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Fuel for Exercise: Bioenergetics and Muscle Metabolism

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Title: Fuel for Exercise: Bioenergetics and Muscle Metabolism


1
Chapter 2
  • Fuel for ExerciseBioenergetics and Muscle
    Metabolism

2
Terminology
  • Substrates
  • 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

3
Measuring Energy Release
  • Can be calculated from heat produced
  • 1 calorie (cal) heat energy required to raise 1
    g of water from 14.5C to 15.5C
  • 1,000 cal 1 kcal 1 Calorie (dietary)

4
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

5
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

6
Table 2.1
7
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

8
Figure 2.1
9
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

10
Figure 2.4
11
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)

12
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

13
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)

14
Figure 2.5
15
Figure 2.6
16
Glycolytic System
  • Anaerobic
  • ATP yield 2 to 3 mol ATP/1 mol substrate
  • Duration 15 s to 2 min
  • Breakdown of glucose via glycolysis

17
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18
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19
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

20
Glycolytic System
  • Cons
  • 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

21
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

22
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

23
Oxidation of Carbohydrate
  • Stage 1 Glycolysis
  • Stage 2 Krebs cycle
  • Stage 3 Electron transport chain

24
Figure 2.8
25
Oxidation of CarbohydrateGlycolysis 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

26
Oxidation of CarbohydrateKrebs 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

27
Figure 2.9
28
Oxidation of CarbohydrateElectron 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

29
Oxidation of CarbohydrateEnergy 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

30
Figure 2.11
31
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

32
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

33
Oxidation of FatKrebs 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

34
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36
Table 2.2
37
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

38
Figure 2.12
39
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

40
Figure 2.13
41
Table 2.3
42
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

43
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

44
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

45
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 limiteduse it or lose it
  • O2 levels entering and leaving the lungs accurate
    estimate of O2 use in muscle
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