Pentose Phosphate Pathway Help

[KoalaT]

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Okay so this is something I am not very familiar with. It seems extremely freaking complicated in all the interchanging of molecules into each other, so I would like some help sorting through all the information and knowing what's important. What do I really need to know here?

Here's what I think is important so far:
- Generates NADPH in the oxidative phase (needed for fat anabolism and to eliminate dangerous oxides)

- Becomes Ribose-5-P in the end of the oxidative phase/beginning of non-oxidative phase. This can be used for DNA and RNA.

- It can interconvert between a bunch of different carbon molecules to become other carbon molecules (they always add to 10C). This is performed by Transketolase and Transaldolase. Besides ribose-5-P, one's worthy of noting are as follows:
- - 3C Glyceraldehyde-3-P for glycolysis
- - 3C DHAP for glycolysis
- - 6C Fructose-6-P for glycolysis
- - 4C erythrose-4-P for amino acid anabolism

What I really seem to be missing is the overarching conditions that decide which direction the PPP decides to go. It would seem to me that if you need energy, you would stop doing the oxidative phase (conserve Glucose-6P) and start converting the ribose-5P toward the glycolysis products. If you are energy plentiful and want to conduct anabolistic mechanisms, you would use the oxidative pathway to create Ribose-5P and NADPH to create DNA/RNA and build up fat cells. You would also want to shift all the glycolysis intermediates toward Ribose-5P. Is this correct? Cause I'm just going purely on what makes sense here.

What is truly important in all this information? Cause it seems like a ton to keep up with and I'm jumbling it all up in my head.

 

[KoalaT]

This is basically what I use to think of it.

Is it correct to say the following?
If you need energy, then you move AWAY from Ribose-5P and toward G6P and glycolysis intermediates.

If you need to build fats/DNA, move towards Ribose-5P and away from glycolysis intermediates.

 

[Necr0sis713]

I think you're right, although i'm not saying i'm 100% sure. I haven't really nailed the PPP yet, but the overall impression I got is that it's an anabolic pathway, so like you said, you would use it to make Rb5P and NADPH to build up fat stores.

Now, since anabolic and catabolic pathways reciprocally regulate each other, it would be plausible to say that when you are energy depleted the pathway wouldn't go in its forward direction. I'm not sure whether it would use the Rb5P toward glycolysis products. What I do know, is that when you are energy depleted you would use glycogen or fat (via ketones) for energy, but i'm not sure about the Rb5P. So with that logic I would say that the G6P just gets used for energy and doesn't enter the PPP

So...to reiterate: The G6P has an option, if you are energy depleted it will be sent forth via glycolysis for ATP. If you have energy surplus, it gets shunted to the PPP.

 

[theonlytycrane]

I think you have it down!

p3 -> generates NADPH for biosynthesis (oxidative phase) and ribose for DNA / RNA from ribulose-5-p. The non-oxidative phase converts ribulose-5-p to glycolytic intermediates (f6p and g3p).

NADPH generation and continued energy generation can occur if you go through both the oxidative phase, non-oxidative phase, and continue on with glycolysis. You can also go backward through the non-oxidative phase to generate ribose if you have enough energy, do not need NADPH, and need to generate DNA / RNA.