AAMC Self-Assessment Bio #85

[DSimone]

The chemical valinomycin inserts into membranes and causes the movement of K+ into the mitochondria. Based on Figure 1, if mitochondria are treated with valinomycin, the rate of ATP synthesis in the mitochondria will most likely:

A) decrease, because K+ will compete with protons at the active site on ATP synthase.
B) decrease, because movement of K+ into the mitochondrial compartments will disrupt proton movement into the intermembrane space.
C) increase, because the net positive charge in the mitochondria will cause increased movement of protons into the intermembrane space.
D) increase, because the additional positive charge will further activate ATP synthase.

So, I can see that A & C are clearly incorrect because K+ will not compete at the active site on ATP synthase and the net positive charge would not increase proton movement into the intermembrane space (they are being actively transported anyway, no?)

My question is: why couldn't D be correct (it's not)?

I was thinking that with additional positive charge in the intermembrane space, an even greater proton gradient would be created, promoting further activation of ATP synthase (so that more protons would go down their gradient into the matrix and reduce that positive charge in the intermembrane space.)

B is the correct answer, so I am also wondering why additional positive charge in the intermembrane space would disrupt proton movement that is taking place via active transport (not dependent on the proton gradient). Why would increased positive charge disrupt this?

Could someone break this down for me? I must be missing something. Thanks!

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[Postictal Raiden]

The chemical valinomycin inserts into membranes and causes the movement of K+ into the mitochondria. Based on Figure 1, if mitochondria are treated with valinomycin, the rate of ATP synthesis in the mitochondria will most likely:

A) decrease, because K+ will compete with protons at the active site on ATP synthase.
B) decrease, because movement of K+ into the mitochondrial compartments will disrupt proton movement into the intermembrane space.
C) increase, because the net positive charge in the mitochondria will cause increased movement of protons into the intermembrane space.
D) increase, because the additional positive charge will further activate ATP synthase.

So, I can see that A & C are clearly incorrect because K+ will not compete at the active site on ATP synthase and the net positive charge would not increase proton movement into the intermembrane space (they are being actively transported anyway, no?)

My question is: why couldn't D be correct (it's not)?

I was thinking that with additional positive charge in the intermembrane space, an even greater proton gradient would be created, promoting further activation of ATP synthase (so that more protons would go down their gradient into the matrix and reduce that positive charge in the intermembrane space.)

B is the correct answer, so I am also wondering why additional positive charge in the intermembrane space would disrupt proton movement that is taking place via active transport (not dependent on the proton gradient). Why would increased positive charge disrupt this?

Could someone break this down for me? I must be missing something. Thanks!

I had trouble with this question because of the same exact reasoning. If answer option B had "movement of into matrix" instead of "movement into the innermembrane space", I would have picked it with a heart beat. However, if positive charges (K+) accumulate inside the mitochondria, wouldn't that makes it harder for protons to diffuse back into the matrix.

With that said, this question can be answer by POE. Options C and D can be eliminated since alteration with the membrane results in less ATP production, so now it's between A and B. A is obviously out, so B should be the answer.

Anyhow, I'll be interested in learning other people's thought on this particular question.

 

[Temperature101]

I had trouble with this question because of the same exact reasoning. If answer option B had "movement of into matrix" instead of "movement into the innermembrane space", I would have picked it with a heart beat. However, if positive charges (K+) accumulate inside the mitochondria, wouldn't that makes it harder for protons to diffuse back into the matrix.

With that said, this question can be answer by POE. Options C and D can be eliminated since alteration with the membrane results in less ATP production, so now it's between A and B. A is obviously out, so B should be the answer.

Anyhow, I'll be interested in learning other people's thought on this particular question.

C and D are definitely wrong.....I think they are right because K+ will compete for the electrons coming out of NADH and FADH2 after their protons are release to the intermembrane space. These electrons wont be able to get to Coenzyme Q, which in turn wont get transferred to Cytochrome C. If there is no electrons to Cytochrome C, more protons(H+) will not get to the intermembrane space. Therefore, protons gradient will be disrupted.That is how i see it. Therefore, B is the correct answer. It is an ambiguous question but it was not that difficult because you can narrow it down to two choices (A and B) and A is definitely wrong.

