Block 1 - Mitochondria (L11-13) Flashcards
What are the two major roles of the mitochondria?
cellular respiration/ATP homeostasis
programmed cell death/apoptosis
What is the basic structure of a mitochondria?
- rod shaped/round
- two membranes
- outer membrane is the boundary
- inner membrane is folded into cristae that project inward
What are the three proteins in the inner membrane of the mitochondria?
- electron transport chain molecules (NADH dehydrogenase, cytochrome b-c1, and cytochrome oxidase)
- ATP synthase
- Transport proteins
What is in the mitochondrial matrix?
- enzymes for oxidation of pyruvate and fatty acids and use in citric acid cycle
- mitochondrial DNA
- mitochondrial ribosomes
- tRNAs and enzymes needed for expression of the mitochondrial genes
What things do mitochondria and bacteria have in common?
circular DNA, double membrane, and similar electron transport chain
Mitochondria generate —– from —— for use in ——-.
generate energy from OxPhos for use in many pathways
Where does glycolysis occur?
cytoplasm, outside the mitochondria
Where does the Krebs/citric acid cycle occur?
inside the mitochondria
What drives ATP synthesis in the mitochondria?
NADH and FADH2 transport H+ protons into the intermembrane space (between the inner and outer membranes) via the electron transport chain
The proton gradient is used in chemiosmotic coupling to generate ATP
What is chemiosmotic coupling?
the process mitochondria use to harness energy
link between chemical bond-forming reactions that generate ATP and membrane transport
What are the two main steps of generating ATP in the mitochondria?
- electron transport chain shifts protons to create a gradient
- proton gradient is harnessed by ATP synthase to make ATP
What is the pathway leading up to ATP synthase
complex 1 - NADPH oxidase
complex 2 - succinate reductase
complex 3 - cytochrome bc1
complex 4 - cytochrome oxidase
NADH —> complex 1 —> Q
OR
succinate —–> complex 2 —–> Q
THEN
Q —> complex 3 —-> cytochrome c —-> complex 4
What is the final electron acceptor in the ETC?
oxygen
Is the plasma membrane voltage greater or less than the inner mitochondrial membrane voltage?
less than - the high protein gradient across the inner mitochondria creates a voltage that is much higher than the voltage across the plasma membrane
Where are ATP created, and how many?
glycolysis (2)
ox phos (34, but need O2)
= 36 ATP
A single cell consumes roughly —- ATP per second.
10 million
How many protons cause ATP synthase to spin once and generate 1 ATP?
10 protons = 1 spin = 1 ATP
Mitochondrial proton gradients are good sensors of…
cellular health
Mitochondria do not actually make new ATP, they…..
continually recycle ADP to ATP
Why is ADP recycled instead of just making new ATP?
We would need 10 million molecules of ATP per second, or 60 kg ATP per day
That’s too much to make new every day, so we reuse throughout the day
Does the citric acid/Krebs cycle generate ATP?
no, it makes precursors which are then used to make ATP
Mitochondria can divide in a process called…
fission
How do mitochondria get their proteins?
essential proteins that are coded by mtDNA are synthesized in the mitochondria
but MOST proteins are imported from the cytosol and are coded by nuclear DNA
What happens to old/worn out mitochondria?
autophagy (phagocytosis of large particles):
- endoplasmic reticulum wraps around it
- fusion with a lysosomal vesicle
- mitochondrion is digested
What happens to new mitochondria?
they can be transported around the cell by motor proteins along microtubules of the cytoskeleton
Mitochondrial DNA, as opposed to nuclear DNA, is…
circular
What is needed for proteins to be translocated into the mito?
translocation contact sites and specialized translocon structures are needed
TOM (translocon of the outer membrane)
TIM (translocon of the inner membrane)
How do TOM and TIM work together to translocate proteins?
