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NS FL 10 P/C Q30

Posted: Mon Aug 12, 2019 12:43 pm
by mgarc805
I had the idea that an antioxidant is a molecule capable of scavenging electrons from reactive species as such I thought molecules that act as antioxidants gain electrons which is the definition of a reduction. NADH ---> NAD+ + e- hence NADH can only oxidize (since it will give away its electron). would you explain to me why NADH is consider an antioxidant thank you.

Re: NS FL 10 P/C Q30

Posted: Mon Aug 12, 2019 8:00 pm
by NS_Tutor_Mathias
Oxidation is the loss of electrons from a species, reduction is the gain of electrons in a particular species.

So while NADH itself is indeed most likely to be oxidized, when it does so, it will reduce some substrate. Likewise, the reduction of NAD+ to NADH is guaranteed to be oxidizing something else.

Therefore we expect all our reduced electron carriers to be anti-oxidants (if something does get oxidized, they can just donate their own electron(s) to it!), and the one oxidized species in the group to be pro-oxidative (it is likely to oxidize some other molecule in the cell).

With that in mind, I have a fun exercise for you: Answer choice A is also not quite correct. Could you tell me what the reduced form of answer choice A actually is?

Re: NS FL 10 P/C Q30

Posted: Tue Aug 13, 2019 2:41 pm
by mgarc805
Hahaha the trick answer will be to say ubiquinol but that isn't true either because when it gains the electrons it also gains the hydrogen in the alcohols which is why the molecule remains uncharged. ( you wanted to play with my mind lol) but I can still see why this molecule could act as an antioxidant because it still can get the electrons from the free radical.

but I am still little confused, NADH will oxidize (to NAD+) and will certainly donate that electron to another molecule which will be reduced. but wouldn't a molecule like FAD be able to gain the electrons from the free radical? that's why I don't understand why D ( the molecule that can in fact gain the electron is said to not act as an antioxidant) while NADH which cannot gain any electron is said to be a good antioxidant

Or are you saying that the electron that comes off the NADH is donated to the prooxidative species and that neutralizes it ?

Sorry if I am making it hard to explain

Re: NS FL 10 P/C Q30

Posted: Tue Aug 13, 2019 8:48 pm
by NS_Tutor_Mathias
mgarc805 wrote:
Tue Aug 13, 2019 2:41 pm
Hahaha the trick answer will be to say ubiquinol but that isn't true either because when it gains the electrons it also gains the hydrogen in the alcohols which is why the molecule remains uncharged. ( you wanted to play with my mind lol) but I can still see why this molecule could act as an antioxidant because it still can get the electrons from the free radical.
Naw, you're right. It is ubiquinol. Reduction is the gain of electrons in a species. Whether or not anything else is gained is irrelevant. Trust yourself, and in cases like these, try to hunt for a reliable definition of processes - the MCAT will weigh the ability to generalize concepts more heavily than memorizing individual facts or cases.
but I am still little confused, NADH will oxidize (to NAD+) and will certainly donate that electron to another molecule which will be reduced. but wouldn't a molecule like FAD be able to gain the electrons from the free radical? that's why I don't understand why D ( the molecule that can in fact gain the electron is said to not act as an antioxidant) while NADH which cannot gain any electron is said to be a good antioxidant
Let me denote hydroxyl radical with "*OH" real quick.

Imagine this:

FADH2 + 2 *OH --> 2 H2O + FAD

In this case, FADH2 donated both protons and electrons. So FADH2 was oxidized to FAD, and *OH was reduced to H2O.


And you can imagine that the presence of excess FAD is more likely to shift the equilibrium in the opposite direction, or at least not contribute to this sort of "clean-up" of free radicals. Yes, this is a little weird to think about, but the free radicals we care about are electrophilic.

Re: NS FL 10 P/C Q30

Posted: Wed Aug 14, 2019 11:25 am
by mgarc805
I see, so the definition of an antioxidant that you are proposing here is that: an antioxidant molecule is that molecule that can oxidize to donate its electrons to the reactive species (namely OH* in your example).

I have an example to bounce back at you:

+e^- +e^-
O2 -------> O * ------> O2-

so if we want to prevent the formation of O* or O2- which are the ROS we would need a molecule that can take those electrons from them and turn them back into oxygen, some molecule like FAD or Ubiquinone. because both can take the electrons from the specie they will both reduce.

I don't know if my example makes sense to you but this is what I thought when I was answering that question.

Also, specifically to the question (NS FL10 PC Q30)

A. Ubiquinone (oxidized)
C. NADH (reduced)
D. FAD (oxidized)

is options A and D are both in their oxidized states then why is one the answer and not the other ? if according to what you told me free radicals are electrophiles both answer choice A and D are electron deficient so none would be a good antioxidant.


I know this has been a long discussion but I am trying to wrap my head around this topic

Addendum

I also found a segment of a book that explains which molecules are antioxidants and they mention NAD, FAD, Quinones, Vitamins A and E

https://pdfs.semanticscholar.org/deb1/e ... 8c64ca.pdf

Let me know what you think

Re: NS FL 10 P/C Q30

Posted: Wed Aug 14, 2019 4:47 pm
by NS_Tutor_Mathias
Regarding the book:
Yes, those descriptions are correct. It is however explaining the whole system of how an antioxidant molecule works, including how it is regenerated. Notice that this is why the text describes a lot of redox pairs! Once for example G-S-H acts as an antioxidant, it itself is the molecule that becomes oxidized and forms the G-S-S-G dimer. That dimer is then reduced again at some point into 2x G-S-H and it is ready to act as an antioxidant again.

Similarly, that is why I pointed out that answer choice A should say ubiquinol, not ubiquinone.

Regarding your reaction:
I'm not sure what you're trying to describe here. Throw electrons at molecular oxygen and in most chemical environments you will generate peroxides, most commonly hydrogen peroxide. Don't confuse anions with free radicals: A free radical does not have to have a charge at all. In fact, aqueous anions are often stable and most emphatically not free radicals. A free radical is simply a species with an unpaired electron. Note that our most common free radical, hydroxide radical, has one LESS electron than the much more stable anion.

You can think of the free radicals we talk about as simply very strong electrophiles for that reason. And being strong electrophiles, they will oxidize SOMETHING. The purpose of a so-called "antioxidant" is to have a sacrificial molecule whose oxidation does not have detrimental effects on the cell, and ideally that antioxidant can be regenerated later. Glutathione is the most common such antioxidant and your first and main line of defense from oxidative stress.

Contrast this with what happens when for example an unsaturated fatty acid is oxidized instead: You get runaway lipid peroxidation, which is extremely damaging (one molecule starting a whole cascade!). Don't spend too much time on this mechanism, I've only presented this as a way of explaining why having sacrificial molecules as antioxidants is good and prevents a lot of trouble, and why "antioxidants" don't really prevent oxidation itself, they just change where it happens.