Lecture 5 Flashcards
(19 cards)
How do we resynthesize ATP?
Synthesis of ATP from by-products
ADP + Pi + energy —> ATP (via phosphorylation)
Can occur in either absence or presence of oxygen
“Non-oxidative” energy sources:
- Phosphocreatine
- glycolysis/glycogenolysis
ATP resynthesized through non-oxidative processes referred to as “substrate-level” phosphorylation
- in the cytosol “Sarcoplasm”
“Oxidative” energy sources:
- citric acid cycle/electron transport
- fatty acids from beta-oxidation
ATP resynthesized through oxidative processes referred to as “oxidative” phosphorylation
- in the mitochondria
Moderate-intensity exercise
Oxidative phosphorylation (takes time to ramp up) efficiently sustains all the ATP resynthesis once steady state is achieved
PCr and Glycolysis/Glycogenolysis contribute very little at the beginning and not at all during steady state
- at gym we don’t ever actually reach our max ability (grey line)
Mitochondria
Key components:
1. Outer membrane - contains proteins and lipids and special pores that allow entry to most ions and small molecules (permeable)
2. Inner membrane - load with proteins and enzymes (including ATP synthase) for transferring electrons to O2 and impermeable to most ions and polar molecules unless they have specific transporters (impermeable)
3. Intermembrane space - contains key proteins, including créatine kinase and cytochrome c
4. Matrix (inter mitochondria) - viscous medium containing all enzymes for the citric acid cycle
- in skeletal muscle, mitochondria are found beneath the sarcolemma and between myofibrils
Conversion of Pyruvate to Acetyl-CoA
- the link between glycolysis and the citric acid cycle
- irreversible rxn catalyzed by the enzyme pyruvate dehydrogenase (PDH)
- glycolysis (anaerobic)
- by product
- pyruvate
- lactate
Lactate is reversible with pyruvate
Oxidative Phosphorylation (aerobic)
- pyruvate
- acetyl - CoA
2 pyruvate + 2CoA + 2NAD+ —(PDH)—> 2 acetyl-CoA + 2CO2 + 2NADH + 2H+ (citric acid/Krebs cycle)
Conversion of Pyruvate to Acetyl-CoA
The pyruvate gets brought into the cycle from the cytosol, with the MPC enzyme (PDH), and produces a NADH, CO2, and acetyl-CoA into the mitochondria
Tricarboxylic acid cycle (TCA) - Krebs cycle
- second stage of carbohydrate breakdown
- oxidizes acetyl-CoA (electrons are lost)
- sequence of metabolic events that remove 4 electrons paris from acetyl-groups and attach 3 pairs to coenzyme nicotinamide adenine dinucleotide (NAD+) and 1 pair t flavin adenine dinucleotie (FAD)
- much of the free energy of oxidation of acetyl-CoA is conserved in the reduced coenzymes (NADH and FADH2)
- general rxn:
3NAD+ + FAD + GDP + Pi + acetyl-CoA —> 3NADH + FADH2 + GTP + CoA + 2CO2 + 3H+
Breakdown of a pyruvate molecule
Breakdown of TCA or Krebs cycle
2 pyruvate, so everything is doubled
- gets oxidized
- creates an acetyl-CoA - oxidized (looses electron pairs) goes to NAD+, GDP (ADP + Pi), and FAD
2 pyruvate molecules: produce 2CO2 and 2 reduced coenzymes
2 acetyl-CoA molecules: produce 4CO2 and 8 reduced coenzymes (4 per acetyl-CoA)
Krebs cycle
Acetyl-CoA goes into the TCA cycle - producing 3 NADH, 4 CO2 and 1 FADH2, in the mitochondria
Malate-aspartate shuttle (electron transport chain)
- translocates electrons from glycolysis across the inner membrane of mitochondria (transport NADH across)
- shuttle system s required because the inner membrane is impermeable to NADH (primary electron donor to the electron transport chain)
- to circumvent this, malade carries NADH across the membrane
In the cytosol we create NAD+, electrons go to malate, then in the matrix we create NADH, electron go to aspartate, then in the cytosol, aspartate then changes into oxaloacetate
Electron Transport Chain (ETC)
- a series of proteins and grains molecules packed in the inner membrane of mitochondria that transfer electrons (e-) (from NADH and FADH2) from one member of the ETC to another in a series of redox reactions (gaining electrons)
- energy released in these rxns transfer protons (H+) from matrix to intermembrane space creating a proton gradient across that provides an electrochemical potential energy
- potential energy is harnessed to resynthesize ATP
- O2 is present at the end of the chain where it accepts electrons to form water
- if there is no O2, the ETC will stop running!
