Week 9 Johnson Lecture 7

Question 1

What is the structure of ATP synthase?

Answer 1

Structure is divided into 2 parts; F0, which is within the membrane, and F1, which is on one side of the membrane.

• a-subunit is the proton half channels connecting each side of the membrane to the c-subunits proton binding glutamate residues
• b2 and δ-subunits form the ‘stator’ bridging the F1 and Fo domains and ensuring the two parts remain coupled
• 3 α-subunits and 3 β- subunits form the F1 head. α and β are similar but only β units are catalytically active with α playing structural role.
• 8-15 c subunits act as the proton driven rotor embedded in the membrane
• γ-subunit is the central shaft that transmits torque from Fo to F1 changing the conformation of the β- subunits between three states O, L and T

Question 2

Explain the mechanism of proton pumping through ATP synthase.

Answer 2

Positive side of the membrane allows protonation of a glutamate residue at low pKa’s because of the high proton motive force. Rotation of c-ring one step at a time by the attracting E- and R+. The E and proton rotate around and as the proximity decreases between E and R+, the pKa is lowered again which triggers deprotonation

Question 3

What are the 3 states in beta-subunits that are interconverted by the gamma-subunit?

Answer 3

3 states;

• In the O-state ATP is released
• In the L-state ADP + Pi is bound
• In the T-state ADP + Pi is converted to ATP

Question 4

What is the significance of c-ring size?

Answer 4

Number of c-subunits defines to H+/ATP ratio

• It’s all about “torque” - the act of turning applied force into rotational movement. [1] High (small) gears vs. cgs (2) Low (big cogs) gears If force applied is same, output will be different- faster with (1) slower with (2). If force applied is different, output will be same- easier to peddle uphill with (2) as requires less applied force compared to (1) to do the same work.
• Larger (i.e. lower) the gear the larger the force multiplier
• In this case the force applied is pmf and the rotational movement is of the γ-subunit
• (1) High gear on ATP synthase involves small c-ring (2) Low gear involves large c-ring
• ✦ If pmf applied is same, output will be different- faster ATP synthesis with (1) slower with (2)
• ✦ If pmf applied is different, output will be same- easier to synthesise ATP with (2) as requires less pmf compared to (1) to do the same work.
• Larger the c-ring the larger the pmf multiplier

Question 5

What is the high and low gear optimised for?

Answer 5

High gear optimised for rapid synthesis (speed) sacrificing efficiency at low energy inputs and low gear optimised to work at c14 low energy inputs sacrificing speed

Question 6

Why is the c-ring usually larger in plants than in mammals?

Answer 6

Light levels (energy input) can vary massively throughout the day due to clouds and dynamic canopy shading (light can’t be stored), animal energy supply is regulated (e.g. liver) and relatively constant (storage of sugar, fat etc)