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Pentose phosphate pathway


What is the Pentose Phosphate Pathway? Why is it important?
Created by Jasmine Rana.
Video transcript
when talking about carbohydrate metabolism we can't forget to mention the pentose phosphate pathway so where does the pentose phosphate pathway fit into the breakdown of glucose let's go ahead and review the breakdown of glucose as we normally kind of usually conceive of it as so we go ahead and start out with glucose which I am drawing here just simple I thing with a six-carbon sugar backbone and we usually imagine that glucose begins to be broken down in the cytosol of the cell through a series of reactions that we call glycolysis and then of course it goes through the krebs cycle in the mitochondria also known as the TCA cycle and then finally it goes to the electron transport chain in the mitochondria to produce a teepee so that's kind of usually the end product we think of when we think about breaking down glucose but the pentose phosphate pathway is kind of a unique pathway because it turns out that in this pathway know ATP is consumed or produce the kind of unique to point out so where is it fit into this overall pathway it turned up the linear way I've written cellular respiration is actually only partly true it's a great way to conceptualize it but there are many branches or kind of side reactions that are taking place almost simultaneously with the breakdown of glucose and the pentose phosphate pathway is one of these so it turns out that glucose begins to go through glycolysis some of it is shunted a way to become the pentose phosphate pathway the glucose continues to be broken down but it continues to be broken down to produce different products than it would if it continued through glycolysis and krebs and then to the electron transport chain so as you can see I've written pentose phosphate pathway kind of suggestively by highlighting Pinto and phosphate in different colors to point out to you that there are two primary products in this pathway so the first is the production of a five-carbon Pentos grrrrrr so Pentos is just another word for five carbon sugar and the particular name of this sugar is ribose 5-phosphate and this sugar so the five carbon sugar i'll go ahead and draw that to remind us of that is an important substrate in producing DNA and RNA so if you remember DNA and RNA contain nucleotides and the nucleotides contain a nitrogenous base a phosphate group and a five carbon sugar in the case of DNA is deoxyribose and an RNA it's just ribose but in either case this ribose 5-phosphate is an important precursor to creating DNA and RNA so quite a crucial molecule now this second primary product of this reaction as this phosphate nicely implies a phosphorylated molecule that is usually abbreviated as n a dpp-4 ending of course for the phosphate in this molecule each nadph so this is not to be confused with the nad ph which if you recall go ahead and actually draw that in here if you recall nadh is actually produced in cellular respiration during the break down glucose so this produces and a G H which of course contribute electrons to the electron transport chain so of course the question you might have in your mind is how is nadh different from the easily confused nadph because they sound like similar molecules and in many ways they are so they actually both exist in pairs inside the cells so an ad plus we know with interconverted with nada h and an ad p plus is interconverted with an a dph of course the H forms of these molecules are the reduced form of these molecules and the plus or oxidized form of these molecules are the NAD+ than any DP plus but what's different about these two pairs of molecules is the relative amount as a reduced form and the oxidized form inside the cell so just to give you a sense of that the ratio of any g + to nadh is about a thousand in other words if you took the amount of NAD+ and divided it by the amount of nadh in the body you would have to about 1,000 times more nad+ on the other hand if you took the amount of NAD+ any DP plus divided by the amount of nadph you would get . one so essentially what this is telling us is that there's a lot of NAD+ in the body and a lot of nadph in the body but not much of nadh or any DP plus and knowing this actually helps me remember and differentiate between the role of nadh and nadph inside the body so first I reason out to myself that if there's a lot of NAD+ present in the body most of the NAD+ will want to accept electrons and of course the biggest role in accepting electrons comes in the breakdown of glucose and producing any th so that makes sense on the other hand the primary role of nadph which is what we have the majority of is to donate electrons i'm going to go ahead and write that here so the biggest role of nadph and them in the bodies to donate electrons and that of course would not be very helpful in breaking down glucose right because the breakdown of glucose donates electrons it doesn't accept that now i will remind you that donating electrons is really important in anabolic reactions so remember that anabolic reactions