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Biological redox reactions

Redox reactions of alcohols in biological systems. Role of NADH/NAD+ in these reactions. Created by Jay.

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Video transcript

In this video, we're going to look at the biological redox reactions of alcohols in phenols. Over here on the left, we have the ethanol molecule. So this is our 2-carbon alcohol. And the carbon that we're most concerned with is this carbon right here, which has one bond to this oxygen atom. And in the liver, ethanol is oxidized to ethanal. So over here on the right is the ethanal molecule-- a 2-carbon aldehyde. And once again, we're concerned with that carbon in yellow. And so one easy way to tell that ethanol was oxidized to ethanal is to see that, on the left, we have one bond of that carbon to oxygen. And over here on the right, we now have two bonds of that carbon to oxygen. So an increase in the number of bonds to oxygen is oxidation. You could also assign oxidation states to this carbon. And you will see that there's an increase in the oxidation state of that carbon. And then, you could also think about electrons. LEO the lion goes GER-- loss of electrons is oxidation, gain of electrons is reduction. And so if I think about these electrons here in magenta, you can see that those electrons are lost from the ethanol molecule. So loss of electrons is oxidation, ethanol is oxidized. If ethanol is oxidized, something else must be reduced. That's how redox reactions work. What's reduced is NAD+ over here on the left. So this is NAD+, which stands for "nicotinamide adenine dinucleotide." The adenine is hiding in this R portion. And we have a nitrogenous-based ring with an amide functional group over here on the right for the nicotinamide portion of the molecule. Plus 1 formal charge on this nitrogen gives us NAD+. This is nicotinamide adenine dinucleotide-- NAD+. And since ethanol is oxidized, NAD+ must be reduced. So reduction means gaining of electrons. NAD+ is going to gain those electrons in magenta from ethanol. So if we think about a possible mechanism, if I took these electrons between the oxygen the hydrogen and moved them in here, that would form our double bond between the carbon and the oxygen. But there'd be too many bonds to this carbon right here. So the electrons in magenta are going to move to this carbon down here on NAD+, to this carbon. That would push these electrons over here, and that would push these electrons here off onto the nitrogen. So if we showed what happened with the movement of all of those electrons over here on the right-- this carbon right here at the top already had a hydrogen bonded to it. And it gained another hydrogen with two electrons. The two electrons were the ones in magenta right here. This hydrogen right here is this hydrogen. And the electrons in magenta move over there to our ring. And then, we would also have pi electrons moved over here. And then, we had a lone pair of electrons move off onto the nitrogen. Like that. And then, we still had some pi electrons over here on the right. This molecule is called NADH. So it's a gained the equivalent of a hydride-- hydrogen with two electrons. And so we can see that NAD+ gains two electrons. And gaining electrons is reduction. So NAD+ is reduced to NADH. Since NAD+ is reduced, it allows ethanol to be oxidized. And so we would refer to NAD+ as an oxidizing agent. It is the oxidizing agent for ethanol, even though it itself is being reduced. So that's something that confuses some general chemistry students sometimes. All right. So now over here, we have the NADH molecule. And this reaction is catalyzed by an enzyme, and the enzyme is alcohol dehydrogenase. OK. So this is catalyzed by the alcohol, dehydrogenase enzyme. Like that. And this reaction is reversible. So if we think about the reverse reaction, we think about ethanal being reduced to ethanol. And so if ethanal is reduced to ethanol, NADH would be oxidized to NAD+. And so let's think about a mechanism where we could oxidize NADH and reduce the ethanal. If I took this lone pair of electrons in the nitrogen and move it back in here, that would push these electrons off over here. And now, the electrons-- in magenta on this bond right here-- would attack this carbon right here. So the electrons-- in magenta-- we could think about the electrons as being right here. And you could think about that as being a hydride-- so a hydrogen with two electrons, giving it a negative 1 formal charge. And even though we've seen in some earlier videos that hydride isn't necessarily the best nucleophile. You could think about this as being a nucleophilic attack, if it makes it easier for you, because this carbon right here would be partially positive. The negatively charged electrons would attack that carbon. And in doing so, that would push these pi electrons off to then grab this proton here. And that would give you your ethanol molecule, and that would convert NADH back into NAD+. So you could think about NADH as being oxidized. It is losing two electrons-- the electrons in magenta. Loss of electrons is oxidation. And since NADH is the agent for the reduction of ethanal to ethanol, you would say that NADH would be the reducing agent for this example. And the best way to remember that NADH is the reducing agent is-- it is the one that has the hydrogen on it. So it has the hydride, which is capable of being the agent for the reduction. So therefore, NADH is the reducing agent. This NAD+, NADH conversion-- and vice versa-- is extremely important in biochemistry. This happens in numerous biochemical reactions. And so it's important to understand what's happening with those electrons on these molecules. Let's look at another biochemical example of redox. And here, we have on the left, phenyol. Right? So this is our phenol molecule. And once again, we're most concerned about this carbon, the one that's attached to this oxygen. And there are many ways to oxidize phenols. So if we oxidize phenol with something like the Jones Reagent-- with sodium dichromate, sulfuric acid, and water-- would be capable of oxidizing phenol to this molecule over here on the right, which we call "benzoquinone." This right here is a benzoquinone molecule. And just real fast, you could see that this carbon right now has two bonds of carbon to oxygen so it has been oxidized. So phenol can be oxidized to benzoquinone using numerous organic reagents. Once you make benzoquinone, you could reduce that to this molecule over here on the right, which is called "hydroquinone." There are several, again, organic reagents that can reduce benzoquinone to hydroquinone. Let me change that spelling there. And then, from hydroquinone, you could oxidize hydroquinone back to benzoquinone pretty easily. And so once again, in organic chemistry, there are lots of reagents that can do these redox conversions. And in the body, you're usually talking about the NAD+, NADH system. So we've just studied that. And if we look here at this molecule, you can see it's a quinone. Right? So you can see the benzoquinone portion of this molecule. And this is called "ubiquione." "Ubi" referring to the fact that this is ubiquitous. This compound is found everywhere. It's found in all the cells in nature. And the other name for this would be "coenzyme Q." This is a very important part of the electron transport chain. And if we look at ubiquinone-- going to this molecule over here on the right-- you can see this is like a hydroquinone analog here. So this is ubiquinol. These carbons are being reduced from this chemical reaction that I've drawn here. So ubiquinone is being reduced to ubiquinol. If ubiquinone is being reduced, something else must be oxidized. All right. So the NADH is being oxidized to NAD+. The NAD is the one that has this hydride on here, which can serve as the reducing agent. So here NADH is acting as the reducing agent-- the agent for the reduction of ubiquinone to the ubiquinol molecule over here on the right. And so this is just an oversimplification of part of the electron transport chain where you're transporting electrons, which eventually leads to oxidative phosphorylation and also ATP synthesis, which, of course, gives us energy. This isn't meant to be an exhaustive detail of those biochemical processes, but it's just to show you how you can analyze biochemistry using a simple knowledge of organic chemistry and the importance of NAD+ NADH in biological systems.