Aromatic Amino Acids: Videos & Practice Problems

Aromatic amino acids, including phenylalanine, tyrosine, and tryptophan, are characterized by their large R groups containing benzene rings. Phenylalanine (Phe, F) is alanine with a phenyl group, while tyrosine (Tyr, Y) adds a hydroxyl group to phenylalanine. Tryptophan (Trp, W) features a structure with a five-membered ring and a benzene ring. These amino acids exhibit unique properties, allowing them to fit into various amino acid categories, enhancing their biochemical roles in proteins and metabolic pathways.

Aromatic Amino Acids

In this video, we're going to talk about our second group of amino acids, the aromatic amino acids. So, the aromatic amino acids all have really big R groups with big benzene rings. Recall from your previous organic chemistry courses that a benzene ring is just a 6-membered carbon ring with a conjugated double bond system just like the one that's shown right here. Also, recall that the benzene ring can be shown with a circle in the middle to represent that conjugated double bond system. It turns out that there are only 3 amino acids that are categorized as aromatic, and those are phenylalanine, tyrosine, and tryptophan. Below, we have a mnemonic to help you guys memorize those 3 aromatic amino acids, and that mnemonic is just FYW or fat young whippersnappers. Believe it or not, 'whippersnappers' is actually a real word. If you Google the definition of whippersnappers, you'll see that it's someone who is young, inexperienced, and overconfident. These aromatic amino acids, Phenylalanine, Tyrosine, and Tryptophan, act like young, overconfident whippersnappers because they feel like they can go into whatever amino acid group they want. Phenylalanine could be categorized into the nonpolar group. Tyrosine could be categorized into the polar group. Tryptophan could be categorized into the nonpolar group or into the polar group. You'll even see that some textbooks categorize these amino acids in that way. They really do act like young whippersnappers that feel like they can go into multiple of these amino acid groups. That's why we've categorized them all together as the aromatic amino acids because they have these big benzene rings. Now that we know that they are indeed young whippersnappers, that they are also fat because they have these big benzene rings. Hopefully, by remembering FYW, or fat young whippersnappers, that'll help you guys remember the aromatic amino acids of Phenylalanine, Tyrosine, and Tryptophan.

Down below, we're going to talk about the structures of each of these amino acids. We're going to start with F here, which is for phenylalanine. Notice on the far left block over here, what we have is phenylalanine, and phenylalanine's 3-letter amino acid code is PHE, and its one-letter amino acid code is phonetic with an F. Phenylalanine is trying to tell us what its R group structure is. It's literally telling us that it's alanine, our leader amino acid, with a phenyl group branching off of it. That's exactly what we have down below. Phenylalanine or F here is literally just our leader amino acid alanine with a phenyl group branching off. Recall that a phenyl group is literally just a branching benzene ring. When we're drawing phenylalanine's structure, we can literally just draw alanine, which we know is CH3, but we know that the number of hydrogens are going to change, so it's going to end up being just 2. Then we can draw a branching phenyl group, and so that's just a branching benzene ring. That is phenylalanine's structure. It's literally just alanine, our leader amino acid, shown here with a phenyl group branching off. Easy, right?

Our next aromatic amino acid is Y, and Y is for tyrosine, which we have in our next block over here. Tyrosine's 3-letter code is TYR, and its one-letter code is Y, phonetically for tyrosine. Tyrosine's structure is literally branching directly off of phenylalanine's structure. That's exactly what we have down below, that tyrosine is literally just phenylalanine having a hydroxyl group baby. If we go back up to our image above, the only place that makes sense for a hydroxyl group baby to come out of this benzene ring is from the bottom of the benzene ring. It doesn't make sense for our benzene ring, our hydroxyl group to come off of the sides or come off of the top. Our hydroxyl group baby has to come off of the bottom. Down below, when we draw Tyrosine's structure, because we know that it's literally phenylalanine, we can draw phenylalanine exactly. So, that is phenylalanine having a hydroxyl group baby, which means that the hydroxyl group has to be coming off of the bottom of the benzene ring here. It doesn't come off of the sides or anywhere else; it comes off of the bottom. That can hopefully help you guys remember Tyrosine's R group, because literally, this is Tyrosine's R group. Over here on the right, what we have is an image of a baby's head coming out upside down, and this circular head can help remind you of the circular oxygen atom in the hydroxyl group. Another way to remember Tyrosine's structure is to recognize that Tyrosine kind of sounds like a tire, and tires are circular just like this benzene ring is up here. The tyrosine can help remind you guys of the O group that is branching off of the tire.

