Bioenergetics II:
Aerobic Production of ATP
I. Review:
Modes of ATP Production
A. Limited ATP storage
1. Enough energy to fuel a few seconds of
maximal work
2. For exercise duration greater than ~10 sec,
resynthesis of ATP must equal its rate of usage
B. Muscle cells can produce ATP via:
1. Phosphocreatine/Creatine Phosphate (PC/CP)
breakdown
2. Glycolysis (anaerobic/aerobic)
3. Oxidative phosphorylation (glucose; fatty
acids; amino acids)
C. Advantages and Limitations to anaerobic
glycolysis
II. Aerobic ATP Production: Citric Acid Cycle
and the Electron Transport Chain
A. Overview
1. Occurs within the mitochondria
2. Occurs only in the presence of O2
3. Two parts – Citric Acid Cycle and Electron
Transport Chain (ETC)
B. Aerobic ATP production: Step 1, Formation of Acetyl-CoA
1. Pyruvate can be formed from carbohydrates
(glycolysis) or amino acids
2. Pyruvic Acid is converted to Acetyl-CoA and
one CO2 molecule inside the mitochondrial matrix
3. Fatty acids can also be converted into
Acetyl-CoA by beta-oxidation (inside mitochondria)
4. Alternative to energy production: Acetyl-CoA can be used to build fatty acids,
ketone bodies, or cholesterol
5. Relationship between the metabolism of proteins,
fats, and carbohydrates
C. Aerobic ATP Production: Step 2, Kreb’s Cycle
1. Completes the oxidation of the fuel – i.e.,
H atoms are removed by NAD+ and FAD
2. Involves 8 different enzymes
3. Per glucose, results in production of:
– 6 NADH H+ = 6* 2.5 = 15 ATP via ETC
– 2 FADH2 = 2* 1.5= 3 ATP via ETC
– 2 GTP = 2 ATP
TOTAL = 20 ATP
D. The Krebs Cycle - Oxidation-Reduction
Reactions
1. pneumonic:
“LEO says GER”
Loss
of Electrons is Oxidation
Gain
of Electrons is Reduction
2. Redox reactions in cells often involve
transfer of hydrogen atoms (1 electron, 1 proton) rather than free electrons
3. Electrons, or hydrogen atoms, are often
transferred in pairs
NAD+ + 2H ® NADH + H+
(reduction of NAD)
FAD + 2H ® FADH2
(reduction of FAD)
E. Aerobic Production of ATP: Step 3, Electron Transport Chain
1. The ETC is located inside mitochondria (in
the cristae = inner membrane)
2. The electrons are removed from 2 H and
passed through a series of cytochromes (iron containing molecules)
3. As electrons are passed, one cytochrome is
oxidized while the next is reduced (gains electrons)
4. THREE ATP are produced from each NADH H+
while TWO ATP are produced from each FADH2
5. Oxygen is the final hydrogen acceptor
Oxygen
is readily reduced (gains electrons) and is therefore a potent “oxidizing
agent”
The
two electrons from the ETC rejoin with two protons (H+ ions) and ˝ O2 to form
H2O
F. The Chemiosmotic Hypothesis of ATP Formation
1. ETC releases energy that is used to pump H+
ions across the inner mitochondrial membrane - Results in H+ gradient across
the inner membrane
2. When 2H+ diffuse back across the membrane,
the potential energy released is used by ATP synthase to form ATP
3. Two protons rejoin with two electrons (from
ETC) and ˝ O2 to form water – this prevents accumulation of H+ in the matrix
space
III. Aerobic ATP Tally: Efficiency of Oxidative
Phosphorylation
1. Aerobic metabolism of one molecule of glucose
yields 32 usable ATP
2. Aerobic metabolism of one molecule of glycogen
yields 33 usable ATP
3. The efficiency of respiration =
32
moles ATP/mole glucose x 7.3 kcal/mol ATP
= 34%
686
kcal/mol glucose
IV. Control of Bioenergetics
A. The rate of energy production by metabolic
pathways are controlled by altering the activity of one or more enzymes in a
particular pathway
B. In general, metabolic pathways operate via negative
feedback, such that high levels of ATP inhibit ATP production and low
levels of ATP and high levels of ADP+Pi stimulate ATP production
V. Interaction Between Aerobic and Anaerobic
ATP Production
A. Energy to perform exercise comes from a
combination of aerobic and anaerobic pathways
B. Effect of duration and intensity
1. Short-term, high-intensity activities =
greater contribution of anaerobic energy systems
2. Long-term, low to moderate-intensity
exercise = majority of ATP produced from aerobic sources