Two metabolic routes convert precursor Y to product X, an otherwise unfavorable step with ΔG∘=+85kJmol−1.
• Pathway A couples the step to the hydrolysis of 2 ATP and 1 GTP.
• Pathway B couples the step to the hydrolysis of 4 ATP.\nStandard free energies of hydrolysis are ΔG∘(ATP)=−30.5kJmol−1 and ΔG∘(GTP)=−31.5kJmol−1.\nWhich statement about the energetic efficiency of the two pathways is most accurate?
a) Pathway B is more efficient because the larger negative overall free energy change (about −37kJ mol−1) drives the reaction further.
b) Both pathways are equally efficient because, after accounting for stoichiometry, each hydrolyzes three high-energy phosphate bonds.
c) Pathway A is more efficient because it supplies only a small excess free energy (about −7.5kJ mol−1) beyond what is needed to drive the reaction.
d) Pathway A is less efficient because its overall standard free energy change is positive (about +7.5kJ mol−1), so the reaction would still require input of energy.