- the cytosolic protein has a signal sequence on the end
- signal sequence is recognized by TOM
- TOM inserts the protein into the membrane, where it contacts TIM
- TIM translocates the protein into the matrix of the mito
- signal peptidase (inside the mito matrix) cleaves the signal sequence from the protein to create the mature mitochondrial protein
Why is mito DNA inherited from the mother?
sperm has very few mito compared to the oocyte, and most are in the tail which does not even become part of the embryo
also, sperm mito may be programmed to die after fertilization
What cytoskeletal element helps sperm swim?
microtubules (because the tail is a flagella)
Why is mitochondrial inheritance risky business? Why is everything still okay though?
Risky: while nuclear genes are recombined every generation, mito DNA is passed asexually and therefore is 50x as prone to mutations
However: mito defects are so lethal that natural selection tends to prevent dysfunctional mito from establishing in a population
What is heteroplasmy? Why is it able to happen in mito?
Heteroplasmy: a cell or tissue contains more than one type of mitochondrial genome
Each mitochondrion contains multiple copies of mtDNA, and a mutation does not necessarily affect all the copies, so some can be mutated while others are not. The dose of a mutation is important when considering the inheritance pattern of a mitochondrial disease.
Besides energy homeostasis, what other functions do the mito have?
- programmed cell death (apoptosis)
- heat production
- generation of reactive oxygen
- calcium store
- role in some diseases (impaired function from ntDNA mutations)
What is apoptosis?
programmed cell death, orderly cellular self-destruction
about 100 thousand cells are produced every second, so 100 thousand also die every second by apoptosis
Evolutionarily, why do cells undergo apoptosis?
bacteria evolved a mechanism to ensure that they died quickly when infected with a virus
this evolved into the mitochondrial pore which is part of the apoptosis machinery
Describe the basic mechanism of mitochondrial apoptosis
- the permeability transition pore opens and cytochrome c exits
- cyt c activates Apaf-1
- Apaf-1 activates caspases (family of proteases that begin the process of degrading cellular components)
Where is cytochrome c located in the mito?
on the intermembrane space side of the inner membrane
cytochrome c is crucial for apoptosis, but also…
the electron transport chain
What are the stages of classic apoptosis?
- healthy cell receives a death signal (extrinsic or intrinsic)
- cell undergoes commitment to die (reversible)
- cell is executed (irreversible) and condenses/crosslinks to become a dead cell
- engulfment by macrophages and neighboring cells where it is degraded
What role do mito play in glucose sensing/insulin release?
- in pancreatic b cells, mito link glucose exposure to insulin release
1. GLUT2 transporters introduce glucose to the cell, and mito increases the ATP:ADP ratio
2. K+ gates close which depolarizes the cell
3. voltage sensitive Ca++ channels open
4. exocytosis of ins. storage granules to the bloodstream
How are the B-cells impaired (causing loss of glucose-stimulated insulin release)?
- superoxide radicals induce UCP-2 (uncoupling protein 2) upregulation
- uncoupling causes proton leak, decreased ATP:ADP ratio, and loss of glucose stim. insulin release
What are UCPs (uncoupling proteins)?
- transmembrane proteins in mito that decrease the proton gradient generated in oxidative phosphorylation
- they create a leak pathway that allows protons to cross the mito inner membrane, bypassing ATP synthase
- basically, H+ gradient is uncoupled from ATP generation
What is UCP1 vs. UCP2?
UCP1:
- brown adipose tissue
- babies and hibernating mammals
- diverts energy from ATP generation to thermogenesis (heat release) in response to cold
- decrease mito production of damaging reactive oxygen species
UCP2:
- white adipose tissue
- humans during feeding
- metabolic adaptation to fasting regardless of weight
- non-thermogenic uncoupling
Mitochondrial mutation can cause disease in what systems?
nervous, cardiovascular, liver, kidneys, eyes, skeletal muscle, digestive, and pancreas
How can a three person baby be made (in order to eliminate mito mutations)?
- mother egg is enucleated
- donor egg is enucleated
- the nucleus of the mother’s egg is put into the enucleated donor egg with normal mito
- sperm fertilizes that “new” egg
When a three person baby is made, where does each part come from?
nucleus comes from mother, mito/rest of the egg comes from donor, sperm from father
