|Capturing the Essence of Energy for Exercise
Roger Vaughan and Len Kravitz, Ph.D.
Hill,Exercise professionals ardently expend a great amount of time developing purposeful exercise programs for clients. Fervent interest is spent learning the intricate mechanisms of different muscle actions, and understanding how the contractile proteins (i.e., myosin and actin) create force to do an array of many types of exercise. Yet, biochemistry of energy concepts are difficult constructs to learn for many professionals. This column will discuss recent explanations how cells channel energy stockpiled in food substances to be used for the work accomplished in exercise.
ATP: The Energy Currency of Exercise
All life forms need energy to grow, move and maintain. Thousands of energy-requiring processes are continuously occurring in cells to meet life's demands. Energy can take on many forms in biological systems, but the energy currency that is most useful is known as adenosine triphosphate (ATP).
The cells cannot create ATP from scratch. According to the first law of thermodynamics the total amount of energy in the universe remains constant. Therefore, from the dietary foods eaten and digested a potential energy resides within cells in the chemical bonds of organic (i.e., carbon-containing) compounds such as glucose (a simple sugar or a monosaccharide), glycogen (a complex sugar or a polysaccharide composed of hundreds or thousands of glucose molecules stored in the muscles, liver and brain) and fatty acids (saturated or unsaturated acids produced during the breakdown of triglycerides). When these compounds enter energy pathways some of the atomic bonds break or become rearranged, with energy released and captured in the formation of ATP. The ATP molecule is then used for cell functions such as supplying the energy for muscle contraction, building other complex molecules (in conjunction with enzymes), generating electrochemical messages in nerves, transporting substances across cell membranes and powering every activity in the cell. The energy for all these processes is liberated from ATP by removing the terminal inorganic phosphate (Pi) group from the molecule, leaving adenosine diphosphate (ADP) plus one proton (H+). This ADP is readily recycled in the mitochondria (power source organelle in cells) and also in the cytoplasm where it is recharged again to ATP.
Why is ATP referred to as a High Energy Molecule?
When the terminal Pi is broken from the ATP a high level of energy is released (which is why ATP is called a high energy molecule) that very closely meets the needs of a specific biological reaction. The outermost Pi groups on the ATP are held together with unstable bonds, meaning the energy is readily released when the ATP is cleaved of its Pi (called hydrolysis because water is the splitting molecule that removes the Pi). During this molecular commotion a little heat energy is lost to the cell surroundings, which the cell does not recapture. ATP is not much of a storage fuel. Rather it is produced in one set of reactions and almost immediately consumed in another set of reactions, which is a process called coupling.
What is the Difference in Anaerobic and Aerobic ATP Pathways?
The first major distinction that is important to make when differentiating types of energy yielding pathways is whether or not oxygen is essential for ATP synthesis. Some metabolic pathways require oxygen and are said to be aerobic and will not proceed unless oxygen is present in sufficient concentrations. Other processes do not require oxygen to proceed to completion and are said to be anaerobic. The important message is that oxygen can play a major role in some pathways, and have little influence on others. It is ideal to have this diversification in cells so they can adapt to cellular energy needs (at least temporarily) independent of oxygen.
The Anaerobic Glycolytic Story: Overcoming a Bioenergetics Challenge
Glycolysis, the breakdown of glucose by enzymes, is one of the most studied metabolic pathways in exercise science. It is a series of 10 sequential reactions that allow the conversion of glucose to pyruvate. If the reactions begin with glycogen, the storage form of glucose, there are 11 ordered reactions (called glycogenolysis). Glycolysis can occur in most cells and it does not require oxygen. It is the preferred energy yielding process for most cells, and is used when blood glucose levels are normal.
