Oxidative Phosphorylation Flashcards
Electron Transfer
Driven by potential difference between electron donor and acceptor
Linked to ATP synthesis by Chemiosmosis
- Electrons from NADH and FADH move through electron transport chain, producing a proton gradient to drive ATP synthesis
Electron Transport Chain
1) NADH
2) NADH-Q Oxidoreductase (complex I)
3) Succinate-Q Reductase (complex II)
4) Q-cytochrome c oxidoreductase (complex III)
5) cyt c
6) Cytochrome c oxidase (complex IV)
7) Oxygen
Flavins
Reduced flavins act as hydrogen (proton + electron) carriers
Iron-Sulphur Clusters
Iron atom can undergo oxidation and reduction and therefore carry electrons
The Cysteine residues in the polypeptide bind the FeS cluster
Quinones
Similar to flavins, carry electrons and protons (a.k.a. hydrogen atoms)
Due to lipophilic nature, Q can act as a mobile carrier between protein complexes
COMPLEX 1: NADH-Q oxidoreductase (NADH dehydrogenase)
1 MDa in size, 14 central and 30 peripheral subunits (some mitochondrially encoded, others nuclear)
1) NADH donates 2 electrons onto Flavins
2) Flavin also accepts 2 protons (1 from matrix, 1 from NADH) to become reduced
3) The 2 electrons move (redox) across iron-sulfur clusters and finally into coenzyme Q (ubiquinone)
4) Ubiquinone takes up 2 protons (from matrix) to become reduced (QH2)
NADH + Q + 5H (matrix) —-> NAD + QH2 + 4H (cytosol)
Proton pumping occurs
COMPLEX 2: Succinate Dehydrogenase (reductase)
1) Succinate is oxidised into fumarate, producing FADH2
2) FADH2 reduces ubiquinone via an FeS centre to produce QH2
3) In reducing ubiquinone the FADH2 is recycled as FAD (Flavin Adenine Dinucleotide)
No proton pumping
COMPLEX 3: Q-Cytochrome c oxidoreductase
Dimer structure made of 11 polypeptide chains with an unusual His co-ordination to stabilise the reduced form (raising complex’s reduction potential)
1) Reduced Ubiquinone (QH2) binds the Q0 site, releasing 2 protons (enter the intermembrane space, proton pumping) and 2 electrons
2) First of 2 electrons binds 2Fe2S in Rieske centre before transferring to haem group of cytochrome c1. This passes to water soluble cytochrome c which transfers to COMPLEX IV
3) The second electron (from QH2) is passed via haem groups of cytochromes bh and bl to a ubiquinone at site Qi.
4) Qi site is reduced to a semi-quinone anion
5) pathway repeats as second QH2 binds the Q0 site.
4 protons pumped into intermembrane space, 2 electrons transported to complex IV via cytochrome c
COMPLEX 4: Cytochrome c Oxidase
Contains 2 haem groups (a & a3) and three copper atoms (CuA/CuA and CuB)
1) The 2 reduced cytochrome c molecules from complex 3 transfer an electron to CuB (via CuA, haem a and haem a3) and another electron to haem a3 (via CuA and haem a).
2) Oxygen can bind with CuB and Haem a3 in reduced state to from a peroxide bridge between the 2
3) 2 more cytochrome c molecules are oxidised, transferring 2 electrons to complex 4 (2 protons taken from matrix to help break peroxide bridge in order to form CuB-OH and haem a3-OH)
4) 2 more protons taken from matrix: oxidise haem a3 and CuB into their original state whilst releasing 2 water molecules
Transfer of 4 electrons and 4 protons
Comparing Complex 3 and Complex 4
Complex 3:
- donates 2 protons into matrix
- transfers 1 electron for every 2 protons
- oxidise substrate by reducing ubiquinone
- pass electrons onto iron
- cytochrome c = electron acceptor
Complex 4:
- takes up 4 protons from matrix
- transfers 1 electron per proton
- oxidise substrate by reducing cytochrome c
- pass electrons onto an iron complexed with haem a3
- oxygen = electron receptor
ATP Synthase (F1F0) - basic subunit function
The Binding Change Mechanism
Enzymatic F1 part in matrix bound to inner membrane by the F0 part which facilitates the protons crossing the membrane
ATP is synthesised by the Binding Change mechanism
- central gamma subunit transmits energy from the reaction of the F0 domain
- the beta subunits undergo conformational changes to accept ADP+Pi and release ATP
ATPase: F1 part
Composed of 3 alpha and 3 beta subunits (hexamer)
- Beta subunits = catalytic
- alpha subunits = regulatory
Gamma subunits connect F1 to F0
ATPase: F0 part
Comprises a ring of hydrophobic proteins acting as a H+ channel
- as protons flow through the F0 channel rotates, driving the rotation of the gamma subunit to drive conformational changes in the alpha and beta subunits
In vertebrates, the ATPase 8C ring subunits each carry a H+ during 1 complete rotation
- 3x ATP produced per rotation
P:O ratios
Describes how many ATP can be made per oxygen atom reduced to water (established using the principal that 1 oxygen atom needs 2 electrons trnaslocated)
Vertebrates:
- making 3x ATP requires 8H (FO subunits): 8/3 = 2.7 + 1 (substrate transport) = 3.7 H/ATP
- 2e- from NADH to oxygen yields 10H+ (FADH yields 6H)
- NADH P:O ratio = 10/3.7 = 2.7
- FADH P:O ratio = 6/3.7 = 1.6
P:O ratio is lower in invertebrates as they have a larger number of c subunits in F0 part
