Cytochrome c oxidase

Question 1

Enzyme name for cytc oxidase

Answer 1

Ferrocytochrome c/ oxygen oxidoreductase

Question 2

What type of reaction is the cytochrome c oxidase RX

Answer 2

Exergonic - energy released

Question 3

Cyrochrome oxidation Em

Answer 3

+270mV (actual = +300mV)

Question 4

Oxygen reduction Em

Answer 4

+820mV (actual =+800mV)

Question 5

DeltaE for cyctc oxidation

Answer 5

+550meV (actual = +500meV)

Question 6

RT/nF

Answer 6

59

Question 7

Components of cytc oxidase

Answer 7

Subunit I, II, III and 10 (nuclear encoded) more

Question 8

Subunit I of cyt c

Answer 8

Location of active site (contains 2 heme and Cu B) - 12 TM helix

Question 9

Subunit II of cyt c

Answer 9

Absent in quinone oxidases (contains CuA and cytc binding site) - 2 TM helix

Question 10

Subunit III of cyt c

Answer 10

Absent in same bacteria - forms part of channel when present

Question 11

Location of cytc oxidase

Answer 11

Spans inner mitochondrial membrane

Question 12

Cofactors of cytc oxidase

Answer 12

1. CuA (dimer of Cu ions) 2. Heme a (6 coord) 3. Heme a3 (5 coord) 4. CuB

Question 13

How do cyt c oxidases vary

Answer 13

In terms of structure and cofactors

Question 14

CuA Em

Answer 14

+280mV

Question 15

Structure of Heme A

Answer 15

Acid groups which are useful for dealing w/ protons - no open ligand binding points

Question 16

Structure of Heme C

Answer 16

Has sulphur groups which connect covalently to protein

Question 17

Fe6 ligands (heme A) and Em

Answer 17

Coordinated by 2 imidazoles (histidine side chains) - electron transfer role . Fe2/Fe3+ Em = 265mV

Question 18

Fe5 ligands (heme A3) and Em

Answer 18

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

Question 19

Which cofactors are close together

Answer 19

Heme A3, tyrosine and CuB

Question 20

CuB positioning

Answer 20

3 imidazoles bound and one open ligand binding site - 1 imidazole is cross linked to Tyr

Question 21

CuB Em

Answer 21

400mV at rest, 700mV functional

Question 22

When is there a large drop in redox potential

Answer 22

Oxygen reduction

Question 23

How many electrons are involved in cytc oxidation

Answer 23

1 (but O2 is a 4e- reaction)

Question 24

3 cytc groups of interest

Answer 24

Fe2+, Cu1+ and HO-Tyr

Question 25

What happens when oxygen binds

Answer 25

Forms “oxyferrous” species

Question 26

Order of reduction

Answer 26

Tyr radical, Fe4+, Cu2+, Fe3+

Question 27

Where are chemical protons taken from

Answer 27

The matrix (-ve side)

Question 28

When is the active site oxidised

Answer 28

In the first step by 4 electrons

Question 29

Delta G for the whole reaction

Answer 29

4 x 500 meV = 2eV

Question 30

Where do the electrons com from

Answer 30

Cytc from inter membrane space (+ve side)

Question 31

What happens when an e- meets a proton in the middle

Answer 31

Form H20 and equivalent to moving an electron across membrane therefore 200 mV generated

Question 32

What happens to the energy released from the reduction of O2

Answer 32

Mainly captured by generation of electrochemical and proton gradients (protons transported from matrix side)

Question 33

How much energy is lost

Answer 33

About 50 meV

Question 34

How is the proton to be pumped transported

Answer 34

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

Question 35

How do quinone oxidases differ from cytc oxidase

Answer 35

No cytc binding site or CuA (and heme b instead of hemea and heme o3 instead of heme a3