There’s been a lot of attention paid to PPE, but not enough to PPP (Pentose Phosphate Pathway)! You know how podcasts, TV shows, etc. sometimes do “extras” where they share content they couldn’t fit into the show? Well, today’s post is kinda like that because I have some more details about G6PD (Glucose-6-Phosphate Dehydrogenase) & the PPP  cuz it’s too cool not to tell you. Apologies in advance if it’s too advanced a topic for some people, but I really am a serious, hard-core scientist and these nerdy details fascinate me…

So, we’ve talked a lot about carbohydrates (aka sugars). The “generic” sugar our bodies use a lot is glucose (aka dextrose because doctors like to confuse us with names like dextrose instead of glucose and cc instead of mL…). Glucose gets into our cells through transporters that are regulated by insulin. Once it gets in, our cells need to keep it in. So they stick a phosphate on it. This provides a negative charge that keeps it from diffusing out through the lipid membrane surrounding our cells. 

The phosphate doesn’t just get stuck on randomly – instead, like most everything in biochemistry, it involves enzymes, which are usually proteins, sometimes RNA or protein/RNA complexes, that facilitate & speed up reactions by doing things like holding reactants in place, etc. Enzymes are specialized to carry out very specific tasks and the enzymes in this case specialize in transferring a phosphoryl (PO₄³⁻) group from ATP onto “position 6” of glucose to give you Glucose 6 Phosphate (G6P). 

And now you have options… Because although enzymes tend to be super specialized, there are different ones this G6P can go to next. And then those enzymes do different things and they can move on to different different enzymes that do different different things, and various outcomes each pathway brings!

This is the beauty of metabolism, a fancy word for the making (anabolism) & breaking down (catabolism) of molecules – there are sooooo many interconnected routes. Usually when you first learn a metabolic pathway, it’s in isolation. So you learn that G6P goes into a pathway called glycolysis where it gets broken down into pyruvate which enters the citric acid cycle. Those pathways produce ATP & NADH, and those NADH’s enter oxidative phosphorylation where their reducing power (ability to transfer electrons) is used to power a molecular motor that makes ATP. 

So, you finally nail all that down and then you move onto the next chapter and WTF?! What the hell is the Pentose Phosphate Pathway (PPP)? You’re telling me that the G6P doesn’t always go into glycolysis? Yup, an alternative fate for G6P (that original trapped glucose) is to get oxidized by glucose 6 phosphate dehydrogenase (G6PD) to give you glucose-6-phosphogluconate. This reaction has the added benefit of reducing NADP⁺ to NADPH. This NADPH is one of the key outputs of the PPP – the other is ribulose-5-phosphate (which can be used to make DNA, RNA, etc.)

WARNING! Do NOT confuse NADH & NADPH. The “P” is for phosphate. This phosphate doesn’t affect its reducing ability, so NADH & NADPH both have 1 reducing equivalent (can donate 1 electron), but what that phosphate *does* do is it allows cells to regulate NADH & NADPH separately. And this allows them to specialize in different functions. NADH (made by glycolysis & other catabolic processes) specializes in catabolism and is needed to break things down. NADPH (made by the pentose phosphate pathway) specializes in anabolism and is needed to build things up – it’s needed for the biosynthetic pathways used to make fatty acids, cholesterol, & steroid hormones.  

NADPH is also needed to regenerate the antioxidant glutathione (more on this below). Therefore, it’s really important for controlling oxidative stress (making sure Reactive Oxygen Species (ROS) – basically super energetic oxygen-y things) don’t build up & attack our proteins, lipids, DNA, etc. And people with G6PD deficiency can have problems with oxidative damage to their red blood cells upon certain triggers like eating fava beans, taking certain medications, or having severe infections. A lot of people have it (especially Black males) but don’t even know, and usually it’s no big deal, but it can be. And I think it should be researched more, but not enough attention is paid to things that mainly affect minorities… 

Anyways, why would you want to be able to regulate NADH & NAPH separately? Think about what form you want to keep stores of each in. So we said that we want NADH for catabolism (breaking down molecules). Catabolism typically involves oxidation – a way to remember this is thinking about “burning energy” – “burning” is combustion which is a form of oxidation (loss of electrons)- you can keep things from burning if you keep out oxygen, such as by putting a lid over a stovetop fire. So, when we break down molecules, we’re often oxidizing them. You can’t have oxidation (loss of electrons) without reduction (gain of electrons) because those electrons you’re losing have to go somewhere. In many cases, these electrons go to NAD⁺, which gets reduced to NADH. So, in order for the oxidation to occur, you need NAD⁺ and you want to keep a high NAD⁺/NADH ratio in your cells. 