The ETC
- Delivery of e- by NADH and FADH2
- transfer e- pairs to molecules near the start of the ETC, turn back to NAD+ and FAD - Electron transfer and proton pumping
- as e- pass thru chain, they move from higher to lower energy level, releasing energy
- energy is used to pump H+ out of the matrix to establish an electrochemical gradient - Splitting of O2 to form H2O
- at the end of chain, 2e- are transferred to O2 which splits in half and takes up 2H+ to form H2O (O2 + 4e- + 4H+ —> 2H2O2)
- oxygen is hanging out - ATP resynthesis
- H+ ions flow down their gradient and back into matrix passing thru an ATP synthase (complex V) enzyme which uses the proton flow to synthesize ATP
ETCprocess
Protons pumped by in at complex 5
ADP turns into ATP
Oxygen is produced with e- and 2H+ and removed
Oxidation of Carbohydrate
- stage 1: Glycolysis
- stage 2: Krebs cycle
- stage 3: Electron transport chain
C6H12O6 + 6O2 + 323ADP + 32Pi —> 6CO2 + 6H2O + 32ATP
32 ATP per molecule of glucose oxidized
Reduced coenzymes
1 NADH —> 10 H+ out —> 4 H+ in per ATP synthesized = 2.5 ATP (10/4)
1 FADH2 —> 6 H+ out —> 4 H+ in per ATP synthesized = 1.5 ATP (6/4)
Stage - products (net) - ATP yield (net)
Glycolysis - 2 ATP - 2ATP
2 NADH - 5 ATP
Pyruvate oxidation - 2 NADH - 5 ATP
Citric acid cycle - 2 ATP/GTP - 2 ATP
6 NADH - 15 ATP
2 FADH2 - 3 ATP
Total: 32 ATP
Oxidative Phosphorlation
- continual ATP resynthesis during coupled oxidative phosphorylation of macronutrients has three prerequisites:
1. Availability of reducing agents NADH or FADH2
2. Presence of a terminal oxidizing agent as oxygen
3. Sufficient quantity of enzymes and metabolic machinery in tissues to make energy transfer reactions “go” at appropriate sequence and rate
2 NADH + 2 H + 5 ADP + 5Pi + O2 —> 2 NAD+ + 5 ATP + 7H2O
2 FADH2 + 3 ADP + 3 Pi + O2 —> 2 FAD+ + 3 ATP + 5 H2O
Créatine Kinase and the PCr Shuttle
- the CK/PCr energy shuttle connects sites of ATP production (mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases)
- transfers energy from where it’s made to where it’s needed - CK in both the intermembrane space and cytosol
Cytosol if CK = CK-MM isozyme
1. PCr -Cr diffusion
2. ATP-ADP diffusion
Mitochondrial CK = CK-Mi isozyme
Créatine kinase and the PCr shuttle process
In mitochondria
- Cr produces PCr
- ATP produces ADP
In cytosol
- PCr produces Cr
- ADP produces ATP
Integrated metabolism
Low exercise intensities
PCr and Oxidative Phosphorylation
Measures Ox Phos vs PCr
They work together
Moderate intensity
- PCr - falls but reaches a plateau
Heavy intensity
- VO2 - takes more time than moderate intensity
