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KIN 364

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  1. KIN 364 Bioenergetics Exercise Metabolism

  2. Reading • Powers CH3 – Bioenergetics (p23-49) • Powers CH4 – Exercise Metabolism (p52-69)

  3. Bioenergetics • Metabolic processes by which foods (CHO, fats, proteins) are converted to energy • Muscle contraction limited by ATP availability

  4. Chemical Basis • 95% of human body composed 4 elements: • Oxygen (65%) • Carbon (18%) • Hydrogen (10%) • Nitrogen (3%) • Elements linked together by chemical bonds to form compounds (molecules) • Organic compounds (contain carbon – CHO, fats, protein) • Inorganic compounds (lack carbon – H2O)

  5. Cellular Organization • Cell membrane • Regulates passage of substances into and out of cell • Nucleus • Contains genes which regulate protein synthesis • Determines cell composition and controls cellular activity • Cytoplasm • Contains organelles for specific functions • Mitochondria – site of oxidative metabolism • Enzymes – regulate glucose breakdown (“glycolysis”)

  6. Metabolism • Total of all cellular reactions occurring in the body • Anabolic – synthesis of larger molecules from smaller ones (process requires energy) • Catabolic – breakdown of larger molecules into small (associated with release of energy) • Chemical reactions - release of energy stored in chemical bonds • Endergonic – require energy to proceed • Exergonic – give off energy • Coupled Reactions – endergonic reactions driven by exergonic reactants coupled with them in chemical processes

  7. Metabolism • Enzymes • Large proteins molecules acting as catalysts to regulate rate of chemical reactions • Ridges / grooves arranged in lock and key configuration between specific enzyme and reactant molecule (substrate) • Reduce energy needed for chemical reaction (lower energy of activation)

  8. Metabolism • Factors affecting enzyme activity: • Core Body Temperture • slight rise, as with exercise, elevates enzyme activity - increases rate of chemical reactions – enhances ATP production (low or excessively high CBT decreases metabolism) • pH • Increases in lactic acid results in lowered pH and sub-optimal environment for bioenergetic enzymes (and decreased ATP for muscle contraction)

  9. Fuels for Exercise • Carbohydrates • Fats • Proteins Fats

  10. Carbohydrates • CHO • Composed of carbon, hydrogen, oxygen • 4 kcal of energy per gram 3 forms: • Monosaccharides (glucose, fructose) • Disaccharides (sucrose - gl + fr) • Polysaccharides (3 or more monos) • Complex Carbs: • Cellulose (fiber) - undigestible • Starch (corn, grains, beans, potatoes)

  11. Carbohydrate Metabolism • Exergonic • Polysaccharides (starches) broken down to monosaccharides and used as energy • Endergonic • Glucose molecules linked together to form glycogen • Liver, muscle glycogen stored in limited amounts (depleted ~ 2 hours activity) • Glycogenolysis (breakdown) • Muscle glycogen - energy source for muscle contraction • Liver glycogen - free glucose released into bloodstream

  12. Fats • 9 kcal of energy per gram • 4 general groups: • Fatty acids • Triglycerides • Phospholipids (cell membrane, schwann cells,…) • Steroids (cholesterol - sex hormone synthesis) • Phospholipids, steroids not energy sources

  13. Fat Metabolism • Lipolysis • Breakdown of triglycerides into: • FFAs (3 mols) used as energy substrate • Glycerol (1 mol) used by liver to synthesize glucose

  14. Proteins • 4 kcal of energy per gram • Amino Acids • Building blocks (subunits) of proteins • AAs linked together by peptide bonds to form proteins • 20 needed by body to form enzymes, blood proteins,… • 9 “essential” AAs cannot be synthesized by the body (must be consumed)

  15. Protein Metabolism • Must be broken down into AAs to provide energy • Alanine – converted to glucose – glycogen • Others used as intermediate compounds in bioenergetic pathways of cellular metabolism

  16. ATP • Adenosine Triphosphate (ATP) – high energy phosphate compound • ADP + Inorganic Phosphate (Pi) = ATP • Endergonic reaction producing high energy bond • ATP ADP + Pi + energy • High energy bond broken under influence of enzyme ATPase releases energy for muscle contraction (Exergonic) ATPase

  17. Bioenergetics Anaerobic ATP production Aerobic ATP production

  18. Bioenergetics • Energy - capacity to do work • Bioenergetics • Conversion of food (CHO, fats, proteins) into biologically useable forms of energy (ATP) • Breakdown of chemical bonds releases energy to perform work (muscle contraction) • Chemical energy converted to mechanical energy

  19. Metabolic Pathways • Anaerobic • ATP produced w/o O2 • ATP-PC (Phosphagen) System • Glycolysis • Aerobic • Oxidative formation of ATP • Oxidative phosphorylation

  20. Anaerobic Metabolism • ATP-PC • Donation of phosphate group (and its bond energy) to form ATP • PC + ADP creatine kinase ATP

  21. Anaerobic Metabolism • ATP-PC • Simple, one-enzyme reaction • Most rapid method of ATP production • Limited storage of PC in muscles = limited ATP production via ATP-PC pathway • Used at onset of exercise: • High intensity - short duration (<5-10 sec.)

