Mitochondrial Respiratory Chain Flashcards
Electron Transport chain (respiratory chain)
Comprises four large multi-unit proteins intrinsic to the inner mitochondrial membrane
Catalyse a series of reactions:
NADH + H+ + 1/2O2 = NAD+ + H2O
Energy released from this reaction not released as heat but tightly coupled to the production of ATP
Components of the Electron Transport Chain
Four components: Complex I, II, III and IV
These are linked by 2 soluble proteins
1. Ubiquinone (coenzyme Q) – a lipid soluble benzoquinone with a long isoprenoid tail
2. Cytochrome c
These are free to move in the membrane by diffusion (they are not part of the complexes
Complex I
Initially electrons are passed to FMN to produce FMNH2
Subsequently transfer to a series of iron-sulphur clusters
Then transfer to Coenzyme Q (ubiquinone)
So, the enzyme catalyses the overall reaction:
NADH + H+ + Q = NAD+ + QH2
It is a proton pump, moving protons from the matrix into the intramitochondrial space
Complex II
FAD within complex II is reduced to FADH2 by electrons gained from the conversion of succinate to fumarate in the TCA cycle
Complex II passes electrons to ubiquinone
Other substrates for mitochondrial dehydrogenases also pass on their electrons to ubiquinone but not through complex II – e.g. from the G-3-P shuttle
Complex III and IV
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
ADP3- + Pi2- + H+ ATP4- + H2O (reversible)
Inner mitochondrial leaflet is generally impermeable to charged species, but 3 specific systems in this membrane that
o Transport ADP and Pi into the matrix
o Synthesise ATP
o Transport ATP into the cytosol
Adenine nucleotide translocase
Integral protein of the inner mitochondrial membrane
Transports ADP3- from the intramitochondrial membrane space into the matrix
In exchange for an ATP4- molecule transported in the other direction (Favoured by the electrochemical gradient generated by proton pump)
Known as antiporter
Atractyloside, a glycoside isolated from a thistle, is a specific inhibitor of adenine nucleotide translocase
Phosphotase Translocase
A second membrane transporter is essential for oxidative phosphorylation and synthesis of ATP
Transports both phosphate and hydrogen ions into the matrix: a symporter (favoured by the transmembrane proton gradient)
ATP synthase
An F-type ATPase with two functional domains
1. Fo, an oligomycin-sensitive proton channel
2. F1, an ATP synthase
Fo comprises three different types of subunit: a, b, and 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, and epsilon
Forms a complex of 9 subunits
The 3 beta subunits have catalytic sites for ATP synthesis
Theory of Rotational Crisis
3 β subunits take it in turns catalysing the synthesis of ATP.
Any given beta subunit starts in a 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 (‘beta-empty’ conformation) so ATP is released
Rotational Catalysis: the gamma-unit rotates, and the properties of the beta-catalytic units change
Summary of Energy Changes
Highly exergonic reaction
NADH + H+ +1/2 O2 -> NAD + H20
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
Uncoupling Agents
Normally e- flow and phosphorylation of ADP are tightly coupled
Uncouplers dissipate the pH gradient by transporting H+ back into the matrix so bypassing the ATP synthase
Thus an uncoupler (e.g. DNP) severs the link between e- flow and ATP synthesis, with the energy being released as heat
Can occur naturally, e.g. UCP1 (thermogenin) is found in brown adipose tissue and has a specific H+ channel through which the [H+] may be dissipated - energy released as heat
Brown adipose tissue: high numbers of mitochondria which contain thermogenin; specialised for heat generation; important in new borns, possible role in obesity/diabetes
DNP - an exogenous coupler
Weak acid that crosses membrane, ferrying H+ across
Each DNP colecule 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
Effects: many deaths of body builders/slimmers, thinking of it as a weight loss strategy; toxicity arises from liver damage, respiratory acidosis and hyperthermia
