Metabolism 5: Mitochondria and Oxidative Phosphorylation Flashcards
Explain the evidence supporting Endosymbiosis Theory
Mitochondria can only arise from pre-existing mitochondria and chloroplasts.
Mitochondria possess their own genome which resembles that of prokaryotes - circular molecule of DNA with no associated histones hence no chromosomes.
Mitochondria have their own protein-synthesising machinery which resembles that of prokaryotes not that of eukaryotes.
The first amino acid of their transcripts is always fMet (formylmethionine) as it is in bacteria and not methionine (Met) which is the first amino acid in eukaryotic proteins.
A number of antibiotics (e.g. streptomycin) that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.
What is the Chemiosmotic Theory
Movement of protons from within the matrix of the mitochondria into the space in between mitochondrial membranes - controlled by the electron transport or respiratory chain.
Pumped protons are allowed back into the mitochondria through a specific channel which is coupled with an enzyme which can synthesise ATP called ATP synthase.
What does the electron transport chain consist of?
Consists of three complexes and two mobile carriers which act as electron carriers:
Membrane Complexes:
NADH Dehydrogenase Complex
Cytochrome b-c1 Complex
Cytochrome Oxidase Complex
Mobile Carriers
Co-enzyme Q (ubiquinone)
Cytochrome C
What does ubiquinone do?
transfers electron from NADH Dehydrogenase Complex to Cytochrome b-c1.
It can pick up one or two electrons (together with an H+ from solution).
What is Cytochrome Oxidase
Cytochrome Oxidase is the last membrane complex in the electron transport chain. It initially receives 2 electrons from cytochrome C in the first cycle of the electron transport chain, then the cycle repeats so that cytochrome oxidase has 4 electrons in total.
Cytochrome oxidase passes the electrons to Oxygen to generate water.
Furthermore, 4 protons are pumped into the intermembrane space, thus enhancing the proton gradient.
What is oxygens role in the ETC
Oxygen is the ideal terminal electron acceptor
has a high affinity for electrons
driving force for oxidative phosphorylation.
What is the ability of a redox couple to accept or donate electrons is known as
the reduction potential or redox potential.
Standard Redox Potentials
NEGATIVE redox potential = tendency to DONATE
POSITIVE redox potential = tendency to ACCEPT
Why do electrons move down the ETC
Each successive membrane complex or carrier has a more positive redox potential than the previous component of the ETC.
The transfer of electrons from one complex to the next is energetically favourable.
As they progress along the chain, the electrons lose energy.
Describe ATP synthase
A multimeric enzyme consisting of a membrane bound part (F0) and a part which projects into the matrix space (F1).
F1 and F0 consist of three different subunits:
F0 = a, b and c
F1 = alpha, beta and gamma
Which parts of ATP synthase can move?
When H+ flows through the membrane via the pore, the disc of c subunits rotate.
The gamma-subunit in the F1 unit is fixed to the disc and so rotates with it.
HOWEVER, alpha and beta-subunits in the F1 unit CANNOT ROTATE because they are locked in position by the b subunit which is anchored to subunit a in the membrane.
To summarise:
C and gamma subunits can rotate
alpha and beta-subunits CANNOT rotate because they are fixed in position by a and b subunits
How is ATP formed in ATP Synthase
As the gamma subunit functions as an asymmetrical axle, the beta-subunits are compelled to undergo structural changes. This rotation drives transitions of the catalytic portions of the beta-subunits, which alters their affinities for ATP and ADP.
So, TORSIONAL ENERGY flows from the catalytic subunit into the bound ADP and Pi to promote the formation of ATP.
What is the entry point for electrons donated by FADH2.
Ubiquinone
What are the 5 metabolic poisons and how do they work
Cyanide - binds to Fe3+ in haem group in cytochrome oxidase
Carbon Monoxide - binds to Fe2+ in haem group and blocks flow of electrons
Malonate - competitive inhibitor to succinate dehydrogenase - slows down flow of electrons from succinate to ubiquinone
Oligomycin - binds to stalk of ATP synthase - blocks flow of protons through the enzyme
Dinitrophenol - uncouples oxidative phosphorylation from ATP production - provides alternate pathway for protons to travel through the membrane
How is mitochondrial activity measured
Using an oxygen electrode,
0.6volts is applied
