Biochemistry - ATP Synthase

Question 1

Chemiosmotic coupling hypothesis

Answer 1

A theory by Sir Peter Mitchell in 1961 to describe ATP synthesis by way of a proton electrochemical coupling.

• Protons are pumped from the mitochondrial matrix to the intermembrane space via Complex 1, 2, 3, 4 while the electrons are transferred along the electron transport chain in the mitochondrial inner membrane.
• Proton gradient is established.
• Hydrogen ions tend to diffuse back to the matrix (where hydrogen ions are less) via the ATP synthase
• As the hydrogen ions diffuse through the ATP synthase energy is released which is then used to drive the conversion of ADP to ATP (by phosphorylation).

Question 2

ATP Synthase Structure

Answer 2

• Sits in the inner mitochondria membrane as dimers
• Stator which is through the membrane and holds ATP Synthase together
• Alpha and Beta units accept the protons and change shape and turn around the gamma unit
• Gamma unit which is an internal rod which extends to catalytic sites in the c-ring
• For each rotation of the gamma unit 3 ATP are made
• c-ring in human has 8 subunits

Question 3

Proton Motive Force

Answer 3

Electrochemical gradient between IMS and Matrix

• IMS has only a slightly lower pH than the matrix however negative phosphate heads of the lipids will attract positive protons thus the protons flow along the membrane
• pH is lowered only around the edges of IMS
• ATP Synthase dimers fold the membrane forming cristae creating an area where protons can congregate (significantly lowering the pH) to move through ATP Synthase

Question 4

ATP output

Answer 4

• Results in 30ATP molecules at 37 degrees
• 36-38ATP at 25 degrees
• IF there is no proton gradient then ATP Synthase will consume ATP (anoxia no O2)

Question 5

ATP Synthase Action

Answer 5

1. H+ ions flow down their electrochemical gradient and enter a half channel in a stator which is anchored in the membrane
2. H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane
3. Each H+ ion makes one complete turn before leaving the rotor and passing through a second half channel in the stator in the mitochondrial matrix
4. Spinning of the rotor causes an internal rod to spin as well. This rod extends like a stalk into the knob below it which is held stationary by part of the stator
5. Turning of the rod activates catalytic sites in the knob that produce ATP from ADP and Pi

• Can work in reverse as a proton pump

Question 6

Mitochondrial vs bacterial ATP Synthase

Answer 6

• Extreme conservation of ATP synthase across organisms
• in the c-ring in animals there are 8 subunits but in fungi, eubacteria, chloroplasts there are 10-15 subunits
• Bigger number of subunits means more protons but less ATP is produced (harder to spin etc)

Question 7

Leigh’s Disease

Answer 7

• Mutation in the c-ring of ATP Synthase
• Results in proton slippage
• Less ATP made per H+
• Results in damage to the brain