Question

Consider a mitochondrion in which the $\text{F}_0$ subunit of ATP synthase is mutated, such that it can still passively conduct protons but is mechanically uncoupled from the $\text{F}_1$ subunit. If such a mitochondrion is provided with succinate, ADP, and Pi in aerobic conditions, the correct outcome based on chemiosmotic theory is:

• A. Continuous oxygen consumption and no ATP synthesis.
• B. No oxygen consumption and no ATP synthesis.
• C. Rapid initial oxygen consumption that plateaus at a low rate and normal ATP synthesis.
• D. Continuous oxygen consumption and normal ATP synthesis.

Hint:

An uncoupled mitochondrion behaves like a short circuit in an electrical system. The flow of current (protons/electrons) is high, consuming fuel (oxygen) rapidly, but no useful work (ATP synthesis) is done.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the physiological outcome in a mitochondrion with a mutated $F_0$ subunit of ATP synthase that can still conduct protons but is mechanically uncoupled from the $F_1$ catalytic subunit.

Step 2: Detailed Explanation:

• According to Peter Mitchell's chemiosmotic hypothesis, ATP synthesis in mitochondria is coupled to the transfer of electrons through the electron transport chain (ETC).

• As electrons are transferred, protons ($\text{H}^+$) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton-motive force (an electrochemical proton gradient).

• Normally, these protons flow back into the matrix through the $F_0$ channel of the ATP synthase complex, driving the mechanical rotation of the stalk that triggers ATP synthesis in the $F_1$ subunit.

• In the given mutant, the $F_0$ subunit still allows passive proton flow, but it is mechanically uncoupled from $F_1$.

• Because the $F_0$ channel is open, protons can freely leak back into the matrix down their electrochemical gradient. This dissipates the proton-motive force.

• Since the proton gradient is constantly dissipated, there is no "back-pressure" on the electron transport chain. The ETC continues to function at its maximum rate to re-establish the gradient, consuming oxygen continuously as the terminal electron acceptor.

• However, because of the mechanical uncoupling between $F_0$ and $F_1$, the energy released by the proton flow cannot be used to synthesize ATP from ADP and $P_i$. Thus, no ATP is synthesized.

• This condition is functionally identical to the action of mitochondrial uncouplers like 2,4-dinitrophenol (DNP).


Step 3: Final Answer:
Therefore, the outcome is continuous oxygen consumption and no ATP synthesis.