  22. Anaerobic Metabolism • Glycolysis • Breakdown of glucose or glycogen • Rejoins Pi to ATP • Net gain = 2 molecules of ATP + 2 molecules of pyruvate or lactic acid per glucose molecule

  23. Anaerobic Metabolism • Glycolysis • 2 phases • Energy investment (priming) • Stored ATP used to form sugar phosphates • Phosphorylation - add phosphate groups to glucose (endergonic) • Priming not required for glycogen substrate • Energy generation • Net gain = 2 ATP (from glucose substrate), 3 ATP from glycogen substrate

  24. Glycolysis • Hydrogen (by-product of substrate metabolism) • O2 available? • Transported into mitochondria for oxidative metabolism by “carrier molecules” • Nicotinamide adenine dinucleotide - NAD • Flavin adenine dinucleotide - FAD • O2 unavailable? • Pyruvic acid accepts hydrogens to form lactic acid (lactate dehydrogenase - LDH) • Lactic acid (lactate = conjugate base of LA) formed to “recycle” NAD and allow glycolysis to continue

  25. Aerobic Metabolism • Oxiative Phosphorylation • Mitochondria - site of aerobic metabolism • Krebs (citric acid) cycle • Electron transport chain • 3 stages: • Generation of acetyl-CoA • Oxidation of acetyl-CoA in Krebs cycle • Oxidative phosphorylation (ATP formation) in the ETC (“respiratory” chain)

  26. Oxidative Phosphorylation • Krebs Cycle • Completes oxidation (hydrogen removal) of CHO, fats, and proteins using hydrogen carriers NAD, FAD • Hydrogens contain potential energy in food molecules used in ETC to combine ADP and P Note: KC is non-oxidative - O2 is used at end of ETC as final hydrogen acceptor (H2O produced)

  27. Krebs Cycle • CHO • Pyruvate broken down to form acetyl-CoA • 1 glucose mol = 2 pyruvate mol (glycolysis) converted to 2 mol acetyl-CoA (in presence of O2) • Fats • Triglycerides broken down to form fatty acids (3) and glycerol • Beta-oxidation - series of reactions to convert fatty acids to acetyl-CoA for entry into krebs cycle • Proteins • Tertiary fuel source (2-15% of fuel during exercise)

  28. Electron Transport Chain • Electrons removed from hydrogen atoms are passed down a series of electron carriers (cytochromes) • Energy is released to rephosphorylate ADP to ATP (endergonic reaction) • Series of oxidation-reduction reactions with oxygen allowing this to continue by acting as final electron acceptor in ETC (ETC end result = formation of ATP and H2O) • As electrons pass down ETC, reactive molecules (free radicals) are formed • large quantities may be harmful to muscles, cause fatigue)

  29. Metabolic Outcome • Aerobic metabolism: • 1 mol glucose = 32 ATP • 1 mol glycogen = 33 ATP

  30. Rate-Limiting Enzymes • Controls speed of metabolic activity by a specific pathway • Cellular levels of ATP or ADP + Pi regulaterate of metabolic pathways involved in ATP production by negative feedback • ATP-PC • Creatine kinase - catalyzes endergonic reaction: ADP + Pi = ATP • Increase in ADP concentrations stimulates ck to trigger breakdown of PC to resynthesize ATP (negative feedback) • Glycolysis • Phosphofructokinase – PFK (activity enhanced as ADP + Pi levels rise • KC - ETC • Isocitrate dehydrogenase (KC) • Cytochrome oxidase (ETC)

  31. Summary • In general, the shorter the activity (higher intensity), the greater the relative contribution of anaerobic energy systems to ATP production • Relative percentage of metabolic system used determined by nature of exercise / event • Powers (ch3, p 49 – Figure 3.24)

  32. Exercise Metabolism

  33. Homeostasis / Baseline • ATP for bodily functions at rest supplied by aerobic metabolism • O2 consumption ~ 3.5 ml O2 / kg BW

  34. Rest-to-Exercise Transition • Anaerobic energy systems contribute to initiation of exercise: • ATP-PC • Glycolysis • Oxidative • O2 consumption in moderate exercise rises rapildy – reaches steady state in 1-4 min Ox G A 0 1 2 3 4 5 6 min.

