12. The mitochondrial F-Type ATPase is a remarkable protein machine in which a subunit rotates
during H+ transport, and 1 ATP is generated for 120 degrees of revolution. ATP hydrolysis releases
~50kJ/mol, and the transport of H+ costs about 19 kJ/mol. While the exact number depends on cell
conditions, approximately how many protons need to cross the membrane to achieve a full
revolution?
a. 1/3
b. 1
c. 3
d. 9
Energy Balance Calculation
The F1 domain synthesizes 3 ATP during one 360° rotation of the γ-subunit, with each 120° step corresponding to 1 ATP. Energy from proton translocation through Fo powers this: each H+ provides ~19 kJ/mol against the gradient. Full revolution demands 150 kJ/mol (3 ATP × 50 kJ/mol), so protons needed = 150/19 ≈ 7.89, approximating 8 protons for the c-ring rotation in animal mitochondria. This exceeds 3 due to c-ring stoichiometry (8 c-subunits), not 1 per ATP.
Option Analysis
- a. 1/3: Incorrect; implies fractional protons per revolution, impossible as c-ring steps are integer (minimum 8-15 total).
- b. 1: Far too low; 1 proton yields ~19 kJ/mol, insufficient for 150 kJ/mol revolution or even one 50 kJ/mol ATP.
- c. 3: Common textbook approximation (3 protons ≈ 3 ATP), but underestimates actual ~8 protons needed per structural data; matches 120° steps but ignores full energetics.
- d. 9: Closest to ~7.89 calculation; reasonable approximation, though precise vertebrate value is 8 protons/turn.
Mechanism Overview
F-Type ATPase (FoF1) rotates via proton flow through Fo’s c-ring against subunit a, driving F1’s catalytic β-subunits. Vertebrate c8-ring translocates 8 H+ per 360° turn for 3 ATP (2.7 H+/ATP), ensuring directionality via protonation/deprotonation of c-subunit carboxylates. CSIR NET contexts often simplify to 3-4 H+/ATP, but energy values here point to ~8 total.
