Flashcards in Normal mitochondrial function - lecture 1 Deck (33):
What is the meaning of the word mitochondria?
2 greek words:
- MITOS= thread
- KHONDRION= granule
mitochondria have both granular and thread like structure
What is the proposed origin of mitochondria and how did this process occur?
1) mitochondria were originally archaea or proteobacteria that were autotrophs surviving by the use of photosynthesis to generate energy
2) it was engulfed but not digested by a primitive eukaryotic cell
3) eukaryote then began a symbiotic relationship, where the eukaryote provided nutrients and protection while the prokaryote provided additional energy to the eurkaroyte via its respiratory cellular machinery
What is the "PROOF" behind endosymbiosis?
No actual proof, and its believed to have occurred 2.5 thousand years ago
1) amoeba are eukaryotes that lack mitochondria so require a symbiotic relationship with an aerobic bacteria
2) mitochondria have their own DNA, RNA and ribosomes so they are pretty self-sufficient although they are not
3) mitochondria and purple aerobic bacteria both consume oxygen and produce ATP via krebs cycle and oxidative phosphorylation
4) mitochondria and bacteria are the same size- poor evidence
5) mitochondria have circular DNA like bacteria
6) mitochondria divide independently of the cell they are in- constantly dividing to make new mitochondria
7) mitochondria have a double phospholipid membrane and although purple aerobic bacteria have a single membrane they acquire a second one when they are endocytosed
What are cristae?
folds and invaginations in mitochondria which increase its surface area
What act as power sources for mitochondria ?
glucose or fatty acids as they produce pyruvate which can enter the mitochondria
What happens in oxidative phosphorylation?
acetyl-coA is a common substrate for the krebs cycle
- it drives the production of reducing equivalents NADH and FADH2
- electrons pass through the ECT causing redox reactions that cause proton translocation across the inner mitochondrial membrane to create an electrical potential and pH gradient to drive ATP synthesis by mitochondrial ATP synthase
selective and non-elective ion channels dissipate energy and alter ionic balance and volume of matrix, partly compensated by antiporters coupled to proton movement
What is present in the outer mitochondrial membrane?
- inwardly rectifying potassium channels
- voltage dependent anion channels
- BCl and Bax for apoptosis
- TOMS= transporters of the outer membrane
- SAMS= sorting and assembling machinery
- maybe AChRs
What is present in the inner mitochondrial membrane?
- Ca2+ activate potassium channels
- intermediate conductance channels
- small conductance potassium mediated calcium channels
- K+ATP channel- though some people dont believe this is present
-acid sensing channels = important due to the way mitochondria function and generate energy
- Kb1.3- could be involved in survival as its thought targeting this channel leads to mitochondrial death
- adenine nucleotide transporter
- anion channels
-PTP= permeability transition pore - allows calcium to flood in when opened
- TIMS= transporters of the inner membrane - important for protein import
- magnesium sensnitive channels
- calcium uniporter- important for modifying calcium levels within the cell
- ryanodine receptor
- maybe NMDA receptors
Why is calcium uniporter important ?
important for buffering calcium in the cytosol when large amounts of calcium enter the cytosol- almost communicates directly with the ER
- can actually move calcium from the mitochondria to the ER without it contacting the cytosol so you dont get any large rises in calcium in the cytosol
What do ROS do to PTP?
a ROS surge causes PTP opening and cell death whereas the inhibition of ROS keeps the pore locked and protects cells from noxious stimuli
What are some key facts about the genome of human mitochondria ?
it codes for 37 genes
first significant part of the human genome that was sequenced
inherited exclusively from the mother- unsure exactly why but the sperm mitochondria are thought to be targeted for ubiquitination
2 strands of circular DNA with a heavy guanine rich strand and a light cytosine rich strand
What genes do the mitochondria DNA encode?
mainly encodes genes for tRNA but also for polypeptides and rRNA
Why does mitochondrial DNA have a greater rate of mutations compared to nuclear DNA?
because of its close proximity to ROS
- thought to be involved in cause of aging as experiments in c.elegans showed that by preventing this mutations their life span was increased by 3-4 times
What does glycolysis produce?
produces 2 ATP molecules which enter the citric acid cycle and another 2 ATP molecules are produced
If cells can evoke glycolysis why do we need mitochondria?
because oxidative phosphorylation enables NADH to be transported into the electron transport system and this generates 32 ATP molecules
"NADH" is the currency for oxidative phosphorylation
What does the citric acid cycle do ?
it is carried out all the time and it produces 4 molecules of NADH
- only complex 2 is part of the citric acid cycle because it produces fumerate from succinate
- fatty acids can also enter this cycle
What is ubiquinone good at?
good at accepting electrons and being reduced to ubiquinol
What are the 4 complexes involved in oxidative phosphorylation?
