Mitochondrial respiratory chain Flashcards
Structure of mitochondria
Outer membrane (freely permeable to small molecules and ions)
Inner membrane (impermeable to small molecules and ions including H+)
Matrix
Cristae
Structure of electron transport chain
Complex I (NADH-CoQ oxidoreductase)
Complex II (succinate dehydrogenase)
Complex III (cytochrome bc1 complex)
Complex IV (cytochrome x oxidase)
Complex I
FMN becomes FMNH2 becomes QH2
NADH + H+ becomes NAD+
4H+ move in
Enzyme is NADH dehydrogenase
NADH dehydrogenase
Initially electrons passed to FMN to produce FMNH2
Transfer to a series of iron sulphur clusters
Transfer to coenzyme Q or ubiquinone
Overall reaction is:
NADH + H+ + Q = NAD+ + QH2
It is a proton pump
Complex II
Succinate dehydrogenase
Electrons of FADH2 pass on their electrons to complex II
Complex II passes them to ubiquinone
Other substrates for mitochondrial dehydrogenases also pass on their electrons but not through complex II
Path of electrons entering the electron transfer chain
Electrons from NADH pass through a flavoprotein to a series of iron sulphur proteins in complex I then to Q
Electrons from succinate then pass through a flavoprotein and several Fe-S centres in complex II on the way to Q
G3P donates electrons to a flavoprotein on the outer face of the inner mitochondrial membrane where they pass to Q
Acyl-CoA dehydrogenase transfers electrons to electron transferring flavoprotein where they pass to Q via ETF
Complex III
Ubiquinone: cytochrome c oxidoreductase
Second of three proton pumps in the respiratory chain
Complex IV
Cytochrome oxidase
Third and final proton pump
Carries electrons from cytochrome c to molecular oxygen
Produces water
Synthesis of ATP
Purpose of the whole process
Inner mitochondrial leaflet is generally impermeable to charged species but 3 specific systems in this membrane that:
- Transport ADP and Pi into the matrix
- Synthesise ATP
- Transport ATP into the cytosol
Synthesis of ATP equation
ADP3- + Pi2- + H+ = ATP4- + H2O
Actual substrates are the Mg2+ complexes of ADP and ATP
Adenonsine nucleotide translocase
Integral protein of the inner mitochondrial membrane
Transport ADP3- from the intramitochondrial membrane space into the matrix
In exchange for an ATP4- molecule transported in the other direction
Known as an antiporter
Phosphate translocase
A second membrane transport is essential for oxidative phosphorylation and synthesis of ATP
Transport both phosphate and hydrogen ions into the matrix: a symporter
Favoured by the transmembrane proton gradient
ATP synthase
An F-type ATPase
Two functional domains:
- F0, an oligomycin- sensitive proton channel
- F1, an ATP synthase
F0
Comprises three different types of subunit: a, b, c
Forms a complex of 13-15 subunits
Subunits c1-10 arranged in a circle
F1
Comprises five different types of subunit: alpha3, beta3, gamma, delta, epsilon
Forms a complex of 9 subunits
The 3 beta subunits have catalytic sites for ATP synthesis
Beta subunits
Are arranged alternately with alpha subunits like segments of an orange
Form a knob like structure held by the gamma and epsilon subunuts
Delta subunit interacts with the two b subunits of F0
Theory of rotational catalysis
3 subunits take turns catalysing the synthesis of ATP
Any given beta subunit starts in conformation for binding ADP and Pi
Then changes conformation so the active site now binds the product ATP tightly
Then changes conformation to give the active site a very low affinity for ATP so ATP is released
Summary of energy changes
Highly exergonic reaction
Energy released is coupled to the movement of H+ across the inner membrane
Electrochemical energy generated represents temporary conservation of the energy of electron transfer
Protons flow spontaneously down their electrochemical gradient releasing energy available to do work
Energy yield from oxidation of 1 molecule of glucose
Glycolysis
- 2 NADH (3 or 5 ATP depending on shuttle used)
- 2 ATP
Pyruvate oxidation
- 2 NADH (5 ATP)
Acetyl-CoA oxidation in CAC
- 6 NADH (15 ATP)
- 2 FADH2 (3 ATP)
- 2 ATP or 2 GTP
Total yield of 30 or 32 ATP
Uncoupling reagents
Normally e- flow and phosphorylation are tightly coupled
Uncouplers dissipate the pH gradient by transporting H+ back into the matrix of mitochondria bypassing the ATP synthase
Uncoupler severs the link between e- flow and ATP synthesis with energy being released as heat
Can occur naturally
Uncoupling and brown fat thermogenesis
Brown adipose tissue:
- specialised for heat generation
- high numbers of mitochondria
- mitochondria contain thermogenin
- important in new borns, possible role in obesity/ diabetes
DNP- an exogenous uncoupler
Weak acid that crosses membrane ‘ferrying’ H+ across
Each DNP molecule collects a proton from the IMS and moves through the membrane with it, depositing it in the matrix
Can then return through the membrane to collect another proton
DNP effects
Leeds medical sudent took 2,4- DNP as weight loss aid
62 deaths due to DNP reported including body builders and slimmers
Toxicity arises from liver damage, respiratory acidosis and hyperthermia
