C H A P T E R 4

1. C H A P T E R 4 METABOLISM, ENERGY, AND THE BASIC ENERGY SYSTEMS

2. w Explore how energy production and availability can limit performance. (continued) Learning Objectives w Learn how our bodies change the food we eat into ATP to provide our muscles with the energy they need to move. w Examine three systems that generate energy for muscles.

3. Learning Objectives w Learn how exercise affects metabolism and how metabolism can be monitored to determine energy expenditure. w Discover the underlying causes and sites of fatigue in muscles.

4. Calorie and Kilocalorie w Energy in biological systems is measured in calories (cal). • 1 cal is the amount of energy required to raise the temperature of 1 g of water 1°C (from 14.5°C to 15.5°C). • In humans, energy is expressed in kilocalories (kcal), where 1 kcal equals 1,000 cal. • 1,000cal = 1kcal≈ 1Cal • People often mistakenly say “calories” when they mean kilocalories. When we speak of someone expending 3,000 cal per day, we really mean that person is expending 3,000 kcal per day.

5. Energy for Cellular Activity w Food sources are processed via catabolism—the process of “breaking down.” w Energy is derived from food sources and stored as adenosine triphosphate (ATP). • ATP = high-energy currency, a form of energy that cells can use

6. ATP

7. ATP MOLECULE

8. Energy Sources w At rest and during low intensity exercise, the body (skeletal muscle) primarily uses fats for energy. w Protein provides little energy for cellular activity, but serves as building blocks for the body's tissues. w During moderate to severe muscular effort, the body relies mostly on carbohydrate for fuel.

9. Digestion & Absorption • Digestion: • Absorption:

10. Carbohydrate • Starches, sugars, bread, pasta, rice, cereal, etc. • - monosaccharide, disaccharide, polysaccharide • Ultimately converted to glucose, which is then metabolized by most tissues. Belk & Borden, Biology, Pearson Prentice Hall, 2004

11. Carbohydrate Belk & Borden, Biology, Pearson Prentice Hall, 2004

12. Carbohydrate wGlucose(C6H12O6) is transported in the blood to the tissues for ATP productionand can also be converted to glycogen in liver and muscle, or converted to fat in liver. w Glycogen stored in the liver can be converted back to glucose as needed and transported by the blood to the tissues where it is needed to form ATP. w Unlike fat stores, glycogen stores are limited, which can affect performance.

13. Fat w Cells catabolize free fatty acids (FFAs) to produce ATP – fats provide substantial energy at rest and during prolonged, low-intensity activity w FFAs are stored as triglycerides (3 FFAs attached to a glycerol molecule) in cells, particularly adipocytes - body stores of fat are much larger than for carbohydrate (i.e., muscle and liver glycogen) w Fat provides more energy per unit (9.4kcal/g) than carbohydrate (4.1kcal/g) but the rate of energy release is very slow – it’s metabolism requires oxygen; thus, it becomes less important as a fuel at higher exercise intensities

14. Fatty acids

15. Triglyceride

16. Adipocyte

17. Protein w Can be used as an energy source if broken down to amino acids, which are then converted in the liver to glucose via gluconeogenesis (important during starvation) • Only the basic units of protein — amino acids — can be used for energy: ~4.1 kcal of energy per g of protein • NOT a major source of energy for exercise

18. Total Body Stores of Fuels and Energy g kcal Carbohydrates Liver glycogen 110 451 Muscle glycogen 500 2,050 Glucose in body fluids 15 62 Total625 2,563 Fat Subcutaneous and visceral 7,800 73,320 Intramuscular 161 1,513 Total7,961 74,833 Note. These estimates are based on an average body weight of 65 kg (143 lb) with 12% body fat.

19. Fuel use during exercise Romijn et al. Am J Physiol E389, 1993

20. Enzymes w Specific protein molecules thatcatalyze chemical reactions by lowering the activation energy w Names always end in “ase” w Work at different rates and can limit flux through a pathway w Glycolytic enzymes are located in the cytoplasm, while oxidative enzymes are located primarily in the mitochondria

21. ACTION OF ENZYMES For example, ATP ADP + Pi + Energy ATPase Brooks, Fahey, & Baldwin, Exercise Physiology, McGraw-Hill, 2005

22. Basic Energy Systems 1. ATP-PCr system (phosphagen system)—cytoplasm 2. Glycolytic system—cytoplasm 3. Oxidative system—mitochondria (“powerhouses” of cell) * Anaerobic vs. Aerobic * Phosphorylation

23. ATP-PCr System: Creatine Kinase reaction w This system can prevent energy depletion by quickly resynthesizing ATP from ADP and Pi. w This process is anaerobic—it occurs without oxygen. w 1 mole of ATP is produced per 1 mole of phosphocreatine (PCr), or creatine phosphate (CP) . The energy from the breakdown of PCr is not used for cellular work, but solely for regenerating ATP.

