Oxidative Phosphorylation Flashcards
What is oxidative phosphorylation
Mechanism of generating large amounts of ATP in a cell by chemiosmosis
Electron transfer and ATP synthesis is coupled to a proton gradient set up across IMM
Proton motive force
PMF = chemical gradient (change in pH) + charge gradient (membrane potential)
PMF drives ATP synthesis
What is the ETC
Present in IMM
Proteins free to move, dont have to be next to each other due to mobile electron carriers
E- from NADH and FADH2 passed down a series of protein complexes
Complex 1 overall reaction
NADH + Q + 5H+ —> NAD+ + QH2 + 4H+
Description of complex 1
NADH-Q Oxioreductase
FMN and FeS clusters
FMN covalently bound to protein
Accepts 2e- from NADH
Reduced to FMNH2
E- from FMNH2 passed one at a time through FeS clusters to reduce Q
Need many clusters as rate of e- movement through space very slow is carriers are more than 15A apart
When Q is reduced to QH2 it causes conformational change that opens water filled channels that allow 4H+ to pass from matrix to IMS
Ubiquinone (coenzyme Q)
Q (oxidised) —> semiquinone radical —> ubiquinol (QH2)
2 ketones reduced to 2 hydroxyl groups
Quinone ring can carry 2e-
Soluble in membrane as very hydrophobic so can move very fast
Acceptance and loss of e- is coupled to protein binding and release
Moves from complex 1 and 2 to 3
Cytochrome c
Small protein with haem group
Carries 1e-
No protons associated with its reduction
Complex 3 overall
Reduces cyt c and oxidises QH2
QH2 + 2CytCox + 2H+ —> Q + 2CytCred + 4H+
Complex 3 description
Ubiquinone-cytochrome c Oxioreductase
Whole complex is a dimer of complexes
Accepts 2e- from QH2 and passes them to cyt c
3 haem groups
Haem bH
Haem bL
Haem c1
Rieske 2Fe2S cluster with 2His and 2Cys
2 binding sites for Q
Q cycle takes place here
What is the Q cycle
Needed as QH2 carries 2e- but cyt c only accepts 1
QH2 binds to q0 site (near IMS)
1e- goes to FeS centre as it has higher reduction poteintial, then cyt c1, then to cyt c
2nd e- goes to haem bL as reduction of FeS cluster causes conformation change so it swings away from Q0, then to head bH, then to Q bound at Q1 site to form semiquinone
A new QH2 binds at Q0 site
E- go in same directions
Semiquinone fully reduced to QH2
Q cycle overall equation
2QH2 + Q + 2CtyCox + 2H+ —> 2Q + QH2 + 2CytCred + 4H+
Complex 4 overall equation
4cytCred + 8H+ + O2 —> 4CytCox + 2H2O + 4H+
For 2 e- coming from NADH or FADH2, 2 protons are pumped through complex 4 to IMS
Complex 4 description
Cytochrome c oxidase
2 haem groups
Haem A
Haem A3
2 copper centres
CUA (has 2Cu but only accepts 1 e- still)
CuB (has 1 Cu)
4 protons pumped per 4e-
Pumped through protein directly
Reduces O2 to H2O
Complex 2 info
Succinate dehydrogenase
Passes 2e- from FADH2 to QH2
FAD covalently bound to enzyme
When succinate is oxidised to fumarate FAD—>FADH2
E- pass through FeS cluster and haem to Q bound near membrane
Q picks up 2 proteins from matrix
No protons pumped or transferred
Number of H+ pumped or transferred by each complex
Complex 1 - 4H+
Complex 3 - 4H+
Complex 4 - 2H+ (per 2 e-)
What are uncouplers
Carry protons through IMM to matrix
Uncouple e- transfer from ATP synthesis
E- still pass to O2
No effect on e- transport
No ATP synthesised because protein gradient destroyed
Energy of proton gradient released as heat
E.g DNP (lipid soluble and can pick up protons in IMS)
Can be important in keeping babies and animals warm
General ATP synthase structure
F1 sticks out into matrix
F0 is hydrophobic and is in the membrane
E- transport generates a PMF and ATP synthesis by ATP synthase is driven by the PMF
F1 structure
3 alpha subunits
3 beta subunits
1 gamma, 1 delta, 1 epsilon
3 alpha and beta subunits arranged alternating in a ring
Gamma subunit is a long alpha helix that goes up through the middle of the ring
Alpha and beta subunits can both bind nucleotides but only beta subunits are catalytic
3 beta subunits have different conformations due to interaction with gamma subunit but have identical sequences
F0 structure
Ring of identical c subunits
Number of c subunits depends on species but mammals have 8
3 subunits connect F1 to F0, collectively called the stator
Made up of hydrophobic a subunit next to c ring in IMM and 2 b subunits
Ab2 stator is connected to F0 and delta subunit of F1
Whole molecule is a rotary motor
Rotation drives atp synthesis
