📒 Chinopoulos 2011
Mitochondrial consumption of cytosolic ATP: Not so fast1
- ΔpH is very small (<0.15) 7-9, therefore, the directionality of F0–F1 ATP synthase is mostly controlled by ΔΨm
n = H+/ATP coupling ratio (roughly 3)
However, MgATP2− can be expressed as [ATP4−]free × [Mg2+]/K M(ATP) and the similar relation applies to ADP:
The E rev_ATPase (black triangles) is more negative than E rev_ANT (white triangles), implying that progressively depolarizing mitochondria will first exhibit reversal of the F0–F1 ATP synthase, followed by reversal of the ANT
ADP–ATP exchange rate of intact mitochondria as a function of ΔΨm is shown:
Challenges prior to cytosolic ATP consumption
- When ΔΨm reaches E_rev_ATPase => => the reversal of the F1FO ATPase , the less negative ΔΨm becomes, the greater will be the reverse activity of the F0–F1 ATP synthase. Substrate-level phosphorylation to keep matrix ATP at a certain level
- When ΔΨm reaches E_rev_ANT => the reversal of the ANT, the less negative ΔΨm becomes, the greater will be the reverse activity it becomes
- in the absence of substrate‐level phosphorylation, the reverse operation of the ANT was indeed supporting ΔΨm (maintained by the reversals of both the ANT and the F0–F1 ATP synthase), but yielding only a very slow rate of ATP entry into the matrix; in the presence of substrate‐level phosphorylation, ANT was operating near its thermodynamic equilibrium. a: with substrate phosphorylation. b: without substrate phosphorylation
- The maintenance of ΔΨm ‘cost’ only 13% of the ATP produced by the glycolysis
- Drop in pH from accumulation of lactate impairs the activity of the ANT 9 and F0–F1 ATP synthase 43 in addition to affecting the concentrations of deprotonated ATP and ADP
IF1 and hysteretic inhibition of ATP consumption
- four mechanisms are in place aiming at blocking it upon decrease in pmf, mediated by:
- a decrease in matrix ATP concentration,
- a decrease in ATP/ADP ratio
- an increase in matrix Pi
- the endogenous factor IF1
- The binding and release of IF1 in F1 are governed by both matrix [ATP] and ΔΨm, inhibition of F1‐ATPase by IF1 may not be complete; matrix ATP inhibits the binding of IF1 to F1‐ATPase with a Ki of 0.14 mM.
- IF1 dissociates from F1‐ATPase upon generation of sufficient proton electrochemical gradient
Do depolarized in situ mitochondria consume cytosolic ATP?
- Respiration‐impaired depolarized mitochondria cannot deplete cytosolic ATP levels
- Upon reversal of F1FO ATP synthase, ΔΨm is generated to a level similar to that of E_rev_ANT
- ATP imported from the cytosol is spared from hydrolysis by the F1‐ATPase due to the inhibition from IF1
- During repolarization (whenever possible), E rev_ANT is less negative than E rev_ATPase. Therefore, the ANT would expel matrix ATP before the uninhibited F1‐ATPase seizes the opportunity to hydrolyze it.
In respiration‐impaired mitochondria with intact inner mitochondrial membranes, if the reverse operation of F0–F1 ATP synthase is sustained by ATP provided by matrix substrate‐level phosphorylation, ΔΨm will be maintained at a value not more negative than E rev_ATPase, while if only cytosolic ATP reserves are being used, the maximum ΔΨm value attainable cannot be more negative than that of E_rev_ANT, imposing very slow reversal ATP–ADP exchange rates on the translocase.