1. Fuel for Exercise: Bioenergetics and Muscle Metabolism
Chapter 2
Fuel for Exercise: Bioenergetics and Muscle Metabolism

2. Chapter 2 Overview Substrates: fuel for exercise
Controlling the rate of energy production
Stored energy: high-energy phosphates
Bioenergetics: basic energy systems
Interaction among energy systems

3. Terminology Substrates Bioenergetics
Fuel sources from which we make energy (adenosine triphosphate [ATP])
Carbohydrate, fat, protein
Bioenergetics
Process of converting substrates into energy
Performed at cellular level
Metabolism: chemical reactions in the body

4. Measuring Energy Release
Can be calculated from heat produced
1 calorie (cal) = heat energy required to raise 1 g of water from 14.5°C to 15.5°C
1,000 cal = 1 kcal = 1 Calorie (dietary)

5. Substrates: Fuel for Exercise
Carbohydrate, fat, protein
Carbon, hydrogen, oxygen, nitrogen
Energy from chemical bonds in food stored in high-energy compound ATP
Resting: 50% carbohydrate, 50% fat
Exercise (short): more carbohydrate
Exercise (long): carbohydrate, fat

6. Carbohydrate All carbohydrate converted to glucose
4.1 kcal/g; ~2,500 kcal stored in body
Primary ATP substrate for muscles, brain
Extra glucose stored as glycogen in liver, muscles
Glycogen converted back to glucose when needed to make more ATP
Glycogen stores limited (2,500 kcal), must rely on dietary carbohydrate to replenish

7. Fat Efficient substrate, efficient storage
9.4 kcal/g
+70,000 kcal stored in body
Energy substrate for prolonged, less intense exercise
High net ATP yield but slow ATP production
Must be broken down into free fatty acids (FFAs) and glycerol
Only FFAs are used to make ATP

8. Table 2.1

9. Protein Energy substrate during starvation
4.1 kcal/g
Must be converted into glucose (gluconeogenesis)
Can also convert into FFAs (lipogenesis)
For energy storage
For cellular energy substrate

10. Figure 2.1

11. Controlling Rate of Energy Production by Substrate Availability
Energy released at a controlled rate based on availability of primary substrate
Mass action effect
Substrate availability affects metabolic rate
More available substrate = higher pathway activity
Excess of given substrate = cells rely on that energy substrate more than others

12. Controlling Rate of Energy Production by Enzyme Activity
Energy released at a controlled rate based on enzyme activity in metabolic pathway
Enzymes
Do not start chemical reactions or set ATP yield
Do facilitate breakdown (catabolism) of substrates
Lower the activation energy for a chemical reaction
End with suffix -ase
ATP broken down by ATPase

13. Figure 2.2

14. Controlling Rate of Energy Production by Enzyme Activity
Each step in a biochemical pathway requires specific enzyme(s)
More enzyme activity = more product
Rate-limiting enzyme
Can create bottleneck at an early step
Activity influenced by negative feedback
Slows overall reaction, prevents runaway reaction

15. Figure 2.3

16. Stored Energy: High-Energy Phosphates
ATP stored in small amounts until needed
Breakdown of ATP to release energy
ATP + water + ATPase  ADP + Pi + energy
ADP: lower-energy compound, less useful
Synthesis of ATP from by-products
ADP + Pi + energy  ATP (via phosphorylation)
Can occur in absence or presence of O2

17. Figure 2.4

18. Bioenergetics: Basic Energy Systems
ATP storage limited
Body must constantly synthesize new ATP
Three ATP synthesis pathways
ATP-PCr system (anaerobic metabolism)
Glycolytic system (anaerobic metabolism)
Oxidative system (aerobic metabolism)

19. ATP-PCr System Anaerobic, substrate-level metabolism
ATP yield: 1 mol ATP/1 mol PCr
Duration: 3 to 15 s
Because ATP stores are very limited, this pathway is used to reassemble ATP

20. ATP-PCr System Phosphocreatine (PCr): ATP recycling
PCr + creatine kinase  Cr + Pi + energy
PCr energy cannot be used for cellular work
PCr energy can be used to reassemble ATP
Replenishes ATP stores during rest
Recycles ATP during exercise until used up (~3-15 s maximal exercise)

21. Figure 2.5

22. Figure 2.6

23. Control of ATP-PCr System: Creatine Kinase (CK)
PCr breakdown catalyzed by CK
CK controls rate of ATP production
Negative feedback system
When ATP levels  (ADP ), CK activity 
When ATP levels , CK activity 

24. Glycolytic System Anaerobic ATP yield: 2 to 3 mol ATP/1 mol substrate
Duration: 15 s to 2 min
Breakdown of glucose via glycolysis

25. Glycolytic System Uses glucose or glycogen as its substrate
Must convert to glucose-6-phosphate
Costs 1 ATP for glucose, 0 ATP for glycogen
Pathway starts with glucose-6-phosphate, ends with pyruvic acid
10 to 12 enzymatic reactions total
All steps occur in cytoplasm
ATP yield: 2 ATP for glucose, 3 ATP for glycogen