  35. Oxygen Deficit • Lag in O2 uptake at beginning of exercise • Difference in O2 uptake for 1st few minutes of exercise compared to and equal time interval after steady state is reached • Training effect (cardiovascular or muscular adaptations): • Shorter time to steady state • Lower O2 deficit • Earlier aerobic ATP production • Lower production of lactic acid at a given intensity

  36. Recovery from Exercise • Metabolism elevated several minutes following exercise • O2 uptake greater, remains elevated longer post-exercise when intensity of exercise is higher • EPOC • excess post-exercise O2 consumption • Amount of O2 consumed during recovery in excess of that which would have ordinarily been consumed at rest

  37. O2 Debt / EPOC • Excess oxygen uptake above rest • Resynthesis of muscle PC • Lactate removal (converted to glucose – gluconeogenesis) • Restoration of muscle and Blood O2 stores • Elevated body temp • Post-exercise elevation of HR, breathing • Elevated hormones • (epinephrine, norepinephrine) • Reoxygenation of blood hemoglobin

  38. Summary • Anaereobic energy systems (ATP-PC and glycolysis) supply need for ATP for first few minutes of CV exercise • Energy supplied anaerobically during O2 Deficit phase until steady state of O2 consumption is utilized • O2 utilization indicates that mitochondrial respiration takes over energy production • Note: energy production is not the result of systems “switching on” and “switching off” but a smooth blending and overlap of the body’s 3 energy systems

  39. Summary • ATP-PC • Primary energy source for events less than 10 sec. duration • Events 10 sec. – 10 min. use combination of anaerobic and aerobic pathways for energy production • Events 45 sec. or longer use combination of all 3 systems (ex: 60 sec. intense exercise ~ 70% anaerobic / 30% aerobic • Event of 2 min. utilize anaerobic and aerobic pathways equally

  40. Anaerobic Threshold • Anaerobic Threshold (Lactate Threshold) • Point of systematic rise in blood lactate during exercise • Occurs at 50-60% VO2 Max in untrained, 65-80% VO2 Max in trained • Indicates failure of mitochondrial “hydrogen shuttle” system to keep pace with rate of glycolytic production of NADH + H+ • Result = conversion of pyruvic acid to lactic acid Powers - Fig. 4.9 p59

  41. Lactate Threshold

  42. OBLA • Onset of blood lactate accumulation • Reflects sudden rise (inflection) in blood lactate concentrations during incremental exercise • Level of exercise intensity (O2 consumption or %VO2 Max) at which blood lactate levels reach 4 millimols / liter Powers - Fig. 4.8 p59 OBLA

  43. Mechanisms for Lactate Threshold • Low muscle O2 (hypoxia) • Accelerated glycolysis • Rising levels of epinephrine / nor-epinephrine (@50-65% VO2 Max) stimulates glycolytic rate and NADH production - if hydrogen shuttle system into mitochondria cannot keep up, pyruvate accepts “unshuttled” hydrogen and LA is formed • Recruitment of fast-twitch fibers • FTF contain a form of LDH that attaches to pyruvic acid - promotes LA formation • Reduced rate of lactate removal

  44. Lactic Acid • Lactic Acid Removal • 70% is oxidized • Light exercise (warm down) following strenuous (glycolytic) exercise enhances oxidation (H2 removal) of lactic acid by working muscles • Optimum warm down intensity = 30-40% VO2 Max (higher intensity = increase in LA production by glycolysis, by necessity for ATP production) • 20% converted to glucose • 10% converted to AAs

  45. Management of Lactic Acid • Training Effect: • LA accumulated in muscles less rapidly (due to earlier / more efficient initiation of aerobic metabolism) • Persist longer in intense exercise at a given muscle lactate level • Greater capacity to remove LA during recovery??

  46. Substrate Utilization • Respiratory Exchange Ratio (R) • Non-invasive technique used to estimate % contribution of CHO or fat to energy metabolism during steady state exercise • Contribution of protein at steady state is negligible - therefore, it is not considered in calculation • R = ratio of VCO2 output to VO2 consumed (Respiratory Quotient - RO)

  47. Fat: C16 H32 O2 Oxidation: C16 H32 O2 + 23 O2 = 16 CO2 + 16 H2O Therefore, R = 16 CO2 / 23 O2 = .70 Glucose: C6 H12 O6 Oxidation: C6 H12 O6 + 6 O2 = 6 CO2 + 6 H2O Therefore, R = 6 CO2 / 6 O2 = 1.0 CHO / Fat Oxidation • CHO contains more O2 - Fat oxidation requires more O2 • R = .70 for Fat, 1.0 for CHO at steady state exercise R = VCO2 output / VO2 consumed Oxidation: O2 combines with Carbon to form CO2; combines with Hydrogen to form H2O

  48. Substrate Utilization • Non-protein R • Range: • .70 (100% fat metabolism) - 1.0 (100% CHO metabolism) • Both substrates contribute at sub-maximal exercise Powers - Table 4.1 p62

  49. Fuel Selection • CHO primary fuel source during high-intensity exercise • Blood glucose • Glycogen (at higher work rates) • Gradual shift from CHO to fat metabolism as primary source during prolonged bouts of exercise (30 min +) • Plasma FFA • Mucsle triglycerides (at higher work rates) • Protein metabolism negligible in duration less than 1 hour (less than 2% of substrate fuel contribution • may be 5-10% of fuel supply during final minutes of prolonged work • BCAAs • Alanine