Oxidative Phosphorylation Flashcards
(19 cards)
What is oxidative phosphorylation?
Electrons transferred from NADH and FADH2 to oxygen and ATP is formed
- The ETC involves oxidation energy to transport protons and to establish a proton gradient
- ATP synthase uses the free energy of the proton gradient to produce ATP
5 oligomeric complexes in the ETC
Complexes I-IV
- involved in electron transport
- I = NADH-ubiquinone oxidoreductase
- II = Succinate-ubiquinone oxidoreductase
- III = Ubiquinol-cytochrome C reductase
- IV = cytochrome C oxidase
Complex V
- ATP synthesis
- V = ATP synthase
What is ubiquinone?
- An electron acceptor
- Abbreviated to Q or coenzyme Q
- exists in 3 forms
- Ubiquinone is the oxidised form and can easily cross the membrane due to its polyisoprene chain
- This is reduced 1 electron at a time
- Semiubiquinone is the free radical version
- Dihydroubiquinone is the fully reduced form and is oxidised 1 electron at a time
What is the first step?
Energy transfer from NADH to complex I and the electrics are transferred to ubiquinone
- NADH gives 2 electrons at a time to NADH-Q reductase
- 4H+ is also given from free energy
- Ubiquinone (Q) accepts these electrons
Describe complex I
- NADH-Ubiquinone oxidoreductase
- Also known as NADH-dehydrogenase
- Largest of the complexes
- Site of NADH oxidation to NAD+
What is the second step?
Electrons transfer from complex II
- Succinate-Q reductase passes electrons to FADH2
- FADH2 passes the electrons to ubiquinone
- This then gets reduced
Describe complex II
Succinate-Q oxidoreductase
- only enzyme common to the citric acid cycle and respiratory chain
- does not contribute to proton gradient
- protons for QH2 come from FADH2 being oxidised to FAD
Describe complexes I and II together
- electrons from NADH enter the respiratory chain two at a time via complex I
- complex I transfers 2 electrons one at a time to ubiquinone which is reduced to ubiquinol
- complex I translocates 4H+ per 2 electrons transferred and captured 2H+ from the matrix to form QH2
- complex II doesn’t translocate protons but supplies electrons from succinate via FADH2 to ubiquinone, reducing it to QH2
What is the third step?
- electrons transfer to and from Q
- all elections originally from glucose are now in QH2
- QH2 diffuses freely into the membrane to reach complex III
- electrons transfer to complex III then to cytochrome C
Describe complex III
Cytochrome C reductase
- transfers electrons from QH2 to cytochrome C
- 4x H+ are translocated, 2 from the matrix and 2 from QH2
- electrons are transferred from QH2 to two molecules of cytochrome C
What is the fourth step?
Electrons transfer to cytochrome C to complex IV
Electrons transfer to oxygen
Water is the final product
Describe complex IV
Cytochrome C oxidase
- receives electrons from cytochrome C carrier one at a time
- iron atoms (in haem groups) and copper atoms are both reduced and oxidised as electrons flow to oxygen
- catalyses the reduction of oxygen to water
- two more H+ are translocated
Electron transport via complex III
- QH2 transfers 2 electrics to complex III one electron at a time
- the complex transfers these electrons to cytochrome C via the Q cycle
- free energy is used to translocate 4x H+
Electron transport via complex IV
- each molecule of cytochrome C passes one electron to complex IV
- the complex reduces oxygen to water
- free energy is used to translocate 2x H+ from the matrix to the intermembrabe space
- the complex captures 4x H+ from the matrix to form 2x water
Describe complex V
ATP synthase
- uses the proton gradient for the synthesis of ATP
- protons flow back into the matrix via ATP synthase
- 3H+ needed for each ATP molecule
- Composed of a knob and stalk structure
Describe the knob and stalk structure of ATP synthase
F1 (knob) = In the matrix. Has catalytic subunits where ATP synthesis occurs
F0 (stalk) = imbedded into the inner membrane. Has the proton channel
Structure of ATP synthase
- F1 has 3 alpha, 3 beta and 1 omega and 1 epsilon subunits
- alpha and beta alternate and ß is where the synthesis actually occurs
- gamma is the main component of the central axle
- omega is in the peripheral stalk and keeps the ring structure steady
- each beta subunit has an active site for ATP synthesis
Rotational catalysis mechanism for ATP synthase
Each ß subunit can exist in 3 conformations
- Open state (O) = available to bind ADP + Pi
- Loose state (L) = active site closes loosely on ADP + Pi once bound
- Tight state (T) = converts ADP + Pi to ATP
Flow of protons drives F0 rotation and forces cyclic conformational changes into each ß subunit
Number of protons per ATP
Each rotation of ATP synthase produces 3 ATPs
Number of H+ = number of subunits divided by 3
