Fueling Exercise & Energy Systems

Substrates

Bioenergetics

Measure Energy Release

1 calorie (cal) = heat energy required to raise 1 g of water 1°C

Fuel for Exercise

Carbohydrate/ 4.1 cal/g

All carbohydrate converted to glucose

Glycogen converted back to glucose when needed to make more ATP

Glycogen stores limited (2,500 cal), must rely on dietary carbohydrate to replenish

Fat/ 9.4 kcal/g

Efficient substrate, efficient storage

Energy substrate for prolonged, less intense exercise

Protein/ 4.1 cal/g

Energy substrate during starvation

Can also convert into FFAs (lipogenesis)

Controlling Rate of Energy Production

by Substrate Availability

Energy released at a controlled rate based on availability of primary substrate

Mass action effect

by Enzyme Activity

Energy released at a controlled rate based on enzyme activity in metabolic pathway

Enzymes

ATP broken down by ATPase

Each step in a biochemical pathway requires specific enzyme(s)

More enzyme activity = more product

Rate-limiting enzyme

Stored Energy: High-Energy Phosphates

ATP stored in small amounts until needed

Breakdown of ATP to release energy

Synthesis of ATP from by-products

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)

  • Anaerobic, substrate-level metabolism
  • ATP yield: 1 mol ATP/1 mol PCr
  • Duration: upto 15 s
  • Because ATP stores are very limited, this pathway is used to reassemble ATP
  • Phosphocreatine (PCr): ATP recycling
  • Replenishes ATP stores during rest
  • Recycles ATP during exercise until used up (~3-15 s maximal exercise)

PCr breakdown catalyzed by CK
CK controls rate of ATP production

  • ATP yield: 2 to 3 mol ATP mol substrate
  • Duration: upto 2 mins
  • Breakdown of glucose via glycolysis
  • Uses glucose or glycogen as its substrate
  • Pathway starts with glucose-6-phosphate, ends with pyruvic acid
  • Phosphofructokinase (PFK)
  • Glycolysis = ~2 min maximal exercise
  • Need another pathway for longer durations
  • ATP yield: depends on substrate
  • 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

Glycolysis can occur with or without O2

  • H+, electrons carried to electron transport chain via NADH, FADH molecules
  • H+, electrons travel down the chain

Energy Yield

  • 1 glucose = 32 ATP
  • 1 glycogen = 33 ATP
  • Breakdown of net totals

Oxidation of Fat

Triglycerides: major fat energy source

  • Rate of FFA entry into muscle depends on concentration gradient
  • Yields ~3 to 4 times more ATP than glucose
  • Slower than glucose oxidation

beta-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

Oxidation of Fat:Krebs Cycle, Electron Transport Chain

Metabolism

  • Acetyl-CoA enters Krebs cycle
  • From there, same path as glucose oxidation
  • Different FFAs have different number of carbons

Oxidation of Protein

Rarely used as a substrate

Energy yield not easy to determine

Interaction Among Energy Systems

  • All three systems interact for all activities
  • More cooperation during transition periods