involve building up molecules such as in the synthesis of fatty acids for example and so nadph plays a vital role in kind of providing this reducing power so to say for these anabolic reactions in addition of briefly mention that nadph also uses its reducing power its ability to donate electrons to maintain the store of aught antioxidants inside the body so you know kind of an ironic part about having oxygen as a requirement for cellular respiration is that some of this oxygen can become really reactive if it gains an extra electron and so the goal of kind of some of the molecules in your body or to serve as antioxidants to kind of trap these reactive oxygen species from reacting with important things in your body like DNA or proteins and so once they do that of course some of these antioxidant molecules in the process of reacting with a reactive electron rich oxygen molecule become oxidized and so of course nadph can come in and save the day by donating electrons to reduce the oxidized form of these antioxidants back into the reduced form so that they can again react with any rogue reactive oxygen species alright so now we're ready to look at the pentose phosphate pathway in more detail i'm going to go ahead and bring up a diagram of how the pentose phosphate pathway is usually represented in most textbooks and this is a lot of detail admittedly and I don't want you to get lost in the details so I'm going to try and break it down and hold your attention to the most important details to take away from this so the first of these important details is to note that there are two big pieces of the pentose phosphate pathway so the first is called the oxidative phase and the second is called bean on oxidative phase and you know as the name implies oxidative faith were oxidizing so remember that breakdown of glucose breakdown of carbohydrates is an oxidative processes in general and in this phase the big idea here is that we are producing and a d pH so that is the big product of the oxidative phase so we actually start out with glucose 6-phosphate here so just note that we start off with this molecule here which I'll remind you is one of the first metabolites that's produced in glycolysis so this is essentially shunted from glycolysis which of course starts at with glucose to glucose enters by collis and some of it will continue through cellular respiration but the other part of the glucose will then be shunted through this glucose 6-phosphate into the oxidative face of the pentose phosphate pathway and glucose 6-phosphate is then broken down into a series of steps which are entirely important but the key idea here is that you're producing nadph along the way now the not oxidative face starts with this molecule called triple O's 5 phosphate and it's really not important to know except for the fact that it kind of sounds like ribose 5-phosphate right which I mentioned before was one of the main primary products of the pentose phosphate pathway and indeed it is a precursor for the ribose 5-phosphate let's see how that happens let's go ahead and scroll down here so Regulus 5-phosphate is actually broken down by a enzyme hey I summary so it's essentially switching around the molecules not really changing the chemical formula but it's switching around the structure to ribose 5-phosphate so that's key remember that's one of our main products of the pentose phosphate pathway now another key point at the non oxidative phase so we produce of course ribose 5-phosphate another key point here is that were also able to interconvert various sugars so interconvert sugars and why is this important turns out to be really handy for the cell because in notice here that there are some products like fructose 6 phosphate and glyceraldehyde 3-phosphate and fructose 6-phosphate that you might be familiar with that come from glycolysis and remember that these are not all five carbon sugars right you know glyceraldehyde-3-phosphate is actually a three carbon sugar so the ability to interconvert sugars through enzymes like the trans aldolase and trans Keeley's will eventually allow cell to produce more ribose 5-phosphate for DNA and RNA synthesis if needed and now we do you want to do this with one caveat which is although the glycolytic intermediates can be re interconverted into drivers 5-phosphate they cannot go all the way up the pathway to glucose-6-phosphate so with these oxidative these reactions are irreversible so shown by kind of the unidirectional area arrow but the non oxidative phase of course allows interconversion and hence is kind of thought of is more of a reversible path way so that in a nutshell is the pentose phosphate pathway and i'll return to the kind of main slide at the beginning and just remind you that the key takeaway is that we are producing a pentose sugar ribose and a phosphorylated molecule nadph in this pathway and that the most unique part of this pathway is that even though we classify it as part of carbohydrate metabolism because it utilizes the metabolites from the breakdown of glucose there is no ATP consumed or produced in this cycle so that's what makes the pentose phosphate pathway unique