Download Ch2

Fuel for Exercise:
Bioenergetics and Muscle
Metabolism
CHAPTER 2 Objectives
• Explain how energy substrates provide energy
during metabolism
– Carbohydrates
– Proteins
– Fats
• Compare and contrast the 3 basic energy systems
– Structure
– Enzymes
– Aerobic vs Anaerobic
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
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)
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
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
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
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
Figure 2.1
Enzyme Activity in Bioenergetics
• 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
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
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
Figure 2.4
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)
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
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)
Figure 2.5
Figure 2.6
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 
Glycolytic System
• Anaerobic
• ATP yield: 2 to 3 mol ATP/1 mol substrate
• Duration: 15 s to 2 min
• Breakdown of glucose via glycolysis
Fig 2.7, p.57
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
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
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
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
Oxidation of Carbohydrate
• Stage 1: Glycolysis
• Stage 2: Krebs cycle
• Stage 3: Electron transport chain
Figure 2.8
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
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
Figure 2.9
Krebs Cycle
• https://www.youtube.com/watch?v=JPCs5p
n7UNI
**ExPhysRules
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
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
Figure 2.10
Figure 2.11
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
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
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
Table 2.2
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
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
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
Figure 2.13
Table 2.3
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
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
Figure 2.14
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
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