glycolysis question

[TypeSH07]

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I'm doing the EK Bio 1001 questions and it says that increased blood glucose will decrease glycolysis? I thought that if you had more glucose available for breakdown then there would be increased glycolysis. Am I missing something? Thanks.

 

[Bacchus]

If you have an increase in glucose then the hormone insulin will initiate glycogen synthesis.

Or so biochem tells me.

Also, I believe the main regulatory point of glycolysis, enzyme PFK, will face negative feedback if an excess of ATP is generated.

Edit: synthesis not uptake.

 

[coffeebythelake]

I'm doing the EK Bio 1001 questions and it says that increased blood glucose will decrease glycolysis? I thought that if you had more glucose available for breakdown then there would be increased glycolysis. Am I missing something? Thanks.

Increased blood glucose should stimulate glycolysis and glycogenesis.

 

[RSAgator]

I'm doing the EK Bio 1001 questions and it says that increased blood glucose will decrease glycolysis? I thought that if you had more glucose available for breakdown then there would be increased glycolysis. Am I missing something? Thanks.

I would say that in the presence of too much glucose the body doesn't produce more energy, but rather it stores the excess glucose. Thus in the presence of too much glucose the glucose would be stored as glycogen and fat. The only way I could justify an increase in glycolysis is that the main component in fatty acid synthesis is acetyl CoA, a product of the pyruvates formed via glycolysis. It's important to note that there is very little energy produced from glycolysis alone, so in this case glycolysis is just a necessary pathway on the way to fatty acid synthesis.

 

[Vihsadas]

Let me re-edit my response to more accurately reflect what happens in the body. I guess I think that this question is probably just asking for: "Yes, too high blood glucose levels inhibit glycolysis and stimulate glycogen and fatty acid synthesis, but moderate levels of glucose will stimulate glycolysis". For that reason, I'm not convinced that you'd get this on MCAT because the answer is not that simple.

Some facts:
- ATP and F-1,6-BP are negative regulators of Pyruvate Kinase. So while excess blood sugar will initially stimulate gylcolysis, it will quickly cause an excess of F-1,6-BP, thereby inhibiting Pyruvate kinase, and thus glycolysis as a whole.

- ATP is also a negative regulator of phophofructokinase (which is the enzyme responsible for forming F-1,6-BP). This is also the major regulatory point of glycolysis and will have an inhibtory effect on glycolysis.

- An increase in glucose will also cause an increase in F-2,6-BP which is the major regulatory molecule of the phosphofructokinase step. This would lead to a stimulation of glycolysis.

The major regulatory steps in glycolysis are hexokinase/glucokinase, phosphofructokinase and pyruvate kinase. The product of the phosphofructokinase step is fructose-1,6-bisphosphate (F-1,6-BP) which comes from the phosphorylation of F6P. F6P also forms F-2,6-BP in a side reaction of glycolysis which is a stimulator of F-1,6-BP production. F-1,6-BP is in turn a major negative regulator of the next irreversible step in glycolysis which is the final step: pyruvate kinase (which catalyzes the formation of pyruvate).

So whether or not high blood glucose stimulates or inhibts glycolysis is dependent on just how high the energy charge of the cell is. Without knowing just how much ATP there is and how much glucose there is, I'm not sure that we can say for sure whether glycolysis will be inhibited or stimulated.

If there was a question on the MCAT that asked, 'Does glucose inhibit glycolysis?' the answer is debateable depending on how you interpret the question. There is no allosteric site for glucose to directly regulate glycolysis, but it is a high concentration reactant for HK/GK which will drive the first step of glycolysis toward production of G6P. The eventual products of this reaction both stimulate and inhibit glycolysis as we've outlined above.

If I was asked this question on the MCAT and no answer said, "Glucose stimulates glycolysis but only to a point", I would be very hesitant about answering it. The answer I would guess the MCAT would be looking for might be 'Yes glucose inhibits glycolysis and stimulates glycogen synthesis and fatty acid synthesis.' I would, however, be pretty weary of my answer and not very happy about the question. Hope this helps, (and I hope I just didn't confuse you more. 🙁 )

 

[Kaustikos]

I agree completely. That question bothered me because of how it was worded, hence the hesitation in answering it. But then again, the MCAT has been known to ask questions worded like that before....to me anyways.
It's all relative is my answer. Glycogen synthesis/insulin secretion is all modulated by blood glucose levels, but so are glucagon and somatostatin levels.
Could we get the exact question and answer choices?


