Week 8 Flashcards
Where is the majority of the energy in eukaryotes generated?
By mitochondria
Oxidation and reduction in terms of electron transfer
Oxidation refers to loss of electrons. Reduction refers to gain of electrons.
Main supply of electrons to reduce oxygen are supplied predominantly by what?
NADH
Oxidative phosphorylation
The process of reducing oxygen in mitochondria is called oxidative phosphorylation because oxidation of electrons is coupled to the phosphorylation of ADP to generate ATP.
Two major sources of acetyl CoA
- Fatty acid beta-oxidation
- Glycolysis
Fatty acid beta-oxidation
Breakdown of even-chained fatty acids. Generates acetyl CoA
Glycolysis
Breakdown of glucose (6 carbons) into pyruvate (3 carbons). Pyruvate dehydrogenase oxidizes pyruvate to generate acetyl CoA
What generates NADH?
NADH is generated from the TCA (citric acid) cycle.
Via what process do mitochondria carry out oxidative phosphorylation?
The electron transport chain.
Four major complexes involved in the electron transport chain (in order)
- NADH dehydrogenase
- Succinate dehydrogenase
- Cytochrome C reductase
- Cytochrome C oxidase
Role of Complex I
Complex I (NADH dehydrogenase) facilitates transfer of electrons from NADH to quinone (Q)
Role of Complex III
Complex III (cytochrome c reductase) allows for transfer of electrons from quinol to cytochrome C
What is the problem solved by the Q cycle?
Quinol carries 2 electrons but cytochrome c can only accept 1.
Role of Complex IV
Complex IV (cytochrome c oxidase) allows for transfer of electrons from cytochrome C to oxygen.
Which complex accounts for 90% of oxygen use in cells?
Complex IV. This makes aerobic life possible.
How do poisons such as cyanide and azide work?
Poisons cyanide and azide bind to heme irons in complex IV more strongly than oxygen
In which complex is oxygen reduced?
Complex 4 is where oxygen is reduced.
Need four electrons to give rise to 2 molecules of water.
What is the energy from the oxidation of NADH and FADH2 used for?
To pump protons into the intermembrane.
What generates ATP in the electron transport chain?
Electrochemical proton gradient is used by the ATP synthase (sometimes also referred to as Complex V) to generate ATP
How does MPP+ work?
- Chemical called MPP+ derived from a contaminant in synthetically made heroin is taken into dopamine neurons by the dopamine transporter, where it acts as a competitive inhibitor of Complex I in mitochondria
- Besides compromising ATP production, MPP+ also increase generation of free radicals (i.e., reactive oxygen species)
What role did MPP+ play in Parkinson’s Disease research?
Provided paradigm-shifting insight into Parkinson’s Disease by pinpointing mitochondrial dysfunction as the likely culprit
What’s so special about mitochondria using an electrochemical gradient to generate ATP?
Frees eukaryotic cells from the surface area: volume ratio constraint, allowing them to produce greater amount of ATP
Multiple lines of evidence support endosymbiotic origins of mitochondria
- Own genome
- Sensitivity of mitochondrial ribosomes to antibiotics
- Protein synthesis starts with N-formylmethionine like in bacteria
What did the Fzo experiment reveal?
Fuzzy onion (Fzo) mutant yeast cells have fragmented mitochondria compared to the wild type, suggesting impaired fusion. Concluded that the mutant was unable to undergo fusion.
Model of fusion via fuzzy onion (Fzo)
To fuse the two mitochondria, the fuzzy onion molecules interact.
The GTP is hydrolyzed to GDP, which results in a conformational change such that the fuzzy onion molecules snap back on themselves and curl up (still hanging onto each other).
This brings them so close that the membranes will fuse.
Role of OPA1 (Optic Atrophy 1)
OPA1 (Optic Atrophy 1), also a GTPase, mediates inner membrane fusion. OPA1 is mutated in individuals who suffer from optic nerve atrophy
Drp1
A type of dynamin and GTPase involved in the constriction model of mitochondrial fission. It pinches off membranes, resulting in two distinct organelles.
Hypothesis on why fusion and fission exists in mitochondria
Working hypothesis: Fusion and fission help maintain balance between having highly efficient mitochondria on the one hand (hyperfused), and eliminating defective mitochondria on the other (fragmented)
Impact on fusion and fission when cells are starved
Temporarily starve the cells—creates demand for mitochondria to produce ATP more efficiently. Causes fusion.
Impact on fusion and fission when demands for energy are low .
When demands for energy are low, the mitochondria fragment, allowing the cell to assess each organelle to determine if it’s functional.
Can then target for degradation if necessary. Causes fission.
NADH in Complex I vs in Complex II
NADH is a more critical electron carrier for complex I than complex II
How does mtDNA cut down unnecessary genes?
The mt genome sheds any genes that are not critical to the nuclear genome.
The mt genome conserves only the most critical genes
Which complex does not have any subunits coded for by mtDNA?
mtDNA does not encode any complex II subunits.
How is mtDNA inherited?
Maternally.l
Five crucial concepts to understand mtDNA disease
- Polyploidy
- Heteroplasmy
- Threshold effect
- Relaxed replication
- Meiotic and mitotic segregation
Polyploidy
Each cell has multiple copies of mtDNA. In contrast, the nuclear genome is diploid.
Heteroplasmy
- Cells with a mixture of WT and mutant mtDNA.
- Because it is polyploid, the mitochondria has a mixed population of alleles.
- Can have a mix of genotypes
- Many ways to be heteroplasmic-–exists on a continuum
Threshold effect
Mutant mtDNA causes pathogenicity when its levels are above a critical threshold.
Relaxed replication
Mitochondria (along with the mtDNA in it) undergo turnover independent of the cell cycle
Compare nuclear DNA and mtDNA replication.
- Nuclear DNA only replicates during the cell cycle and is tightly regulated.
- In contrast, mtDNA can replicate independent of the cell cycle.
Why is it important that the genomes inside mitochondria can undergo cycles of replication and degradation?
Means you can have a cell (like a neuron) in a hetereoplasmic state where mutants are low, but then the ratio of mut : WT can change.
Meiotic and mitotic segregation
Asymmetric inheritance of mutant mtDNA