Glucose in the blood can be transported into the cell with specialized GLUT carriers. Once in the muscle cell the glucose is trapped within that cell by the attachment of a Pi group on glucose's 6th carbon. It is interesting to note that glucose trapped in upper body muscles cannot be removed to help supply energy needs in the lower body (and vice versa). So, during an challenging workout a client may become glycogen energy depleted in one area of the body but have plenty of stored glycogen in other areas of the body, but unable to obtain it. Interestingly, intense training bouts (i.e., sprints) lasting greater than 10 seconds will result in greater glycogen storage and thus positively affect exercise performance (Kraemer, Fleck, & Deschenes, 2012).
Glycolysis is a story of struggle, although healthy cells can perform it with ease. The first five steps in glycolysis are designed to weaken the atomic bonds, making the carbohydrate compound less stable and more willing to liberate its energy. Think of glycolysis as going uphill on a bicycle. Getting up the hill is somewhat challenging but once on top it's easy pedaling the rest of the way back down. Glycolysis reactions function in the same way, with the second five steps being the energy-yielding phase.
Aerobic Metabolism: The Citric Acid Cycle
With sufficient oxygen present the carbohydrate breakdown (called carbohydrate oxidation) will continue until completion. The initial step of aerobic metabolism begins with the conversion of pyruvate into acetyl coenzyme A or acetyl-CoA in the mitochondria of the cell. Acetyl-CoA then combines with oxaloacetate to form citrate. Citrate is the first metabolite of several reactions called the citric acid cycle or Krebs cycle. During the citric acid cycle, citrate undergoes several reactions that generate CO2 (which is metabolic waste that is expired during an exhalation), some hydrogen carriers known as NADH and FADH2 (which transport energy to synthesize ATP in the next metabolic pathway), and a little ATP (via help of a GTP molecule).
Aerobic Metabolism: The Electron Transport Chain
The NADH and FADH2 hydrogen carrier compounds proceed to the electron transport chain (ETC) where a sequence of cytochromes (iron-containing proteins) harvest the chemoelectric energy through specialized reactions (during which, protons are pumped into the intermembrane space of the mitochondrion). During this process, oxygen is the driving force that causes electrons to be shuffled through the cytochromes. Eventually, the electrons combine with oxygen to form metabolic water. The protons (H+) that were pumped into the intermembrane space are then be pumped into the mitochondrial matrix by an enzyme called ATP synthetase, which liberates energy to synthesize ATP.
Wait, What About Fat Breakdown?
Like aerobic carbohydrate breakdown, fat degradation (or fat oxidation) requires oxygen. Because fats are long carbon chains, fats begin their disassembly with a metabolic process called beta-oxidation. Beta-oxidation is analogous to a lumberjack chopping down some long 'carbon' logs into more manageable acetyl-CoA, NADH and FADH2 compounds. These compounds go directly into the mitochondrion to yield ATP (via the same metabolic processes explained above).
Side Bar 1. Why Does the Body Prefer Carbohydrates As Exercise Intensity Increases?
As exercise intensity increases from rest to near maximal levels, there is a gradual transition to use more glucose and glycogen as the predominant sources of ATP (Kraemer, Fleck, & Deschenes 2012). From a metabolic standpoint in the mitochondria, more ATP can be produced aerobically from the breakdown of carbohydrate as composed to fat. However, and most importantly, as exercise intensity increases many more fast-twitch muscle fibers are recruited which are much more suited (because of their enzymes) to utilize carbohydrates for the needed ATP production. In addition, higher intensity exercise stimulates epinephrine production, which also enhances carbohydrate metabolism (Kraemer, Fleck, & Deschenes 2012).
Closing Energy Tributes
There are several diverse energy systems within the body working in unison to meet ATP needs as shown in Figure 4. Depending on exercise intensity and oxygen availability, one system may be used more than others. However, let the final message of this column be a tribute to both the genius and the exceedingly complex nature of the 'essence of energy for exercise'.
Jones, D., Round, J., de Haan, A. (2004). Skeletal muscle from Molecules to Movement, Churchill Livingstone, Edinburgh
Kraemer, W.J., Fleck, S.J., and Deschenes, M.R. (2012). Exercise Physiology: Integrating Theory and Application, Wolters Kluwer/Lippincott Williams & Wilkins, Philadelphia