26. Glycolytic System Cons Pros
Low ATP yield, inefficient use of substrate
Lack of O2 converts pyruvic acid to lactic acid
Lactic acid impairs glycolysis, muscle contraction
Pros
Allows muscles to contract when O2 limited
Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

27. Glycolytic System Phosphofructokinase (PFK)
Rate-limiting enzyme
 ATP ( ADP)   PFK activity
 ATP   PFK activity
Also regulated by products of Krebs cycle
Glycolysis = ~2 min maximal exercise
Need another pathway for longer durations

28. Oxidative System Aerobic ATP yield: depends on substrate
32 to 33 ATP/1 glucose
100+ ATP/1 FFA
Duration: steady supply for hours
Most complex of three bioenergetic systems
Occurs in the mitochondria, not cytoplasm

29. Oxidation of Carbohydrate
Stage 1: Glycolysis
Stage 2: Krebs cycle
Stage 3: Electron transport chain

30. Figure 2.8

31. Oxidation of Carbohydrate: Glycolysis Revisited
Glycolysis can occur with or without O2
ATP yield same as anaerobic glycolysis
Same general steps as anaerobic glycolysis but, in the presence of oxygen,
Pyruvic acid  acetyl-CoA, enters Krebs cycle

32. Oxidation of Carbohydrate: Krebs Cycle
1 Molecule glucose  2 acetyl-CoA
1 molecule glucose  2 complete Krebs cycles
1 molecule glucose  double ATP yield
2 Acetyl-CoA  2 GTP  2 ATP
Also produces NADH, FADH, H+
Too many H+ in the cell = too acidic
H+ moved to electron transport chain

33. Figure 2.9

34. Oxidation of Carbohydrate: Electron Transport Chain
H+, electrons carried to electron transport chain via NADH, FADH molecules
H+, electrons travel down the chain
H+ combines with O2 (neutralized, forms H2O)
Electrons + O2 help form ATP
2.5 ATP per NADH
1.5 ATP per FADH

35. Oxidation of Carbohydrate: Energy Yield
1 glucose = 32 ATP
1 glycogen = 33 ATP
Breakdown of net totals
Glycolysis = +2 (or +3) ATP
GTP from Krebs cycle = +2 ATP
10 NADH = +25 ATP
2 FADH = +3 ATP

36. Figure 2.10

37. Figure 2.11

38. Oxidation of Fat Triglycerides: major fat energy source
Broken down to 1 glycerol + 3 FFAs
Lipolysis, carried out by lipases
Rate of FFA entry into muscle depends on concentration gradient
Yields ~3 to 4 times more ATP than glucose
Slower than glucose oxidation

39. b-Oxidation of Fat
Process of converting FFAs to acetyl-CoA before entering Krebs cycle
Requires up-front expenditure of 2 ATP
Number of steps depends on number of carbons on FFA
16-carbon FFA yields 8 acetyl-CoA
Compare: 1 glucose yields 2 acetyl-CoA
Fat oxidation requires more O2 now, yields far more ATP later

40. Oxidation of Fat: Krebs Cycle, Electron Transport Chain
Acetyl-CoA enters Krebs cycle
From there, same path as glucose oxidation
Different FFAs have different number of carbons
Will yield different number of acetyl-CoA molecules
ATP yield will be different for different FFAs
Example: for palmitic acid (16 C): 129 ATP net yield

41. Table 2.2

42. Oxidation of Protein Rarely used as a substrate
Starvation
Can be converted to glucose (gluconeogenesis)
Can be converted to acetyl-CoA
Energy yield not easy to determine
Nitrogen presence unique
Nitrogen excretion requires ATP expenditure
Generally minimal, estimates therefore ignore protein metabolism

43. Control of Oxidative Phosphorylation: Negative Feedback
Negative feedback regulates Krebs cycle
Isocitrate dehydrogenase: rate-limiting enzyme
Similar to PFK for glycolysis
Regulates electron transport chain
Inhibited by ATP, activated by ADP

44. Figure 2.12

45. Interaction Among Energy Systems
All three systems interact for all activities
No one system contributes 100%, but
One system often dominates for a given task
More cooperation during transition periods

46. Figure 2.13

47. Table 2.3

48. Oxidative Capacity of Muscle
Not all muscles exhibit maximal oxidative capabilities
Factors that determine oxidative capacity
Enzyme activity
Fiber type composition, endurance training
O2 availability versus O2 need

49. Enzyme Activity
Not all muscles exhibit optimal activity of oxidative enzymes
Enzyme activity predicts oxidative potential
Representative enzymes
Succinate dehydrogenase
Citrate synthase
Endurance trained versus untrained

50. Figure 2.14

51. Fiber Type Composition and Endurance Training
Type I fibers: greater oxidative capacity
More mitochondria
High oxidative enzyme concentrations
Type II better for glycolytic energy production
Endurance training
Enhances oxidative capacity of type II fibers
Develops more (and larger) mitochondria
More oxidative enzymes per mitochondrion

52. Oxygen Needs of Muscle As intensity , so does ATP demand In response
Rate of oxidative ATP production 
O2 intake at lungs 
O2 delivery by heart, vessels 
O2 storage limited—use it or lose it
O2 levels entering and leaving the lungs accurate estimate of O2 use in muscle