KNPE 452 CH2 Fuel for exercise: bioenergetics and muscle metabolism

For unit 1 qui

Bioenergetics

Foundation of all voluntary human movement

Conversion of food into ATP

Transfer of energy via chemical bonds using metabolic pathways (purpose is to capture energy stored in food → convert to energy stored in ATP molecule)

ATP

The energy stored in molecules that powers all biological work, energy is the currency of cells

The only energy source that can directly be used for muscle contraction

Our ability to produce movement is equal to:

Our ability to produce ATP

Cellular chemical reactions order

Glycolysis → krebs cycle → electron transport chain

Substrates

Fuel sources from which we make energy

Reactant

Carbohydrate + fat + protein

Bioenergetics simple def

Process of converting substrates into energy

Performed at a cellular level

Metabolism

1st law of thermodynamics

Chemical reactions in body

Anabolism (small → large molecule [protein synthesis])

Catabolism (large → small molecule)

@ rest the body burns

50% carbohydrates, 50% fat

During short exercise the body burns

More carbohydrate

During long exercise the body burns

More fat

Carbohydrate

All are converted to glucose

Primary ATP substrate for muscles, brain

4.1 kcal/g; ~2,500 kcal stored in body

Extra is stored as glycogen in liver and muscles, polymer

Glycogen

Extra glucose that is stored in the liver and muscles

Is converted back to glucose when needed to make more ATP

Stores are limited (2,500 kcal) and must rely on dietary carbohydrate to replenish

Fat

Efficient substrate, efficient storage

9.4 kcal/g

+70,000 kcal stored in body

Stored as tryglycerides, glyceral backbone with 3 fatty acid

Energy substrate for prolonged, less intense exercise 

High net ATP yield but slow ATP production

Triglycerides broken down into free fatty acids and glycerol

Lypolysis

Breaking down tryglyceride

Protein

Energy substrate during starvation

@ rest its estimated protein only contributes to ~2% to our metabolism, up to 10% during exercise

4.1 kcal/g

Must be converted into glucose

Can be converted to free fatty acids (FFAs) for energy storage and cellular energy substrate

Enzymes

The production of ATP involves anaerobic and aerobic energy systems

Energy systems

Metabolic pathways that include a number of enzymatic steps

Anaerobically

W/o oxygen

Aerobically

W/ oxygen

Benefit of enzymes

Lowers activation energy required to cause a reaction

Factors that alter enzyme activity

Temperature (optimal rage between 37C[@ rest] and 40C[during exercise])

pH (optimal range ~7.5-8)

Basic energy systems

ATP storage is limited

Body must consistently synthesize new ATP

Estimated that the amount of ATP stored in skeletal muscle could only fuel 3-4s of all out intensity

Three ATP synthesis (metabolic) pathways

ATP-PCr system (anaerobic metabolism, high intensity)

Glycolytic system (anaerobic metabolism, high intensity)

Oxidative system (aerobic metabolism)

(phosphogen system is the most powerful)

ATP-PCr system

Simplest and fastest method of producing ATP (1 step)

Very short term, high intensity exercise (explosive movements)

<10-15s

Exercise intensity

Measured by duration

Regeneration of ATP by PCr equation

Pcr +ADP —creatine kinase—> ATP + Cr

PCr

Phosphocreatine or creatine phosphate

Regeneration of ATP by PCr

Creative kinase takes the phosphate group off of PCr and adds it to ADP

Reaction switches from going right to going left during rest

Creatine kinase

PCr breakdown catalyst

Controls rate of ATP production

When ATP levels decrease (ADP increases) activity of this increases, and vice versa

Glycolytic system simple

Anaerobic

Net yield ATP 2-3 ATP

20s-3min

Breakdown of glucose via glycolysis

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: 10-12 enzymatic reactions in total, all steps occur in cytoplasm, net ATP yield 2 ATP (glucose) 3 ATP (glycogen)

End product it lactate

How many carbons does glucose and glucose-6-phosphate has

6

Cons of glycolytic system

Low ATP yield

Inefficient use of substrates

H+ ions (byproduct) impairs glycolysis and muscle contraction

Glycolysis is short lived because of the associated decrease in pH

Pros of glycolytic system

Allows muscles to contract when O2 is limited

Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

Phosphofructokinase (PFK)

Slowest enzyme in the glycolytic pathway

Rate-limiting enzyme

Also regulated by products of Krebs cycle

Decrease in ATP (increase ADP) → increased PFK activity (and vice versa)

Time duration for glycolysis

~2 min maximal exercise

Oxidative system

Aerobic

ATP yield depends on the substrate: 32 ATP per molecule of glucose, 100+ ATP per 1 FFA

Duration: steady supply for hours

Most complex of three bioenergetic systems

Occurs in mitochondria, not cytoplasm

Stages of oxidation of carbohydrate

Stage 1: glycolysis

Stage 2: krebs cycle 

Stage 3: electron transport chain

High energy electron carriers

NADH and FADH2

Where are the most NADH and FADH2 produced

The Krebs cycle

How NADH and FADH2 produce ATP

Is processed through the electron transport chain

End product of aerobic glycolysis with O2

Pyruvate

Steps of the Krebs cycle

Step 1: 1 molecule glucose → 2 acetyl-CoA

(1 molecule glucose → 2 complete krebs cycles, 1 molecule glucose → double ATP yield)

Step 2: 2 acetyl-CoA → 2 GTP → 2 ATP

Also produces NADH and FADH2

Electron transport chain

H+, electrons carried to electron transport chain via NADH, FADH molecules

H+, electrons travel down the chain: H+ combines with O2, electrons + O2 help form ATP, 2.5 ATP per NADH produced and 1.5 per FADH2 produced (AVERAGES)

Why does FADH2 give less ATP than NADH

It skips the first molecule in the electron transport chain, therefore less energy is harnessed

Energy yield of oxidation of carbohydrate

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

Oxidation of fat

Triglycerides: major fat energy source (broken down to 1 glycerol + 3 FFAs)

Yields ~3-4 times more ATP than glucose

Slower than glucose oxidation

β (beta) - oxidation of fat

Process of converting FFAs to acetyl-CoA before entering Krebs cycle

Number of steps depends on number of carbons on FFA: 16-carbon FFA yields 8 acetyl-CoA

1 acetyl-CoA is produced for every

2 carbons in the fatty acid

Krebs cycle and Electron transport chain of the oxidation of fat

Step 1: acetyl-CoA enter Krebs cycle

Step 2: follows the same path as glucose oxidation

(Different FFAs have different numbers of carbons and will yield different numbers of acetyl-CoA molecules and ATP yield)

Net yield of palmitic acid

106 ATP

Oxidation of protein

Rarely used as a substrate

Can be converted to glucose (gluconeogenesis) and acetyl-CoA

Energy yield is not easy to determine

Requires oxygen to be metabolized

Lactate utilization

Is NOT a waste product and DOES NOT cause muscle soreness

Is an important fuel during exercise

Muscles can utilize in 3 ways:

• Converted back to pyruvate and used in the cell it was produced in or sent back into the blood and taken up by another cell

• Can recirculate back to the liver, reconverted to pyruvate and then to glucose through gluconeogenesis

Interaction of the energy systems

All three systems interact for all activities

No one system contributes 100%, but one system often dominates for a given task