Mitochondria and Death Pathway Flashcards
Give an overview of the mitochondria?
Major intracellular organelle - involved in oxidative metabolism
Around 0.5 - 1µm in length and 100-1000 per cell
Distinctive structure in the electron microscope but are large enough to be seen under the electron microscope
They are very mobile - constantly changing their shape
They can be in a fixed position or moving a long microtubules to other regions within a cell
Describe the structure of the mitochondria?
Matrix - highly concentrated mixture of hundreds of enzymes - including ones required for pyruvate/fatty acid oxidation and the citric acid cycle
Also contains mitochondrial DNA genome
Inner membrane - folded into numerous cristae (increase surface area), this contains embedded proteins that carry out oxidation reactions of the electron transport chain and ATP synthesis
Impermeable membrane - cardiolipin (with 4 fatty acid chains) helps with this
Transport proteins - for selective permeability of small molecules needed for mitochondrial enzymes
Outer membrane - contains large channel-forming protein (porins), this membrane is permeable to all molecules 5 kDa or less
Contains enzymes involved in mitochondrial lipid synthesis/enzymes for the matrix
Intermembrane space - contains several enzymes that use ATP passing out of the matrix to phosphorylate other nucleotides
Give an overview of the mitochondria’s involvment in oxidative metabolism?
Pyruvate - generated from sugars in glycolysis
Fatty acid - mobilised from stores and diet
Both are taken up into the matrix of the mitochondria
They can there produced acetyl CoA, which is oxidised in the citric acid cycle, and releases CO2 as waste
The electrons produced are carried by NADH and FADH2 - pass along the electron transport chain within the inner membrane - generating a proton gradient
This is then involved in ATP synthesis - NADH oxidation is coupled to ATP synthesis
NAD+ is needed for continued oxidative metabolism
What are the components involved in the electron transport chain?
Made up of redox carriers associated with proteins and cofactors in the inner membrane
It is made up of 4 complexes but 6 elements:
Complex I NADH dehydrogenase - oxidises NADH
Complex II Succinate dehydrogenase - oxidises FADH (succinate -> fumarate)
Ubiquinone
Complex III Q cytochrome c oxidoreductase
Complex IV Cytochrome oxidase - reduction of oxygen
Cytochrome C
What are some types of redox carriers?
Haems - carry electrons, found in complexes I, II and IV
Iron sulphur centres - carry electrons, found in complexes I, II and III
Flavins - carry hydrogen, found in complexes I and II
Quinones - carry hydrogen and UQ links the complexes I&III and II&III
Why can the electron transport chain take place?
Electron transport chain components are organised in the inner membrane such that their individual redox potentials become successively more positive
This allows electrons to be transferred along the chain
Oxygen, with a large positive redox potential, is the final acceptor of the electrons
How does the electron transport chain generate a membrane potential?
There is a chemical (ΔpH) and electrical difference (Δψ) between the matrix and the intermembrane space - due to a proton being an ion
Matrix - fewer H+ ions and higher pH
Outside the mitochondrion - more H+ ions and lower pH
Therefore endergonic
However, protons moving down the gradient is exergonic - providing energy for ATP synthesis
What is the chemiosmotic theory?
The free energy of electron transport is conserved by pumping H+ from the mitochondrial matrix to the intermembrane space to create an electrochemical H+ gradient across the inner mitochondrial membrane
The electrochemical potential of this gradient is harnessed to synthesize ATP
How is ATP synthesised in the mitochondria?
As H+ has just been transported into the intermembrane space by the complexes of the electron transport chain, there is a higher concentration of H+ ions in the intermembrane space
- Protons travel down their concentration gradient back into the matrix through the ATPase
- For every three protons passing through the ATPase, one molecule of ATP is formed via ATP synthase
ADP and Pi are needed as substrates by the ATPase, therefore they enter the matrix via transporters
What is the structure of ATPase?
It is made up of two parts
The first sits in the membrane, called F0 - essentially a proton channel
F0 comprises a transmembrane ring of hydrophobic proteins that act as a H+ channel
As protons flow through the F0 channel it rotates
This in turn drives rotation of the gamma subunit which drives conformational changes in a and b subunits
The second sits in the matrix, called F1 - acts as an enzyme to catalyse the synthesis of ATP
F1 is composed of 3 alpha and 3 beta subunits
b is the catalytic subunit, a is regulatory
gamma connects F1 to F0
What is the mechanism of ATP synthesis?
Within the beta subunit there are different conformations that are more preferable for ATP synthesis
There is a T, O and L conformation (in a clockwise circle order) - tight, open, loose
1. ADP + Pi bind in the L (loose) binding site. there is a conformational change to T state
2. The T conformation has such a high affinity for ATP that bound ADP +Pi are converted to ATP (forming the phosphoanhydride bond)
3. This ATP is released after the conformational change driven by the rotation of the gamma subunit to the O conformation (driven 120° anticlockwise)
4. ADP and Pi bind to the vacant L site after further rotation converting O to L and L to T resulting in the synthesis of a second molecule of ATP
What can the mitochondria also do in varying conditions?
Where food is abundant:
Mitochondria generate NADPH and both carbon skeletons that are needed for cell growth
From excess citrate (from the citric acid cycle) it is transferred down its electrochemical gradient to the cytosol where it is metabolised for biosynthesis
Starvation:
Uses amino acids to fuel ATP production
Describe the mitochondrial genome?
Each mitochondrion contains 2-10 copies of a covalently-closed circular double-stranded DNA molecule (mtDNA)
mtDNA has limited coding capacity – needs proteins encoded by the nuclear genome
Maternally-inherited - from egg cell
Endosymbiotic theory - the bacterium was taken up and kept/utilised
This requires some of the organelle DNA to be taken up by nuclear DNA
Mitochondrial genomes vary in size
E.g. Homo sapiens - 17 kbp whereas melon - 2500 kbp
Number of genes it encodes also varies greatly
Describe mitochondrial protein synthesis?
There are different genes for parts of mitochondria - ATP subunits, cytochrome C oxidase subunits etc…
The mitochondrial polymerase is encoded for in nuclear DNA so is made elsewhere and brought to the mitochondria
The mechanism to take up mitochondrial proteins into the mitochondria is encoded in the nucleus and synthesised in the cytoplasm
Protein synthesis starts with N-formyl methionine
What are some significant features of mitochondrial DNA?
Dense gene packing - most nucleotides seem to be part of a coding sequence but still contains some introns removed in RNA splicing
Relaxed codon usage
Variant genetic code - from the universal code