Lastly, we have our last aromatic amino acid of the W, and the W is for tryptophan, which we have in our last block over here. Tryptophan's three-letter code is TRP, and its one-letter code is phonetic with a W because the way Bugs Bunny would say it is, twitophan with a W in it. Down below, what you'll see is that tryptophan's structure, the W here, is literally just our leader amino acid again, alanine with two joined rings. The first joined ring is a 5-membered ring, and the second joined ring is a 6-membered carbon ring, which is a benzene ring. It's important to note that the benzene ring is going to go below. We're going to draw the benzene ring last, so first, we're going to draw our 5-membered ring. If we start to draw tryptophan's structure, we know that it's literally just our leader amino acid alanine with two joined rings. If we draw alanine, we know it's CH3, but we know that the number of hydrogens are going to change here, it's going to end up being 2. Then it has two joined rings, a 5-membered ring, and a 6-membered ring. But that 6-membered benzene ring goes below, so we draw it last, which means that we draw the 5-membered ring first. Here is our 5-membered ring. Perfect. The next part we need to realize is that our R group in Tryptophan's structure has a nitrogen atom in it. To figure out where this nitrogen atom actually goes, we can remember that tryptophan trips on a tripod, and we know that tripods have three legs, so that can remind us that there's a 3-carbon start to this nitrogen atom. Looking above at our R group, notice the R group starts to begin right here, so our 3-carbon start would be 1 carbon right here, 2 carbons here, and then we've got two different pathways that we could take. We could take this pathway over here, which is our third carbon and then that would mean that our nitrogen goes right here. So, a 1-2-3 carbon start and then our nitrogen atom. The other possibility would have 1, 2 carbons, and then have our third carbon be over here, and that would mean that our nitrogen would go right here. Either possibility is possible, and it doesn't really matter which one you pick. I'm going to pick this second possibility here just because of space reasons. So, let's get rid of these dots and let's put our nitrogen over here. Now that we know where our nitrogen atom is, the next thing that we need to realize is that there's a 3-carbon start to our nitrogen, and so we just identified that. The next part that we need to realize is that this nitrogen atom right here is not part of the benzene ring. We know that the benzene ring is going to be below our structure, and so we know that these rings are joined. That means that the benzene ring has to share a side with this other ring that's here. But if the benzene ring shares a side on this side or on this side of the 5-member ring, then the benzene ring is going to be up above the structure. But we know that the benzene ring has to be below the structure, which means that the benzene ring has to share a side on either this side, this side, or this side. We know that the nitrogen atom cannot be part of the benzene, which means that the benzene cannot go on this side or on this side because, otherwise, the nitrogen atom would be part of the benzene. So, the only place that the benzene can go is on this side right here. Then we can draw in our benzene structure here, and that is our benzene. You can see now that it is below the structure. It has the lowest point in our structure here. The last part here is going to be to draw in the double bond, and the double bond does not touch, so the double bond does not touch the nitrogen or the benzene ring, and so the double bond goes right at this position here. Of course, we can draw in our hydrogens coming off of the nitrogen atom, and that is it for tryptophan's structure. That essentially concludes our lesson on the aromatic amino acids, and we'll be able to get some practice utilizing all of these memory tools in our next practice problem. So, I'll see you guys there.

Draw in the R-groups from memory for each of the aromatic amino acids.

