LECTURE 12 - REDOX REACTIONS

Question 1

where does the ETC happen?

Answer 1

in the cristae

Question 2

what is the difference between the outer vs inner membrane of the mitochondrion?

Answer 2

the outer membrane is porous, molecules can easily come in from the cytosol

the inner membrane is not very porous but it has transporters

Question 3

where does the CAC happen?

Answer 3

in the mitochondria matrix

Question 4

where does the energy for proton pumping come from?

Answer 4

comes from electron flow, which come from NADH and FADH

Question 5

how many protons are pumped by each complex?

Answer 5

complex 1: 4
complex 2: 0
complex 3: 4
complex 4: 2

Question 6

how many protons are pumped per two electrons?

Answer 6

per 2 electrons you have 10 protons pumped and 1 oxygen molecules turned into water

Question 7

how does oxygen get all the electrons at the end?

Answer 7

has the highest reduction potential
every subsequent redox center has a higher reduction potential, this keeps the electrons moving

Question 8

what are the different redox centers?

Answer 8

iron-sulfur clusters
cytochromes or flavoproteins

Question 9

what does the pumping of protons inside the intermembrane space cause?

Answer 9

causes a disequilibrium of protons
lots inside the membrane, positive charge, and negative charge outside
this creates an electrochemical gradient, or also a proton motive force

Question 10

what is the difference between the matrix and the intermembrane space and what is that equivalent to?

Answer 10

the difference is 150-200mV
when you take into account the size of the mitochondria, that is equivalent to a lighting bolt

Question 11

which complexes donate to CoQ?

Answer 11

complex I and II

Question 12

what does CoQ donate to?

Answer 12

donates to complex III and cytochrome C

Question 13

what are flavoproteins?

Answer 13

have a flavan mononucleotide (FMN) or dinucleotide (FAD)
can accept and then donate electrons

Question 14

what powers complex V (ATP synthase)?

Answer 14

complex V is powered by the energy released by complexes I, II and IV

Question 15

what does cytochrome C do?

Answer 15

shovelling protein
takes electrons from complex III and than gives them to complex IV

Question 16

what is complex IV?

Answer 16

cytochrome oxidase
the electrons from cytochrome will go to oxygen to make water

Question 17

what are the metals in the ETC?

Answer 17

diverse array of metals like Fe, Cu and heme
the standard reduction potentials depend on the microenvironment

Question 18

what is the structure and function of complex I?

Answer 18

• pumps 4 net protons
• made of 44 proteins, that come from 2 different genomes
• 7 of the proteins are coded in mitochondrial DNA, which means that coordination is needed between mitochondria and nucleus to make and assemble those proteins
• complex I has a hydrophobic arm (alpha helix) in the membrane
• has a hydrophilic arm which points into the matrix
• FMN is the first redox center, then it’s iron sulfur clusters, and finally ubiquinone

Question 19

what does complex II do?

Answer 19

does not pump protons
contributes to membrane potential by feeding electrons into the Q pool

Question 20

what are pathways that reduce CoQ?

Answer 20

• complex I
• complex II
• multiple dehydrogenases which are also FAD bound
  they impinge upon ETF (electron transferring flavoprotein)
  ETF also has an FAD
  passes it onto ETF-QUO
  which finally donates it to CoQ
  succession of redox centers on different molecules

Question 21

what does complex III do?

Answer 21

pumps 4 net protons
receives protons from ubiquinol

Question 22

what are the different forms of Q?

Answer 22

Q=ubiquinone
QH=semi-quinone
QH2=ubiquinol

Question 23

explain the Q cycle

Answer 23

• QH2 comes into complex 3, 2 protons are pumped
• one electron is donated to cytochrome C, the other one goes to a Q molecule to make QH
• the cycle starts again, with another QH2, and 2 protons are pumped again
• one of the electrons from the QH2 goes to cytochrome C and the other one goes to QH, which regenerates QH2
• the cycle can start again
• total 4 electrons: 2 cytochrome C and 2 to make QH2 again
• cytochrome C will donate these electrons to complex IV

Question 24

how are these complexes arranged?

Answer 24

arranged into one super complex called the respirasome
for the same reasons as the PDC, it is more efficient that way, if they are all close together

Question 25

what is the equation for the ETC, and how much energy is released?

Answer 25

NADH+ (H+)+ 1/2O2 –> (NAD+)+H2O=-220kJ/mol

Question 26

what are the reduction potentials of NAD+ and oxygen?

Answer 26

NAD=-0.32V
oxygen=+0.82V

Question 27

in which reduction direction do electrons flow?

Answer 27

from low to high reduction potential

Question 28

what is the energy from ATP hydrolysis and how does it compare to the ETC?

Answer 28

energy from ATP hydrolysis is 50kJ/mol
and if there are 2.5ATP/NADH that is 125kj/mol
there is more that enough harvested to make ATP

Question 29

why are there so many redox reactions instead of one big one?

Answer 29

there are multiple redox reactions, so the energy is slowly harvested over the multiple step rather than having a big explosive step

Question 30

what are the two shuttles for cytosolic NADH to the mitochondria?

Answer 30

G3P shuttle
malate/aspartate shuttle

Question 31

how does the G3P shuttle work?

Answer 31

1. in the cytosol, DHAP is reduced by NADH to make G3P
2. G3P enters through the outer membrane, carrying the electrons from NADH
3. on the inner membrane is G3P dehydrogenase, which will oxidise G3P back to DHAP
4. the electrons from G3P will go onto FADH2 and then onto CoQ in complex III
5. DHAP returns to the cytosol to be used in the next cycle of this reaction

Question 32

how does the malate/aspartate shuttle work?

Answer 32

1. oxaloacetate is reduced by NADH to malate
2. malata enters the mitochondria
3. malate is reoxidated to oxaloacetate, and NAD is reduced, which will then go to complex I
4. oxaloacetate reacts with glutamate to make aspartic acid (amino group is donated)
5. glutamate becomes alpha ketoglutarate
6. aspartic acid and alpha ketoglutarate are co-transported out of the mitochondria into the cytosol
7. by another transamination, alpha KG is made back into glutamate and aspartic acid is made into oxaloacetate
   8 the cycle can start again

Question 33

what are the two parts of ATP synthase?

Answer 33

F0 (in the membrane)
and F1 (in the matrix(
reversible enzyme

Question 34

how many ATPs are made per every 360°C turn?

Answer 34

3 ATPs/turn

Question 35

how many c subunits of F0 in mammals?

Answer 35

8 subunits, therefore 8 protons can come in
8 protons per 3 ATP
other organisms have a different number of subunits

Question 36

explain why 2.5ATPs are made per NADH?

Answer 36

this is also the P/O ratio through complex 1
8 protons go through ATP synthase, which generates 3 ATP
8/3=2.7 protons/ATP
but, the Pi is co transported with another proton, which means that it is actually 3.7 protons/ATP
NADH goes through all the complexes, therefore 10 electrons are pumped through the ETC
10/3.7=2.5
that is where we get 2.5ATP/NADH

Question 37

explain why 1.5ATPs are made per FADH?

Answer 37

this is also the P/O ratio through complex 2
8 protons go through ATP synthase, which generates 3 ATP
8/3=2.7 protons/ATP
but, the Pi is co transported with another proton, which means that it is actually 3.7 protons/ATP
FADH skips complex 1, therefore 6 electrons are pumped through the ETC
6/3.7=1.5
that is where we get 1.5ATP/FADH