 

[Postictal Raiden]

.

 

[Temperature101]

......................

 

[Postictal Raiden]

That is what I said... Did you read my whole explanation?

Sorry, I misread your post using my stupid phone. I amended my previous post.

 

[Temperature101]

Ok...Previous post edited as well....

 

[DSimone]

C and D are definitely wrong.....I think they are right because K+ will compete for the electrons coming out of NADH and FADH2 after their protons are release to the intermembrane space. These electrons wont be able to get to Coenzyme Q, which in turn wont get transferred to Cytochrome C. If there is no electrons to Cytochrome C, more protons(H+) will not get to the intermembrane space. Therefore, protons gradient will be disrupted.That is how i see it. Therefore, B is the correct answer. It is an ambiguous question but it was not that difficult because you can narrow it down to two choices (A and B) and A is definitely wrong.

Huh? Could someone explain this?

I am still a bit unclear on this question. Any alternative explanations out there?

 

[Postictal Raiden]

Huh? Could someone explain this?

I am still a bit unclear on this question. Any alternative explanations out there?

I think what Temp.101 tried to say was that potassium ions act as strong oxidizing agents, oxidizing NADH and FADH2, and compete with the membrane proteins for the electrons provided by these electron carriers. Therefore, instead of donating their electrons to innermembrane proteins, NADH and FADH2 donate their electrons to the potassium ions. Because electrons provide the "pumping" energy of protons across the innermembrane, lack of them will reduce this transport, resulting in less protons being in the innermembrane space.

 

[DSimone]

I think what Temp.101 tried to say was that potassium ions act as strong oxidizing agents, oxidizing NADH and FADH2, and compete with the membrane proteins for the electrons provided by these electron carriers. Therefore, instead of donating their electrons to innermembrane proteins, NADH and FADH2 donate their electrons to the potassium ions. Because electrons provide the "pumping" energy of protons across the innermembrane, lack of them will reduce this transport, resulting in less protons being in the innermembrane space.

Aha! I never even thought of this approach to the problem. Excellent! Thanks Temperature 101 & Ibn Alnafis MD! This just clicked

 

[adrakdavra]

B is correct due to selectivity and electrochemical potential gradient not oxidation

 

[DSimone]

😕

 

[DSimone]

B is correct due to selectivity and electrochemical potential gradient not oxidation

Could you explain how excess positive charge (K+ in the intermembrane space) disrupts proton movement into the intermembrane space? I'd be interested in an alternative take on this...

 

[Incontrol]

I had the same thought process as you, DSimone, but I still chose B.

Because I knew valinomycin was antibiotic, it kills bacteria; and the only way that make sense to me if Valinomycin decreased ATP synthesis.

 

[adrakdavra]

The movement of protons through ATP synthase occurs from the intermembrane space to the matrix.
due the increase In K+ , H+ will have a hard time moving from the matrix to the intermembrane space, NADH and FADH2 will still get oxidized via Cytochromes oxidase but not as much, and not that many electrons that are transferred through the electron transport system initially.

 

[Postictal Raiden]

The movement of protons through ATP synthase occurs from the intermembrane space to the matrix.
due the increase In K+ , H+ will have a hard time moving from the matrix to the intermembrane space, NADH and FADH2 will still get oxidized via Cytochromes oxidase but not as much, and not that many electrons that are transferred through the electron transport system initially.

But how does the presence of K+ in the matrix impedes the movement of protons out of the matrix. This doesn't make sense because increasing the positive charges inside the matrix should facilitate the movement of positively charged ions (H+) out of the matrix.

 

[adrakdavra]

why do you think of the presence of K+ in the matrix?

 

[Postictal Raiden]

why do you think of the presence of K+ in the matrix?

Hmmm, good point. Now, that makes sense. If K+ were accumulated within the innermembrane space, then the pump of protons out of the matrix will be hindered.

 

[maverick145]

I think this is a mistake. If you look up the action of valinomycin - it makes K lipid soluble so it can go across membranes. The electrochemical gradient created by the proton pumps ( + in innermembrane space / - in matrix) is eliminated by the free movement of potassium. With no electrical potential - no movement of H+ through ATP synthase. I think the answer is supposed to say "movement into the matrix". It must be backwards.