But with anabolism, you want the opposite. You want to be able to reduce molecules. So you need something that can get oxidized. So you want the reduced form of your electron carrier, which in this case is NADPH. So you want to keep a high NADPH/NADP⁺ ratio.

By having that extra phosphate distinguishing NADPH & NADH you’re able to keep these oppositely skewed populations! And you’re able to separately control when you want to make stuff and when you want to break stuff. 

Since NADH is made through glycolysis (the pathway by which glucose is broken down to make energy and recycle piece), etc. & NADPH is made by the pentose phosphate and both of those start with G6P (glucose-6-phosphate), determining which pathway it goes into is a sort of values choice. If you want energy, go glycolysis, but if you want ribose, deoxyribose, or other sugars, if you want lipids & steroid hormones – or if you want NADPH, go PPP. 

Which way it goes depends in part on how much NADPH there is. NADPH inhibits G6PD. This negative feedback is good because. If you have a lot of NADPH you probably don’t need more. Without G6PD carrying out the first PPP step, the G6P gets used for glycolysis instead. When there’s more NADP⁺ than NADH however you want to make more. So NADP⁺ stimulates G6PD to work even better. 

As I briefly mentioned, NADPH is important for regenerating glutathione and glutathione acts as a sort of “shock absorber” for ROS. What glutathione does is it plays a sacrificial role – it lets the ROS attack it instead, neutralizing the threat. But then this now-oxidized glutathione has to get reduced (“un-oxidized”) before it can sop up ROS again. And NADPH does this reducing. Glutathione is a tripeptide that can exist as a monomer or a dimer – the dimerization happens through the oxidation of cysteines to form crosslinks between the 2 glutathione. So, you basically have the following situation:

reduced glutathione (-GSH HSG-) + ROS (with help from glutathione peroxidase, etc.) -> oxidized glutathione  (-G-S-S-G-) + non-ROS (water, etc. depending on type of ROS)

oxidized glutathione (-G-S-S-G-) + NADPH  (with help from glutathione dehydrogenases)-> reduced glutathione (-GSH HSG-) + NADP⁺

at this point our glutathione is ready to go again, but the NADP⁺ is not. Which is why we need the PPP to keep regenerating NADPH.

A cool thing about PPP is that you can go a couple of ways with it. The first few steps are referred to as the oxidative phase – they start with G6PD oxidizing G6P to give you a lactone (that’s where you have a carbonyl (C=O) next to another oxygen in a circle) and NADPH. This lactone, 6-phosphogluconolactone, is then further oxidized (by a different enzyme) to give you ribulose-5-phosphate. And then you can go a couple of ways with that. You can shift around the carbonyl to get ribose 5-phosphate which you can use to make nucleotides for DNA & RNA & coenzymes like FADH₂ & coenzyme A, etc. This route is used extensively by cells that are dividing rapidly and thus need to keep copying their DNA. 

But sometimes cells don’t need more of those things. And instead they just want to get as much NADPH out of that G6P as possible (this is the case for cells under oxidative stress and/or needing to make a bunch of those fatty acid type things). Instead of converting ribulose 5-phosphate to ribose 5-phosphate, they take multiple copies of it and shift the carbons from one to another in a series of transketolase & transaldolase reactions that produce G6P again. This is called the non-oxidative phase. You don’t get any more NADPH in this part, but in 6 cycles, you turn 6 ribose 5-phosphates (which are 5 carbon each) into 5 G6Ps (which are 6 carbon each). And each of those G6Ps can be used to make 2 more NADPH. 

So, yup, there are a LOT of decision points in metabolic pathways and instead of pathways their more like giant webs. I remember we had this metabolic pathway chart on the wall of my biochem classroom in undergrad and I found it sooooo intimidating. But now I think it’s really cool! 

Sorry that was kinda rambly and weird, I just like to share thoughts that help me understand things in case they can help other people understand things too. Because I’m definitely not one of those people who thinks that biochem (or any class) should be used to weed out any students. The point of teaching is to teach people – anyone who wants to learn. And I want to do whatever I can to help people succeed. I really hope that I make my dream of becoming an undergrad prof some day, but, til then thanks for letting me practice on you all!

more on glycolysis:

more on NADH: 

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉 

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