Stator keeps alpha beta ring stationary so gamma can rotate within it
Binding change mechanism for ATP synthase
C ring rotates, causing gamma subunit to roatate, causes beta subunit to change through 3 different conformation states
1 active site in each beta subunit
Loose (L) binds ADP+Pi
Tight (T) binds ATP very tightly so ADP—> ATP
Open (O) has more open conformation so releases ATP
Rotates in steps of 120 degrees
Full 360 rotation produces 3 ATP
Each subunit changes T—>O—>L
At any given time only one subunit is in each conformation
How does PMF drive rotation
Each c subunit has 2 alpha helices that span the membrane
In the middle of one of the alpha helices of each c subunit is Asp or Glu so they can pick up or release protons
A subunit has 2 half channels that dont span the membrane
H+ from IMS enter half channel on C subunit nearest half channel
On c subunit nearest matrix half channel H+ are lost
Protonated form more hydrophobic so pushes c subunits into membrane
Gain and loss of protons drives rotation of c ring
C ring rotation causes gamma subunit to rotate
ATP/ADP translocase
Couples entry of ADP into matrix with exit of ATP
Pi and H+ brought into matrix
Therefore need one extra H+ for each ATP synthesised to bring in Pi
2 methods of transport of cytoplasmic NADH from glycolysis into mitochondria
Glycerol-3-phosphate shuttle
Malate-aspartate shuttle
Glycerol - 3 - phosphate shuttle
Dihydroxyacetone phosphate reduced to glycerol-3-p
Reoxidises NADH to NAD+
Glycerol-3-p can move to IMS
Oxidised back to dihydroxyacetone phosphate by mitochondrial glycerol-3-p dehydrogenase
Reduces FAD to FADH2
E- from FADH2 reduce Q to QH2
E- now from FADH2 so only 1.5 ATP
Malate-aspartate shuttle
OAA can be converted to malate, oxidising NADH
Malate passes through IMM
Malate dehydrogenase reduces NAD+ to NADH and converts malate to OAA
OAA converted to aspartate and alpha keto glutarate that are transported back across IMM
Difference between NAD and NADP
NADP has phosphate group on 2’ C of ribose
Phosphate group as no effect on e- carrying abilities
Phosphate tag allows enzymes to distinguish between them
What is NAD for
Activated carrier of e- for fuel oxidation
Reduced in oxidation of fuel molecules
What is NADP
E- donor in reductive biosynthesis reactions
Need high potential e- because precursors are more oxidised than products
Why do we have different e- carriers in oxidative and reductive synthesis
Allows independent regulation
[NAD] can be high for catabolism and [NADPH] can be high for biosynthesis
Rato NAD/NADH kept high and ratio NADP/NADPH kept low
If using same carrier you can’t have both types of reaction occurring at same time
Overall equation for oxidative phase of pentose phosphate pathway
Glucose-6-P + 2NADP + 2H2O —> 2NADPH + ribose-5-p + CO2 + 2H+
Ribose-5-P used to make nucleotides
Allosteric regulation of oxidative phase of pentose phosphate pathway
NADP is an Allosteric activator of glucose-6-p dehydrogenase
Non oxidative phase of pentose phosphate pathway
Just molecular rearrangements occurring
Coverts ribulose-5-p back to 2 molecules of fructose-6-p and one glyceraldehyde-3-p
Using transketolases and transaldolases
If nucleotides not needed, ribulose-5-p converted back to intermediates of glycolysis or gluconeogenesis
What happens in oxidative phase of pentose phosphate pathway
Glucose-6-p —> 6-phosphogluconolactone produces 1 NADPH from NADP
6-phosphogluconolactone —> 6 phosphogluconate
6phosphogluconate —> ribulose-5-p produces 1 NADPH
Ribulose-5-p <—> ribose-5-p
Uses of NADPH
Reductive biosynthesis in tissues that synthesise FAs or for cholesterol/steroid hormone synthesis
Used to prevent oxidative damage in all cells, particularly RBC
What is GSH (reduced)
Tripeptide antioxidant in cytoplasm
Keeps cytoplasm as a reducing environment
Glutamate linked to cysteine by a gamma peptide bond
Gamma-Glu-cys-gly
Reduces ROS and keeps protein SH groups in reduced form
What is GSSG
Glutathione (oxidised)
2 cys form disulphide bond
Role of NADP in maintaining levels of GSH
Reduces GSSG to 2GSH
GSH can then reduce H2O2 to H2O
Important in RBC as they are directly exposed to oxygen
Fates of G-6-P
Key intermediate in carbohydrate metabolism
Converted to G-1-p for UDP-glucose
Used in glycolysis to make pyruvate
Used in pentose phosphate for ribose-5-p and NADPH
Used in liver for glucose