24. Resynthesis of ATP from PCr This occurs in the cytoplasm proximal to the myosin ATPase on the thick filaments

25. ATP AND PCr DURING SPRINTING The muscle fiber does everything it can to maintain its ATP stores!

26. Glycogen Breakdown and Synthesis Glycogenesis—Process by which glycogen is synthesized from glucose to be stored in the muscle or liver Glycogenolysis—Process by which glycogen is broken down into single glucose-1-phosphate molecules for catabolism in glycolysis in the muscles (or release into the blood from the liver)

27. Glycolysis w Requires a series of enzymatic reactions to sequentially break down glucose into pyruvic acid or lactic acid • Glycolysis produces several important products: • A small amount of ATP (substrate-level phosphorylation • Pyruvic acid or lactic acid, which can subsequently be used as an oxidative substrate in the mitochondria • A small amount of H+ (reducing equivalents), that are used in the electron transport chain in the mitochondria to produce ATP

28. Glycolysis G-1-P • Net ATP production • 2. Net H+ production • (NAD+ + 2H+→ NADH + H+) • 3. Net pyruvic acid or lactic acid production during anaerobic conditions

29. Lactic acid or Lactate? An acid is a compound that dissociates into H+ and a salt when placed in water Thus, when lactic acid occurs in the aqueous environment of the cells or blood, lactic acid → lactate + H+ This increases the H+ concentration, or increases the acidity (lowers the pH) • Consequences of low pH • - enzyme function: • - calcium binding:

30. Anaerobic metabolism The combined actions of the ATP-PCr and glycolytic systems allow muscles to resynthesize ATP in the absence of oxygen; thus, these two energy systems are the major energy contributors during the early minutes of high-intensity exercise.

31. Oxidative System w Relies on oxygen for ATP production w Produces ATP in the mitochondria of cells w Can yield much more energy (ATP) than anaerobic systems w Is the primary method of energy production

32. Oxidative Production of ATP from CHO 1. Glycolysis — cytoplasm 2. Krebs cycle (TCA cycle, citric acid cycle) — mitochondria 3. Electron transport chain — mitochondria

33. Aerobic Catabolism of CHO

34. KREBS CYCLE

35. Oxidation of Carbohydrate 1. Pyruvic acid from glycolysis is converted to acetyl coenzyme A (acetyl CoA) by pyruvate dehydrogenase (PDH). 2. Acetyl CoA enters the Krebs cycle; in the cycle, ATP, carbon dioxide, and hydrogen are produced. 3. Hydrogen produced in the cycle combines with coenzymes (NAD+ or FAD) that carry it to the electron transport chain. 4. Hydrogens pass down the electron transport chain producing ATP and combine with oxygen to form water. 5. One molecule of glucose from glycogen can generate up to 39 molecules of ATP.

36. OXIDATIVE PHOSPHORYLATION

37. By oxidative Stage of process Direct phosphorylation Glycolysis (glucose to pyruvic acid) 3 6 Pyruvic acid to acetyl coenzyme A 0 6 Krebs cycle 2 22 Subtotal 5 34 Total 39 ATP Production From the Oxidation of Muscle Glycogen

38. Oxidation of Fat wLypolysis — breakdown of triglycerides into glycerol and free fatty acids (FFAs) in adipocytes, hepatocytes, or muscle. w FFAs travel from adipocytes via blood to muscle fibers and are broken down by enzymes in the β-oxidation pathway in the mitochondria to acetyl CoA. w Acetyl CoA enters the Krebs cycle and hydrogens produced enter the electron transport chain.

39. METABOLISM OF FAT Murray et al., Harper’s Biochemistry, Lange, 1996

40. Β-oxidation

41. Adenosine triphosphate produced from 1 molecule of palmitic acid By oxidative Stage of process Direct phosphorylation Fatty acid activation 0 –2 -oxidation 0 35 Krebs cycle 8 88 Subtotal 8 121 Total 129 ATP Production From the Oxidation of Palmitic Acid (C16H32O2)

42. Protein Metabolism w Body uses little protein during rest and exercise as fuel (contributes less than 5% to ATP production) in normal fed condition – becomes important during starvation. w Some amino acids that form proteins can be converted into glucose. w The nitrogen in amino acids (which cannot be oxidized) makes the energy yield of protein difficult to determine.

43. Overview

44. INTERACTION OF ENERGY SYSTEMS ILLUSTRATING THE PREDOMINANT ENERGY SYSTEM

45. ENERGY SYSTEMS DURING EXERCISE Brooks, Fahey, & Baldwin, Exercise Physiology, McGraw-Hill, 2005

46. What Determines Oxidative Capacity? w Muscle oxidative enzyme activity (mitochondrial density) w Fiber-type composition w Capillary density in the muscles w Oxygen delivery to the muscle capillaries O2 Ventilation Circulation Oxidative phosphorylation Taylor et al., Resp Physiol 69: 1, 1987

47. OXIDATIVE ENZYME ACTIVITY AND OXIDATIVE CAPACITY Succinate dehydrogenase (SDH) is a Krebs cycle enzyme (i.e., located in the mitochondria)