Complex I and Rieske/Cytochrome b Complexes Flashcards
What is the ultrastructure of mitochondria?
What are cristae and what advantage do they provide?
Double membrane
- Outer mitochondrial membrane; Separates mitochondria from cytoplasm
- Inner mitochondrial membrane; Separates inter membrane space from mitochondrial matrix
Cristae are infoldings of membrane that extend into matrix that increase membrane area; These contain electron transport chain complexes – More membrane area means more complexes bound
Mitochondrial complex I is a NADH:ubiquinone Oxidoreductase
What does it do?
Structure?
-Domains?
- Subunits?
Oxidises NADH and reduces ubiquinone
2 major domains
14 core subunits for catalysis and electron transfer
31 supernumerary subunits important for structure, stability and assembly
What are the 2 domains of mitochondrial complex I and their function? (Subunit number) (hint - peripheral, long membrane)
Peripheral hydrophilic domain extends into matrix – Contains the NADH binding site and all the redox cofactors (7 core subunits)
Long membrane domain (64 TMHs) anchors complex in membrane and contains 4 proton translocating pumps (6 core subunits); Generates pmf
What are the electron transfer cofactors in mitochondrial complex I? (hint - FMN and FeS)
Where is the ubiquinone reduction site?
1 FMN group and a wire of 7 FeS centres
Ubiquinone reduction site is at interface between peripheral arm and membrane domain
What is the process of mitochondrial complex I electron transfer?
NADH is oxidised by NAD(P)H dehydrogenase; Provides electrons
FMN accepts 2 electrons and transfers them through FeS clusters to reduce ubiquinone
- Releases free energy
4 protons pumped across inner mitochondrial membrane from matrix to intermembrane space (IMS) using this free energy
What is the structure and mechanism of photosynthetic complex I?
How is it similar and how does it differ to mitochondrial complex I
NAD(P)H dehydrogenase domain at the top of the peripheral arm is missing; Ferredoxin is the electron donor
2Fd needed as it’s a 1 electron carrier
Wire of three 4Fe-4S clusters transfer electrons from Fd to reduce PQ
- Free energy from PQ –> PQH2 is used to pump 4 protons into lumen
What is photosynthetic complex I important for? (hint - pool, ratio)
Important for cyclic photosynthetic electron transport with electrons returned from FNR/Fd back to quinone pool
Quinones are re-reduced and there is an increase in ATP:NADPH ratio
Structural similarities between mitochondrial complex I and photosynthetic complex I? (hint - domains, core subunits)
Similar structures of large hydrophilic arm extended into stroma/matrix, as well as long membrane domain as an anchor for the complex
Share 11 core subunits; Highly conserved
- PS-C1 has 7 oxygenic photosynthesis specific subunits; Regulate Fd interactions and electron transfer
- M-C1 has NADH dehydrogenase specific subunits; Coordinate FMN and FeS clusters
Structural differences between mitochondrial complex I and photosynthetic complex I?
Peripheral arm smaller in PS-C1; Missing NAD(P)H dehydrogenase domain and 4 FeS clusters
- Binds Fd instead
- Remaining 3 FeS clusters are in similar positions between the 2
What is the role of all cytochrome b type complexes in electron transport chains?
Re-oxidise quinols back to quinones, move protons across membrane to generate pmf and reduce a soluble electron carrier on P side of membrane
Structure of mitochondrial complex III?
Key conserved subunits?
Dimeric with each monomer having 11 subunits
Cytochrome b subunit; 8 TMHs
Iron-Sulphur protein (ISP)/Rieske subunit; 1 TMH
Cytochrome c1; 1 TMH
What is the ‘respirasome super-complex’?
Benefits? (hint - travel, packing)
Mitochondrial electron transport complexes I, III, IV form this
Quinols produced by complex I don’t have to travel far to complex III
Electrons from re-oxidising quinols also don’t have to go far
Cyt c also doesn’t need to travel far to complex IV
This arrangement helps with the packing of the supercomplex in the membrane and minimises distance electron carriers must travel between complexes
Out of cytochrome bc1 and b6f, which is involved in respiration and bacterial photosynthesis, and which is involved in plant and cyanobacterial oxygenic photosynthesis
Bc1 is in respiration, working with ubiquinone/ol
B6f is in plant photosynthesis, working with plastoquinone/ol and plastocyanin
What is the structure of cytochrome bc1?
Monomer structure?