And wtf, Vihsadas, are you taking a biochemistry course or do you just know that at the top of your head? I completely forgot about Fructose 1-6 BP as a negative regulator.

 

[Vihsadas]

I agree completely. That question bothered me because of how it was worded, hence the hesitation in answering it. But then again, the MCAT has been known to ask questions worded like that before....to me anyways.
It's all relative is my answer. Glycogen synthesis/insulin secretion is all modulated by blood glucose levels, but so are glucagon and somatostatin levels.
Could we get the exact question and answer choices?


And wtf, Vihsadas, are you taking a biochemistry course or do you just know that at the top of your head? I completely forgot about Fructose 1-6 BP as a negative regulator.

Haha. I took biochem last semester. 😛

 

[BloodySurgeon]

And wtf, Vihsadas, are you taking a biochemistry course or do you just know that at the top of your head? I completely forgot about Fructose 1-6 BP as a negative regulator.

I was thinking the same... 🙂 You sly dog Vihsadas... making us think we have bad memories for not remembering every step of glycolysis/kreb cycle and what not.

 

[Bacchus]

In honesty, is all that actually necessary Vih? I know its good to be covered but I don't imagine having to know all that for the MCAT 😉. Heck, I don't think any review course will cover that in depth. Heh, my answer was from biochem but I guess it is the short and sweet answer.

 

[Vihsadas]

In honesty, is all that actually necessary Vih? I know its good to be covered but I don't imagine having to know all that for the MCAT 😉. Heck, I don't think any review course will cover that in depth. Heh, my answer was from biochem but I guess it is the short and sweet answer.

No, it's definitely not MCAT relevant, but I wanted to demonstrate why I don't think that a question like that would be asked on the MCAT in the first place. It would be an unfair question to ask an MCATee because they aren't given enough info to answer it. 🙁

 

[Kaustikos]

No, it's definitely not MCAT relevant, but I wanted to demonstrate why I don't think that a question like that would be asked on the MCAT in the first place. It would be an unfair question to ask an MCATee because they aren't given enough info to answer it. 🙁

Agreed. This is simply a badly worded question, in my opinion.

And now I reremember the inhibitory mechanism with F-1,6-BP.😎

 

[TheGreatHunt]

Increased blood glucose should stimulate glycolysis and glycogenesis.

Wrong, increased glucose in the system would make your body sense that there is a lot of glucose, telling your body that "I have plenty of food, store it" so your body stores it as a growing part of a glycogen molecule. When it is starving, it will undergo glycolysis(Glycogen-lysis) which frees GLUCOSE so you can use it for energy.

 

[RSAgator]

Wrong, increased glucose in the system would make your body sense that there is a lot of glucose, telling your body that "I have plenty of food, store it" so your body stores it as a growing part of a glycogen molecule. When it is starving, it will undergo glycolysis(Glycogen-lysis) which frees GLUCOSE so you can use it for energy.

Glycolysis is the breaking down of 1 molecule of glucose into 2 molecules of pyruvate. Glycogenolysis is the breaking down of glycogen into glucose. I thought the question was pretty accurately covered in the previous posts, did you even read past the first one? There is no question here that the effect of excess glucose in the blood is for the body to move in the direction of less glucose, the only confusion arose in how glycolysis is involved in the process.

 

[coffeebythelake]

Wrong, increased glucose in the system would make your body sense that there is a lot of glucose, telling your body that "I have plenty of food, store it" so your body stores it as a growing part of a glycogen molecule. When it is starving, it will undergo glycolysis(Glycogen-lysis) which frees GLUCOSE so you can use it for energy.

You are wrong. You realize that glycolysis is a way of creating triglycerides, right? As in the storage molecule found in adipocytes.

 

[Bacchus]

You are wrong. You realize that glycolysis is a way of creating triglycerides, right? As in the storage molecule found in adipocytes.

Wait, you mean that the common intermediates to life (pyruvate, acetyl CoA, and G6P) do more than just participate in glycolysis?

 

[TheGreatHunt]

Oh wow is that a shameful thing I wrote... hahah. Thanks for the rips, I deserved it 😉

I somehow confused Glycogenolysis with glycolysis??? LOL shows what happens after you take the MCAT... memorize and dump =P