L4. Electron Transport Chain and Oxidative Phosphorylation Flashcards
LO
- Describe how measurement of oxygen consumption or carbon dioxide production gives an estimate of whole-body energy expenditure
- Explain the principles of uncoupling and how DNP and UCP-1 can lead to uncontrolled or regulated uncoupling respectively
- Explain how the chemical properties of NAD, FAD, UQ and the complexes allow them to fulfil their roles in electron transport
- Outline how the exchange of electrons between different types of carriers can lead to proton pumping
- Appreciate the electrical and concentration components of the proton motive force
- Appreciate the reasons why cytosolic NAD+ regeneration presents an important challenge and how the glycerol 3-phosphate shuttle and the malate aspartate shuttle can help
- Organise the four separate routes that feed into UQ (Complex I, Complex II, G3P shuttle and beta-oxidation)
- Outline the mechanisms involved in the generation and destruction of free radicals
- List the components and functions of the ATP synthase and explain the mechanism by which it produces ATP
- Recognise the contribution of the proton gradient to processes other than the ATP synthase
- Understand the assumptions made in tables which claim to calculate the yield of ATP from different metabolic pathways and fuels
- Using the fundamental rules of coupling, extrapolate to predict the effects of various interventions on the rates of fuel oxidation, maintenance of the proton gradient and ATP generation
Coupling
- Rate of fuel oxidation is matched to the rate of which the ATP is used (demand driven system)
- Measurung the rate of fuel oxidation will tell us the rate of energy expenditure (O2 consumed = CO2 produced)
Uncoupling
The H+ ions being able to diffuse through the membrane, causing no gradient and NO ATP Synthesis can occur
- H+ no longer flow through ATP Synthase
VERY BAD:
No driving force for ATP synthase
- Therefore no back pressure to stop the pumping and e- movement down the ETC
Instant regeneration of NAD from NADH
- massive fuel oxidation rate
- Massive oxygen consumption
- BUT no ATP synthesis (cells will die)
Dinitrophenol (DNP)
- Hydrophoibic when protonated (move freely across membrane)
- Weak acid
- When H+ comes off the negative charge can be delocalised (shared across the molecule)
- Cells will be producing lots of heat (sweating and fast breathing)
DNP Mechanism
Causes weight loss uncontrollably
Not much H+ in the matrix (high pH, high in OH-)
High H+ ions in cytoplasm (acidic pH, low in OH-)
- DNP picks up H+ ions in cytoplasm
- DNP then diffuses into mitochondria matrix
- Once in the matrix, DNP looses the H+ in the matrix due to low H+ concentraion and high pH
- Repeats cycle as DNP is always hydrophobic
Natural Uncoupler
UCP-1:
- In brown Adipose Tissue (high # mitochondria)
- Can short circuit the ATP Synthase
- DIssipates the proton gradient by opening the inner pore of UCP-1 (Thermogenin)
- Thermogenenin generates heat instead of ATP
Thermogenin
Generates heat (Good for hibernating animals and small mammals)
Under hormonal control
- Noradrenalin binds to β-3 receptors on cell surface
- Stimulating FA release and opens proton channel
e- transport and H+ pumping in the ETC
NADH drops its H/e- at complex 1
- The H/e- skip past complex 2, travels through Q pool, complex 3, cytC and complex 4.
FADH2 drops its H/e- at complex 2
- The H/e- travels through Q pool, complex 3, cytC and complex 4.
Protons are only pumped at complexes 1,3 & 4
- No protons are pumped at complex 1 and the Q pool
- Protons are pumped from the matrix, not the carriers
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ETC
4 complexes all within the mitochondrial inner membrane
Each complex is made of many proteins
- Structural (maintain shape)
- Prosthetic group (transport H/e-)
H+ explelling reactions are on the outside of the membrane
H+ consuming reactions are on the matrix side of the membrane
NAD
UV wavelength absorbance changes when reduced Vs. Oxidised
NAD: CH2-CHOH
into
NADH: CH2-C=O
- NADH drops cargo to complex 1
- 1H & 2xe-
- Gives 10xH pumped in ETC
FAD
Likes saturated Hydrocarbon chains
FAD: CH2-CH2
into
FADH2: CH=CH
- FAD is ALWAYS in complex 2 (not free like NAD)
- FAD is an acceptor and donor of H/e- (2xH 2xe-)
- Gives 6xH pumped in ETC
UQ (Ubiquinone)
UQ is very hydrophobic (lives within inner membrane)
- Electrons move around in complex 1 from one prosthetic group to another until they reach the Q-pool
- UQ picks up H+ from complex 2
- Once UQ obtains a H, it is reduced to UQH2
- UQH2 transfers H+ to complex 3
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Cytochrome C and Iron
- Cyt C picks up e- from complex 3 and gives the e- to complex 4
- Cyt C has a prosthetic group containing an Iron atom
- The Iron converts between Fe3+ and Fe2+ as it picks up and donates off the e-.
- Doesn’t carry H+
- The Iron is held in place by a Heme-group or and iron-sulphur complex
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Mechanism of proton pumping
- When different types of carriers exchange H/e-, H+ can be taken up or released
- The orientation of the uptake/ release allows net translocation (pumping) of H+
- Proton releasing = cytoplasmic side of membrane
- Proton consuming = Matrix side
Net result is pumping from inside the mitochondria to outside
- 10X H pumped per NADH
- 6x H pumped per FADH2
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Outer membrane
Has pores big enough to allow proteins to pass
Electrochemical gradient
- Both a chemical and electrical gradient
- The amount of free energy produced is driven by electrical and chemical gradients.
Getting cytoplasmic NADH to the ETC
NADH generated by glycolysis can’t simply get into the matrix
Glycerol 3-Phosphate Shuttle:
Malate Aspartate Shuttle:
Glycerol 3-Phosphate shuttle
- Dihydroxyacetone Phosphate takes the H+ from NADH and is converted into Glycerol-3-Phosphate.
- G-3-P can drop the H+ to FAD in complex 2
- Therefore only 6H+ will be pumped from the FADH2 through the Q pool, and complex 3&4
Malate Aspartate Shuttle
- NADH passes H/e- to Oxaloacetate forming Malate
- Malate can diffuse into the matrix
- Malate donates its H+ to NAD+, reducing it to NADH
- Malate turns into oxaloacetate and is cycled back out to the cytoplasm
- 10H will be pumped by the NADH
The 4 Routes to Q-pool
- From complex 1
- From complex 2
- From the first step of β-oxidation (FADH2 was created)
- From the glycerol 3-phosphate shuttle
Free Radicals
- Electrons in the UQ pool can react with molecular oxygen producing free radicals
- Very dangerous (causes mutations to DNA and damages proteins)
- Is cleaned out by compounds
- Only occurs when there are ‘traffic jams’ in the ETC
ATP synthase
3 protons = 1 ATP
- Due to the F0-F1 ATPase Structure
The F0 channel is composed of 12 cylindrical proteins
1. As protons enter there is rotation of
stem subunit
2. This causes the β subunit of F1 to change
its conformation in three ways:
* Accepting ADP and Pi
* Reacting them together to give ATP
* Releasing the ATP
Every time three protons come in, the β-subunit changes conformation
- 3 different shapes for 3 different purposes
H+ gradient in transport
H+ are used to transport ATP, ADP and Pi into and out of the cell
- The swapping of ATP/ADP takes negative charge outside
- The import of phosphate consumes protons