Symmetrical dimer
Each monomer contains 3 core subunits and performs its electron transfers reactions independently of other – Needs dimer to function however
What is the structure of cytochrome b, ISP and cytochrome c1 in cytochrome bc1? (hint - quinone binding sites, prosthetic groups)
Cytochrome b
- 8 TMHs, ~45 kDa
- Binds 2 b-type hemes
- Contains 2 quinone binding sites
Rieske iron sulphur protein (ISP)
- 1 THM, largely periplasmic, ~20 kDa
- Binds 1 high potential 2Fe-2S cluster
Cytochrome c1
- 1 THM, largely periplasmic, ~30 kDa; Flanks the core
- Binds 1 c-type heme
What is the structure of cytochrome b6f in Spinacia oleracea? (hint - dimer)
Dimeric with 4 core subunits
Structure of cytochrome b6f monomer units of cytochrome b6, subunit 4, ISP, cytochrome f and Pet G L M and N?
Cytochrome b6
- 4 TMHs, 24 kDa
- Binds 2 b-type hemes and 1 c-type heme
- Contains 2 quinone binding sites
Subunit 4
- 3 TMHs, 18 kDa
Rieske iron sulphur protein (ISP)
- 1 THM, largely luminal, ~20 kDa
- Binds 1 2Fe-2S cluster
Cytochrome f
- 1 THM, largely luminal, ~32 kDa
- Binds 1 c-type heme
Pet G, L, M and N
- Small (3-4 kDa) single TMH subunits
- Stabilise the complex and mediate interactions between b6f and other complexes
What units are homologous between cytochrome b6f and cytochrome bc1?
Together, cytochrome b6 and subunit IV in b6f are homologous to cytochrome b in bc1
Rieske ISPs are largely homologous but differ in the periplasmic subdomain
Despite serving similar functional roles cytochrome c1 and cytochrome f are not homologous structurally
How many b type hemes are there in the cytochrome b complexes?
Which has an extra one and what is its name?
2 b type hemes
B6f has extra heme; heme Cn
What and where are the 2 quinone binding sites in b subunits?
Qi (inside)
Qo (oxidising)
How is the Rieske ISP 2Fe-2S cluster different to normal?
What does this do?
Coordinated by 2 histidine and 2 cysteine residues rather than 4 cysteine residues
Creates more more positive redox potential cluster so it can accept electrons from quinones
Why does chlorophyll tail near Qo site in cytochrome b6f move?
Moves to gate the Qo binding site and let quinones and quinols in and out
Draw the positions of cofactors in cytochrome bc1
What is bifurcation in the Q cycle?
When and where does it occur?
1 electron takes one of two routes
This occurs when quinol is oxidised at Qo site
What is the first step of the Q cycle? (hint - Q pool)
What is then transferred and what does this result in?
Ubiquinol arrives at Qo site from Q pool
Electron is transferred to the Rieske subunit Fe-S centre, releasing 1 proton to the p-side and leaving a semiquinone (SQ) at the Qo site
What happens in Q cycle after SQ is formed at Qo? (hint - 2nd proton, 2nd electron transfer)
What is transferred to cyt c2?
2nd electron is transferred from the SQ to cyt bL, then onto bH, and the 2nd proton is released to the p-side, forming quinone
Electron on the Rieske protein is transferred to cyt c1 and then to cyt c2
What happens to electron once it reaches bH on n-side? (hint - semi-quinone)
Electron at cyt bH is donated to a quinone at the Qi site generating a SQ
What happens to oxidised quinone at Qo and what then arrives? (hint - repeat)
Oxidised quinone leaves the Qo site and a new quinol arrives at Qo
This is oxidised as before and process repeats
How is Ubiquinol generated at Qi site?
2 protons taken up from n-side to generate UQH2
In cytochrome bc1, when is Rieske subunit in position ‘b’?
What does this allow? (hint - transfer)
Why is this now a problem? (hint - c1)
H-bond between Rieske subunit His residue and U10 head group at distance of 8 Å
Distance between U10 and FeS close enough for electron transfer from U10 to FeS
Distance between Fes and c1 is now too far for electron transfer
In cytochrome bc1, when is Rieske subunit in position ‘c’?
No quinone bound and the FeS is in a different position where it is closer to c1
How does Rieske/FeS cluster move?
What does the movement help with?
Moves by dynamic hinge-like movement of Rieske subunit to bring it closer to c1
Helps with electron bifurcation as FeS becomes too far away for electron transfer
