Glycolysis Irreversible Step

[MedPR]

I remember learning that up until a certain point the intermediates of glycolysis can go backwards. I'm trying to figure out which step marks the end of reversibility.

From what I'm reading from wiki, it seems like the irreversible step is the formation of Fructose-1,6-biphosphate from fructose-6-phosphate, which is catalyzed by phosphofructokinase. In other words, all fructose-1,6-biphosphate in the body are destined to move further along in glycolysis.

This is the third (?) step, and also the step where the second ATP is burned.

Since we're on the topic 🙂

One ATP is consumed at step1 and another at step 3 and two ATP are produced at step 6 and two more at step 9.

Also, I'm assuming the color coding in this image is of some significance. I found it online with no associated text or explanation. Can anyone take a guess at why certain steps are colored the same way? I'm especially curious about the area in the pink box...

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[chiddler]

speaking of clueless bio discussions

fructose with one phosphate to fructose with 2 phosphates is the determined step. "Is practically irreversible" according to TPR bio book. the enzyme that catalyzes it is PFK, phosphofructokinase. might be worth remembering.

i spent a bit of time trying to figure out why they are color coded but i've no idea.

 

[MedPR]

speaking of clueless bio discussions

fructose with one phosphate to fructose with 2 phosphates is the determined step. "Is practically irreversible" according to TPR bio book. the enzyme that catalyzes it is PFK, phosphofructokinase. might be worth remembering.

i spent a bit of time trying to figure out why they are color coded but i've no idea.

Hmm, ok thanks for verifying the part about the irreversible step. I was thinking the last two color groupings had something to do with the fact that the products are now 3C instead of 6C.

Still not really sure though.

 

[mcloaf]

You're correct in that once you get through phosphofructokinase you're committed to glycolysis. One thing that makes this easier to see is writing the free energies of all the reactions in the pathway. In glycolysis, as in the rest of metabolism, most of the reactions are easily reversible. What keeps things moving forward is a small subset of highly favorable reactions, such as this one, that pull forward the whole process. This makes it much easier to regulate the pathway as a whole by only altering one or two enzymes, and I believe helps maximize the energy you get during catabolism. Keep in mind that the first step is essentially irreversible (outside of the liver), but since you can still put that G-6-P into glycogen, you aren't yet committed to glycolysis.

I hope that helps a little bit. I really don't know about the color coding. I had the same thought about 6C vs. 3C, but that doesn't totally explain it. Not sure. :/