Energy Metabolism & Utilization
Introduction
Energy is the capacity to do work. The chemical energy needed to support bodily movement (i.e. muscle contraction) is stored in the high-energy phosphate bonds of the molecule, adenosine triphosphate (ATP) which provides 7.6 kcal/mol of ATP to working muscles. Muscular activity entails the conversion of chemical energy to mechanical work utilizing the energy released from the breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate (P). The production of ATP comes from the nutrients obtained in the foods we eat. The low-energy molecular bonds of food are converted by special catalytic proteins called enzymes ("-ase") to the higher-energy phosphate bonds of ATP. There are limited stores of ATP in skeletal muscle: 5-6 mmol/kg of resting muscle, which can provide energy for only 1-3 muscle contractions or for an exercise duration of 1-2 seconds under intense activity. Thus, a continuous supply of ATP is needed for longer durations of exercise.
The body has two very quick systems of ATP reconstitution during continuous muscular activity: ATP-Creatine Phosphate(CP) and Myokinase. To replenish the stores of ATP in the body, there are two metabolic pathways (sequence of enzymatic reactions involved in the transformation of one substance into another) which converts nutrients into ATP: anaerobic (i.e. glycolysis) and aerobic (Oxidative phosphorylation).
ATP-CP & Myokinase Systems
Creatine phosphate is another high-energy phosphate molecule present primarily in the cells of skeletal muscle at a concentration four times greater than muscle ATP. In resting muscle, there is 16-25 mmol of CP/kg of musculature, and 70-80% of this amount can be used for energy, which is very efficient. Under continuous intensive demand, all CP stores can be completely emptied. CP is not used directly by muscles, but is used as a backup supply of ATP once the supply of muscle ATP is depleted [Oly]. Breakdown of creatine phosphate to creatine and inorganic phosphate is carried out by the enzyme, creatine phosphokinase (CPK) and the resulting inorganic phosphate is then added to ADP and ATP is regenerated:
CPK
CP --------------> C + P
ADP + P -------> ATP
The activity of this enzyme is regulated by the amount of ATP currently consumed by the muscle. The breakdown of CP occurs at a linear rate with power production for dynamic exercise (i.e. running, swimming) and with the force developed under static loading (i.e. weight lifting). This source of ATP can only supply enough energy for about 6-8 s during maximal muscle exercise. For maximal load intensities, the enzyme myokinase (or, adenylate kinase and also located in skeletal muscle) converts two molecules of ADP to ATP and AMP (adenosine monophosphate), thus utilizing the energy of ADP to rebuild ATP. The breakdown of ATP from these two systems generates about 10.0 kcal of energy. From 1 mmol of CP, 3.9-5.3 kcal of mechanical work can be obtained. The time it takes for ATP regeneration by these two systems is about 24-28 s. When a higher concentration of ADP and AMP under severe muscle load occurs, and the reconstitution of ATP from available CP stores is not sufficient to compensate, then glycolysis will be activated.
Anaerobic Metabolism
When ATP stores generated from CP and myokinase are depleted and there is insufficient oxygen for aerobic metabolism in the early minute of continuous muscle contraction, glycolysis is activated. This metabolic pathway converts carbohydrates (glycogen) to a metabolic intermediate, pyruvate. Glycogen is a large polymer of glucose units which is found in muscle (up to 1%) and in the liver (up to 10%).
In anaerobic metabolism, pyruvate is converted to lactic acid by lactate dehydrogenase and a reduced coenzyme (a catalytically active component of an enzyme), nicotinamide adenine dinucleotide (NADH+H) which is oxidized to NAD. This NAD is then utilized back in the glycolytic pathway to be reduced again to NADH+H. The ATP's produced from glycolysis is either used directly or stored with the help of the enzyme creatine phosphokinase is creatine phosphate. The glycolysis pathway is inefficient as only 3 mol of ATP (or, 22.8 kcal of energy) is produced from 1 mol (180g) of glucose, and this amount of energy is only sufficient for exercises lasting up to a minute (60 s) such as sprints and weight-lifting. As glycogen is further degraded, the by-products, lactic acid and phosphoric acid, build up in the blood and interfere with muscle contraction which results in muscle cramping and acidosis (decrease in blood pH).
Aerobic Metabolism
If there is sufficient oxygen, then aerobic metabolism is preferred over anaerobic glycolysis because of its high efficiency of ATP production. In this process, carbohydrates are completely oxidized to produce 39 mol of ATP (or, 296.4 kcal of energy) from 1 mol of glucose. Carbohydrates are first converted to glucose and then through glycolysis to pyruvate and to acetyl bound to coenzyme A as acetyl Coenzyme A. When there is not a sufficient supply of oxygen, pyruvate is being converted to lactic acid. Acetyl CoA enters in the tricarboxylic acid cycle (TCA) (also known as Krebs Cycle) where hydrogen atoms (protons plus electrons) are withdrawn by several enzymes and are carried as the reduced forms of co-enzymes NAD and flavin adenine dinucleotide (FAD), that is as NADH and FADH2. Carbon dioxide is a by-product which is removed from the body by venous blood and expired out of the lungs. Two pairs of electrons, one from NADH and one from FADH2 then enter the respiratory or electron transport chain which is located in the inner membrane of the mitochondria. ATP's are produced from the phosphorylation of ADP as electrons flow through the chain, which comprises of a series of cytochromes (a group of heme proteins serving as electron donors and acceptors) with oxygen as the final electron acceptor. The oxygen is reduced by accepting two electrons to produce water keeping the flow of electrons continuous. This coupling of electron transport and ATP synthesis is oxidative phosphorylation which is the main function of aerobic metabolism. The rate of oxidative phosphorylation is regulated by the level of ADP in muscle, so as ATP is consumed as in exercising muscle, electrons flow from carbohydrates and fatty acids to oxygen to regenerate the ATP. Free fatty acids are also involved in oxidative phosphorylation, where fats are oxidized (degraded) to acetyl CoA which enters the TCA Cycle and also fuels oxidative phosphorylation. Aerobic metabolism is the primary method of energy production for long duration exercise, such as endurance events, where an athlete can provide sufficient oxygen to working muscles.
References
Diet and Health: Implications for Reducing Chronic Disease Risk. Committee on Diet and Health Food and Nutrition Board and Commission on Life Sciences and National Research Council. National Academic Press, Washington D.C. 1989. pp. 139-142; 159;259; 273-276; 291-2.
Buskirk, E.R. Some Nutritional Considerations in the Conditioning of Athletes. Ann. Rev of Nutr. Darby, Broquist & Olson, Eds. 1981. 1:319-50.
Linder, Maria. Nutrtional Biochemistry and Metabolism with Clinical Applications. 1985. Elsevier, New York. pp.15-31; 33-39; 51-56; 69-73.
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