 2010). Carbohydrates, fats, and proteins are all used as substrates in this process and have a large relative yield of ATP (32). Protein is a special case when entering the Kreb’s cycle because it must first be broken down into the amino acid subunits (Powers & Howley, 2015). Oxidative phosphorylation utilizes O2 as the final hydrogen acceptor which yields both water and ATP.
The Krebs cycle is the initiator of oxidative phosphorylation beginning with the combination of both acetyl-coenzyme A and oxaloacetic acid which will form citric acid as the final resultant. With the removal of hydrogen from NADH and FADH in the cycle, electrons are transported via the electron transport chain directly to the mitochondria of the cell where energy is released and utilized to phosphorylate ADP to ATP (Powers & Howley, 2015).
Interaction of Anaerobic and Aerobic Pathways
Both pathways work synergistically to perform most types of exercises, as ATP production via the phosphagen system, glycolysis and oxidative phosphorylation occur simultaneously, the contributions of each shift pending the duration of the exercise. Listed below are the percentage contributions of each during activity. Basically, the phosphagen and glycolysis work more quickly for high bursts of energy, while the aerobic pathway is for endurance, as shown below in Table 1.
Table 1: Energy % contribution between pathways
(Powers & Howley, 2015)
Rate-Limiting Enzymes for Glycolysis and Krebs Cycle
The rate limiting enzyme for glycolysis is phosphofructokinase. The rate limiting enzyme for the Krebs cycle is isocitrate dehydrogenase. Both of these enzymes are rate limiting because they operate off of a negative feedback loop as both enzymes are inhibited via high levels of ATP and stimulated through increasing levels of ADP + P1 when exercise commences (Powers & Howley, 2015).
References:
Baechle, T.R. &Earle, R.W. (Eds.) (2008). Bioenergetics of exercise and training. Essentials of strength training and conditioning (3rd ed.) (pp. 23, 33). Champaign, IL: Human Kinetics.
Ehrman, J.K. (Ed.). (2010). Exercise physiology & Cardiorespiratory and health-related physical fitness assessments. ACSM’s resource manual for guidelines for exercise testing and prescription (6th ed.) (pp. 46, 47, 58, 310). Baltimore, MD: American College of Sports Medicine.
Powers, S.K., & Howley, E. T. (2015). Bioenergetics & work tests to evaluate cardiorespiratory fitness. Exercise physiology: Theory and application to fitness and performance (9th ed.) (pp. 62, 63, 336). New York, NY: McGraw-Hill Education.
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