So this practice problem wants us to draw in the R groups from memory for each of the aromatic amino acids. And when you hear aromatic, you should be thinking about those big R groups with those big benzene rings. You can also think about our mnemonic fat young whippersnappers. And so remember that the "f" here stands for phenylalanine, and phenylalanine is literally trying to tell you what its R group is. It's literally alanine with a phenyl group. So we can draw an alanine and we know the hydrogens are going to change, ends up being 2, and then we can just draw the phenyl group. So the phenyl group is just a benzene ring that's branching off. And that's it. That's phenylalanine right there. Alright. So the next one is "y." And remember what "y" stands for - tyrosine. And tyrosine is literally branching right off of phenylalanine structure. So you can remember that it's building off of its structure. And so tyrosine is literally just redrawing phenylalanine exactly, so let's do that. Alright. Alright. Perfect. That's phenylalanine. And then, we need to know that tyrosine is phenylalanine having a hydroxyl group attachment, and of course, the hydroxyl group has to come off the bottom, and so, this is tyrosine's structure. And then last is the "w," and remember the "w" is for tryptophan because the way that Bugs Bunny would say it is, "twiptophan" with the "w." And recall that, tryptophan's structure is literally just alanine with two joined rings. The first ring is a 5-membered ring and the second ring is a 6-membered ring or the benzene ring, but the benzene goes below, so we're going to draw it last. So let's go ahead and draw in our alanine, CH, and we know the hydrogens are going to change. It ends up being 2. And then we're going to draw our first membered ring, which is the 5-membered ring. Perfect. There's our 5-membered ring. Now, the next step is to figure out where this nitrogen goes because we need to know that tryptophan has a nitrogen atom. And so, where does this nitrogen go? And, the way that we remember that is to remember tryptophan trips on a tripod and a tripod has 3 legs, so that helps us remember that there's a 3 carbon start before you actually get to the nitrogen. So we can go ahead and count our carbons. Here's carbon 1. Here's carbon 2. Here's carbon 3. Alright. So there's our nitrogen. So let's get rid of these red dots. And we know our nitrogen goes here. Perfect. Now we know where our nitrogen is. The next step is to remember that the nitrogen is not part of the benzene, and so the benzene also has to be below. So that limits us. It can't be on either of these two sides because then it would be above. It can't be on this side or this side because then it would be, the nitrogen would be part of the benzene. Right? So it can only be on this side here. And so then we go ahead and we draw in our benzene. Perfect. There's our benzene. And then the very last step is to draw in the double bond, and the double bond does not touch benzene ring. So this is the only place for it. And so, that's it. That's tryptophan structure. So hopefully, these memory tools were useful for you, and you guys can get more practice as we move along throughout our lesson. So, I'll see you guys in our next video.

Here’s what students ask on this topic:

What are the aromatic amino acids and their structures?

The aromatic amino acids include phenylalanine, tyrosine, and tryptophan. Phenylalanine (Phe, F) has a structure where a phenyl group (a benzene ring) is attached to the alanine backbone. Tyrosine (Tyr, Y) is similar to phenylalanine but with an additional hydroxyl group (-OH) attached to the benzene ring. Tryptophan (Trp, W) has a more complex structure with a five-membered ring containing a nitrogen atom fused to a benzene ring. These structures give aromatic amino acids their unique properties and roles in proteins and metabolic pathways.

Why are aromatic amino acids important in biochemistry?

Aromatic amino acids are crucial in biochemistry due to their unique structures and properties. The benzene rings in their R groups allow them to participate in π-π interactions, which are important for protein stability and function. They also play significant roles in enzyme active sites and are precursors for various biomolecules. For example, tyrosine is a precursor for neurotransmitters like dopamine, while tryptophan is a precursor for serotonin and melatonin. Their ability to fit into multiple amino acid categories enhances their versatility in biochemical processes.

How can you remember the structures of aromatic amino acids?

A mnemonic to remember the aromatic amino acids is 'FYW' or 'fat young whippersnappers.' Phenylalanine (F) is alanine with a phenyl group. Tyrosine (Y) is phenylalanine with a hydroxyl group added to the benzene ring. Tryptophan (W) has a five-membered ring with a nitrogen atom fused to a benzene ring. Visual aids, such as imagining tyrosine as a tire with an -OH group, can also help. These memory tools can make it easier to recall their structures and properties.

What is the role of tryptophan in the body?

Tryptophan (Trp, W) plays several vital roles in the body. It is a precursor for the neurotransmitter serotonin, which regulates mood, sleep, and appetite. Tryptophan is also a precursor for melatonin, a hormone that controls sleep-wake cycles. Additionally, it is involved in the synthesis of niacin (vitamin B3), which is essential for energy metabolism. The unique structure of tryptophan, with its fused five-membered and benzene rings, allows it to participate in various biochemical pathways, making it a crucial amino acid for overall health.

How do the structures of phenylalanine and tyrosine differ?

Phenylalanine (Phe, F) and tyrosine (Tyr, Y) have similar structures, but with a key difference. Phenylalanine consists of an alanine backbone with a phenyl group (a benzene ring) attached. Tyrosine, on the other hand, is essentially phenylalanine with an additional hydroxyl group (-OH) attached to the benzene ring. This hydroxyl group makes tyrosine more polar than phenylalanine, affecting its solubility and reactivity in biochemical processes. This structural difference also allows tyrosine to participate in different biochemical pathways compared to phenylalanine.