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  • - [Voiceover] When talking about carbohydrate metabolism

  • we can't forget to mention the pentose phosphate pathways.

  • So, where does the pentose phosphate pathway fit into

  • the breakdown of glucose?

  • So, 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'm drawing here to symbolize it 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 ATP.

  • 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 no ATP is consumed or produced.

  • That's kind of unique, to point out.

  • So, where does it fit in to this overall pathway?

  • It turns out that 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, turns out that as glucose begins

  • to go through glycolysis, some of it is shunted away

  • to become the pentose phosphate pathway.

  • So, glucose continues to be broken down,

  • but it continues to be broken down

  • to produce different products than it would

  • if it continued through going 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 pentose 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 pentose sugar.

  • So, pentose is just another word for five-carbon sugar,

  • and the particular name of this sugar

  • is ribose-five-phosphate.

  • And this sugar, so it's a 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.

  • So, in the case of DNA, it's deoxyribose,

  • and in RNA, it's just ribose.

  • But, in either case, this ribose-five-phosphate

  • is an important precursor to creating DNA and RNA,

  • so, quite a crucial molecule.

  • Now, the second primary product of this reaction,

  • as this phosphate nicely implies,

  • is a phosphorylated molecule that is usually abbreviated

  • as N-A-D-P, P standing of course for the phosphate

  • in this molecule, H.

  • NADPH.

  • So, this is not to be confused with the NADH, which,

  • if you recall, I'll go ahead and actually draw that in here,

  • if you recall, NADH is actually produced

  • in cellular respiration during the breakdown of glucose.

  • So, this produces NADH, which, of course, contributes

  • 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 cell,

  • so, NAD-plus we know is inter-converted with NADH,

  • and NADP-plus is inter-converted with NADPH.

  • So, 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-plus and NADP-plus.

  • But, what's different about these two pairs of molecules

  • is the relative amount of the reduced form

  • and the oxidized form inside the cell.

  • So, just to give you a sense of that,

  • the ratio of NAD-plus to NADH is about 1000.

  • In other words, if you took the amount of NAD-plus

  • and divided it by the amount of NADH in the body,

  • you would have about 1000 times more NAD-plus.

  • On the other hand, if you took the amount of NADP-plus

  • divided by the amount of NADPH, you would get 0.1.

  • So, essentially what this is telling us is that

  • there is a lot of NAD-plus in the body

  • and a lot of NADPH in the body, but not much

  • of NADH or NADP-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-plus present in the body,

  • most of the NAD-plus will want to accept electrons.

  • And, of course, the biggest role in accepting electrons

  • comes in the breakdown of glucose

  • and producing NADH, 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, so I'm gonna go ahead

  • and write that here.

  • So, the biggest role of NADPH in the body

  • is 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 them.

  • Now, I will remind you that donating electrons

  • is really important in anabolic reaction.

  • 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, I'll briefly mention that NADPH

  • also uses its reducing power, its ability

  • to donate electrons, to maintain the store

  • of 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 are 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 their 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.

  • So, 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 hone 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 phases

  • of the pentose phosphate pathway.

  • So, the first is called the oxidative phase

  • and the second is called the non-oxidative phase.

  • And, you know, as the name implies,

  • oxidative phase we're oxidizing.

  • So, remember that breakdown of glucose,

  • breakdown of carbohydrates,

  • is an oxidative process in general.

  • And, in this phase, the big idea here is that

  • we are producing NADPH, so that is

  • the big product of the oxidative phase.

  • So, we actually start out with glucose-six-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 out with glucose.

  • So, glucose enters glycolysis and some of it

  • will continue through cellular respiration,

  • but the other part of the glucose will then be shunted

  • through this glucose-six-phosphate into

  • the oxidative phase of the pentose phosphate pathway.

  • And, glucose-six-phosphate is then broken down

  • in a series of steps which aren't entirely important,

  • but the key idea here is that

  • you're producing NADPH along the way.

  • Now, the non-oxidative phase starts with this

  • molecule called ribulose-five-phosphate,

  • and it's really not important to know except

  • the fact that it kind of sounds like ribose-five-phosphate,

  • right, which I mentioned before was one of

  • the main primary products of the pentose phosphate pathway

  • and indeed, it's a precursor for the ribose-five-phosphate.

  • So, let's see how that happens.

  • Let's go ahead and scroll down here.

  • So, ribulose-five-phosphate is actually broken down

  • by an enzyme, an isomerase.

  • So, it's essentially switching around the molecule.

  • It's not really changing the chemical formula,

  • but it's switching around the structure

  • to ribose-five-phosphate.

  • So, that's key.

  • So,remember that's one of our main products

  • of the pentose phosphate pathway.

  • So, another key point of the non-oxidative phase,

  • so we produce, of course, ribose-five-phosphate.

  • Another key point here is that we're also able

  • to interconvert various sugars,

  • so interconvert sugars.

  • And why is this important?

  • This turns out to be really handy for the cell,

  • because notice here that there are some products,

  • like fructose-six-phosphate

  • and glyceraldehyde-three-phosphate

  • and fructose-six-phosphate that you might be familiar with

  • that come from glycolysis.

  • And, remember that these are not all five-carbon sugars,

  • right, you know that glyceraldehyde-three-phosphate

  • is actually a three-carbon sugar.

  • So, the ability to interconvert sugars through enzymes

  • like the transaldolase and the transketolase

  • will essentially allow cells to produce

  • more ribose-five-phosphate for DNA

  • and RNA synthesis if needed.

  • And, we do want to say this with one caveat

  • which is although the glycolytic intermediates

  • can be reinter-converted into ribose-five-phosphate,

  • they cannot go all the way up the pathway

  • to glucose-six-phosphate.

  • So, these oxidative phase reactions are irreversible.

  • So, shown by kind of the unidirectional arrow,

  • but the non-oxidative phase, of course,

  • allows interconversion and hence is kind of thought of

  • as more of a reversible pathway.

  • 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's no ATP consumed

  • or produced in this cycle, so that's

  • what makes the pentose phosphate pathway unique.

- [Voiceover] When talking about carbohydrate metabolism

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B2 中高級 美國腔

磷酸戊糖途徑 - 環狀結構和同分異構體|生物分子|MCAT|可汗學院 (Pentose phosphate pathway - Cyclic structures and anomers | Biomolecules | MCAT | Khan Academy)

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    Jerry 發佈於 2021 年 01 月 14 日
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