Cytochrome c oxidase Flashcards
Enzyme name for cytc oxidase
Ferrocytochrome c/ oxygen oxidoreductase
What type of reaction is the cytochrome c oxidase RX
Exergonic - energy released
Cyrochrome oxidation Em
+270mV (actual = +300mV)
Oxygen reduction Em
+820mV (actual =+800mV)
DeltaE for cyctc oxidation
+550meV (actual = +500meV)
RT/nF
59
Components of cytc oxidase
Subunit I, II, III and 10 (nuclear encoded) more
Subunit I of cyt c
Location of active site (contains 2 heme and Cu B) - 12 TM helix
Subunit II of cyt c
Absent in quinone oxidases (contains CuA and cytc binding site) - 2 TM helix
Subunit III of cyt c
Absent in same bacteria - forms part of channel when present
Location of cytc oxidase
Spans inner mitochondrial membrane
Cofactors of cytc oxidase
- CuA (dimer of Cu ions) 2. Heme a (6 coord) 3. Heme a3 (5 coord) 4. CuB
How do cyt c oxidases vary
In terms of structure and cofactors
CuA Em
+280mV
Structure of Heme A
Acid groups which are useful for dealing w/ protons - no open ligand binding points
Structure of Heme C
Has sulphur groups which connect covalently to protein
Fe6 ligands (heme A) and Em
Coordinated by 2 imidazoles (histidine side chains) - electron transfer role . Fe2/Fe3+ Em = 265mV
Fe5 ligands (heme A3) and Em
Coordinated by 1 imidazole - catalytic role - open binding site for ligand to bind. Fe2+/Fe3+ Em = 275mV (at rest, when functional = 700mV). Fe3+/Fe4+ Em = 762 mV
Which cofactors are close together
Heme A3, tyrosine and CuB
CuB positioning
3 imidazoles bound and one open ligand binding site - 1 imidazole is cross linked to Tyr
CuB Em
400mV at rest, 700mV functional
When is there a large drop in redox potential
Oxygen reduction
How many electrons are involved in cytc oxidation
1 (but O2 is a 4e- reaction)
3 cytc groups of interest
Fe2+, Cu1+ and HO-Tyr
What happens when oxygen binds
Forms “oxyferrous” species
Order of reduction
Tyr radical, Fe4+, Cu2+, Fe3+
Where are chemical protons taken from
The matrix (-ve side)
When is the active site oxidised
In the first step by 4 electrons
Delta G for the whole reaction
4 x 500 meV = 2eV
Where do the electrons com from
Cytc from inter membrane space (+ve side)
What happens when an e- meets a proton in the middle
Form H20 and equivalent to moving an electron across membrane therefore 200 mV generated
What happens to the energy released from the reduction of O2
Mainly captured by generation of electrochemical and proton gradients (protons transported from matrix side)
How much energy is lost
About 50 meV
How is the proton to be pumped transported
Glu gets protonated by it - electron transfer from heme a to the active site triggers H+ movement into a trap near the inner membrane space
How do quinone oxidases differ from cytc oxidase
No cytc binding site or CuA (and heme b instead of hemea and heme o3 instead of heme a3