COMPLEX 1: NADH enters the system (it is a massive structure)
COMPLEX 2: succinate dehydrogenase- this doesn't actually provide movement across the membrane
COMPLEX 3: carried out "Q cycle"
COMPLEX 4: oxygen is consumed
ATP synthase: energy is passed onto this complex
What is oxidative phosphorylation vital for ?
vital for pumping protons into inter membranous space- this is how energy is stored = chemihypotic hypothesis
process can actually run backwards
How does complex 1 work?
1) it is a large redox enzyme that oxidises NADh, reducing ubiquinone and transfers protons across the mitochondrial membrane
2) NADH oxidation occurs by a flavin mononucleotide in the hydrophilic domain and this causes intramolecular electrons to transfer towards the membrane by a chain of iron sulphur clusters
3) reduced flavin reacts with oxygen producing superoxide but it also reacts with many other molecules - of which some increase oxidative stress
it is thought that generally 4 protons are transferred for each NADH molecule oxidised but some people have suggested 3
What does complex 1 look like?
it has 2 clear domains- a hydrophobic domain within the membrane and a hydrophilic domain which sticks into the matrix
What is meant by complex 1 modulation?
it can exist in 2 forms - A and D
ex vivo 5-15% of it exists in D conformation at normal oxygen levels - this indicates that some of the energy released during steady state NADH oxidation in situ is for maintaining catalytcially competent A form
- maintaining some of complex 1 in D form ensures fast changes in conditions like stress, increased ATP demand and changes in oxygen availability
What may A/D transitions be important for?
fine tuning enzyme activity
- under conditions such as ischemia, fine tuning would protect not only complex 1 but the entire respiratory system from oxidative damage
What are the series of events between complex 1 and complex 2?
1) NADH binds complex 1 and donates 2 electrons to flavin monomucletide, FMNH2 then transfers electrons through a series of iron sulphur clusters to ubiquinone reducing it to ubiquinol
2) succinate is oxidised to fumerate reducing a bound flavin adenine dinucleotide to FADH2. the electrons are transferred though iron sulphur clusters to reduce ubiquinone
What is complex 2 part of ?
part of both citric acid cycle and electron transport system
What are the different conformations of ubiquinone?
ubiquinone= fully oxidised
semiquinone= ubstable/reactive state
ubiquinol= fully reduced
What happens in complex 3?
uses 2 cycle process to transfer 4 protons to the inter membranous space
1) ubiquinol binds to Qo of complex 3 giving up 1 electron to each rieske iron sulphur protein and the BL haem producing transient free radical semiquinone
2) semiquinone can be dangerous if it is transferred to oxygen as it can produce superoxide anion but usually oxidation continues and it produces ubiquinone which is transferred to Qi site
3) electron donated to iron sulphur protein is transferred to cytochrome c1 subunit causing it to donate it to an externally bound molecule of oxidised cytochrome c that dissociates from the complex
3) electron donates to BL reduced the Bh which then reduces ubiquinone at Qi to semiquinone radical
4) as semiqunione is dangerous another q cycle occurs to so another electron transfer can occur from cytochrome Bh reducing semiqunone to ubiquinol
total of 4 protons move into intermembraneous space and a further 2 protons from the matrix
What happens at complex 4?
4 electrons are removed from 4 molecules of cytochrome c and transferred to molecular oxygen producing 2 molecules of water
at the same time 4 protons are removed from the matrix but only 2 are transported across the membrane contributing to proton gradient
oxygen is consumed here
What is ATP synthase ?
it is a molecular motor driven by the proton gradient
- primary function is ATP synthesis
- very large structure with a mushroom shaped protein complex
- has 2 portions F0= proton translocating region and F1= catalytic region
works by converting the energy present in the proton gradient to chemical energy stored in ATP
What happens at the F0 portion ?
1) protons flow down their electrochemical gradient and enter a pore in the F0 portion which doesn't penetrate the entire membrane
2) instead the proton binds to rotor subunits, protonating them and this causes them to rotate into the membrane away from the channel
3) once it has completed nearly a full turn and has returned to stationary channel the proton exits through another pore to the other side of the membrane
this process converts the stored energy in the proton gradient to rotational mechanical energy which is transferred to the F1 portion
What happens in the F1 portion of ATP synthase?
this portion catalyses the formation of ATP
it has a 3 site alternating binding site mechanism
the central shaft of this region drives changes in the surrounding subunits so ATP can be synthesised
1) the loose subunit binds ADP and Pi
2) the energy input alters its conformation
3) New ATP forms by condensaion and the original ATP is release
4) then the cycle begins again with the knob rotated by one subunit
Why are mitchondrial poisons important?
because they have aided our knowledge about the electron transport system and its functions