TBR Cellular Structure Lecture Outline handout Question 5 Valinomycin and mtpH

[nUgnoY]

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5. Valinomycin makes a K+ channel across the inner mitochondrial membrane. K+ flows through this channel according to an electrochemical gradient. This will result in:

I. lower pH in the intermembrane space
II. lower pH in the mitochondrial matrix
III. raised [K+] in intermembrane space
IV. raised [K+] in mitochondrial matrix

A. I & III only
B. II and IV only
C. I and IV only
D. III only

There was no answer listed in the handout.

Valinomycin should allow K+ to flow into the mito. matrix since matrix is more negative than intermembrane space. Which means IV is true.

But I don't know how valinomycin would change the pH. My guess would be either
1. No effect on pH
Since inner membrane is still impermeable to H+, and H+ is pumped out by
2. Less pH in intermembrane space
Just as a speculation, since there will be less electric gradient attracting H+ back into the matrix. So same H+ pump rate and less force dragging H+ back in could lead to higher [H+] in intermembrane space and lower pH

The reason I'm confused is that
1. There is no "IV only," which would mean there would have to be a pH change.
2. I looked up an Kaplan explanation of a similar problem, which says

"Since valinomycin disrupts one of the gradients responsible for the proton-motive force (the pH gradient is
unaffected), ATP synthesis will decrease."

Also, the Kaplan source says that "The
insertion of valinomycin causes the movement of K+ into the mitochondrial matrix" and double checks that IV is right.

So it seems to me that the two sources are directly disagreeing, Kaplan saying there's no pH change and Berkeley saying that there IS a pH change. Does anyone know if this was an error or if there is an explanation for this? Thanks!

 

[fizzgig]

my best guess would have been C, 1and4. if the emf of H+ to come back to the matrix is lessened but you're still pumping H out using the ETC then the pH there would go down...

dunno about the first source you quoted...

 

[nUgnoY]

my best guess would have been C, 1and4. if the emf of H+ to come back to the matrix is lessened but you're still pumping H out using the ETC then the pH there would go down...

dunno about the first source you quoted...

I speculate that too, but I'm unsure because:

1. I've spent many hours googling valinomycin and looking into textbooks, research articles (not all of which I understood perfectly), valinomycin and mitochondria was used as an example of measuring mitochondrial membrane voltage w/o using a voltmeter. I couldn't find anything direct about valinomycin and pH change

2. The source is KAPLAN explanation of AAMC MCAT. Although I would normally choose Berkeley over Kaplan, Berkeley text does have a lot of errors, and with my reasoning outline in OP I'm not too confident about this one.

 

[fizzgig]

it's possible that the kaplan text means to look at what is DIRECTLY affected by K going into the mmatrix. it doesn't change H concentration because it's not an H channel, but it alters how MUCH H wants to come back inside.

if the EMF for H to come to the inside of the mmatrix decreases and less is spun through the ATP synthase so ATP synth goes down, then unless something else happens that causes fewer Hs to go into the intermemb space H should build up higher than normal out there.

hopefully someone more experienced/confident will chime in on what the answer should be here.

when they do they're going to remind us though that we're looking for the BEST answer. valinomycin could have been ANY word. all we need to know is K goes in, so IV is right. clearly only IV can't be the answer, so either the pH has to go down in the mmatrix or intermemb space. with a lower emf pulling in, II doesn't make much sense.

the kaplan source may have a bad choice of words or sthg because according to this question, the pH IS affected somewhere, directly OR indirectly. anyway...

 

[nUgnoY]

it's possible that the kaplan text means to look at what is DIRECTLY affected by K going into the mmatrix. it doesn't change H concentration because it's not an H channel, but it alters how MUCH H wants to come back inside.

if the EMF for H to come to the inside of the mmatrix decreases and less is spun through the ATP synthase so ATP synth goes down, then unless something else happens that causes fewer Hs to go into the intermemb space H should build up higher than normal out there.

hopefully someone more experienced/confident will chime in on what the answer should be here.

when they do they're going to remind us though that we're looking for the BEST answer. valinomycin could have been ANY word. all we need to know is K goes in, so IV is right. clearly only IV can't be the answer, so either the pH has to go down in the mmatrix or intermemb space. with a lower emf pulling in, II doesn't make much sense.

the kaplan source may have a bad choice of words or sthg because according to this question, the pH IS affected somewhere, directly OR indirectly. anyway...

I definitely see your POV, and I myself thought about it too (my reasoning in the OP).

I'm just wondering if this is an error on TBR's part
1. There is no listed answer
2. There is no outside support for valinomycin changing mito pH
3. There is opposing argument for valinomycin not changing mito pH
4. BR does have a lot of typos/minor errors as I found in their text. So it could even be that d: III only is supposed to be IV only.

Maybe choosing the best answer is what they're going for is, but I still want to know the REAL answer and the reasoning.

 

[loveoforganic]

I'd go with C too. You're dissipating the electrochemical force (just the electro part of electrochemical in this case) resisting the transport of protons into the intermembrane space, thus allowing for a higher concentration of protons to build up in the intermembrane space before equilibrium is reached.

 

[Charles_Carmichael]

"Since valinomycin disrupts one of the gradients responsible for the proton-motive force (the pH gradient is
unaffected), ATP synthesis will decrease."

This statement essentially describes why the pH in the intermembrane space is lowered. As loveoforganic mentioned, both a chemical and an electrical gradient need to be taken into account for the movement of charged particles. In this case, because of increased K+ in the matrix, the electrical gradient is disrupted, even though the chemical gradient is still maintained.

As a result of a decreased electrical gradient, there will be accumulation of H+ in the intermembrane space, which would decrease the pH. What would ultimately happen is that eventually, a balance will be reached where the chemical gradient becomes sufficiently high enough to overcome the electrical barrier. However, by this point there would be an increased [K+] in the matrix and increased [H+] in the intermembrane space. Thus, answer choice C makes sense.

Hope this helps.

 

[BerkReviewTeach]

There was no answer listed in the handout.

Are you sure you don't have the answer explanations somewhere? The explanations are discussed in lecture and the answers are (usually) left on the board until the next lecture. Also, the teacher passes out the detailed explanations to most of those questions in their next lecture (the idea being that it forces you to review the question before reading the explanation).

Luckily, everything that needs to be covered has been explained in this thread.