Nonpolar Amino Acids - Online Tutor, Practice Problems & Exam Prep

Nonpolar amino acids, characterized by hydrophobic R groups lacking terminal electronegative atoms, include glycine (Gly), alanine (Ala), valine (Val), leucine (Leu), isoleucine (Ile), methionine (Met), and proline (Pro). A mnemonic, "GAV LIMP," aids memorization. Glycine has the smallest R group (a hydrogen), while alanine features a methyl group. Valine and leucine are extensions of alanine, with isoleucine as a lopsided isomer of leucine. Methionine contains a methylthiol group, and proline's cyclic structure connects back to the amino group, making it unique among amino acids.

Memorizing Nonpolar Amino Acids

In this video, we're going to talk about our first group of amino acids, the nonpolar amino acids. So, the nonpolar amino acids all contain r groups that are largely hydrophobic. And again, recall that hydrophobic just means that they're water-fearing and that they do not mix well with water. And so the reason that all of these r groups are hydrophobic is because they all lack terminal electronegative atoms. And by that, what we mean are electronegative atoms that are at the very ends of a molecule and are able to easily interact with the water. So they lack those and that's why they're hydrophobic. It turns out that there are actually 7 amino acids that are grouped as nonpolar amino acids, and these are glycine, alanine, valine, leucine, isoleucine, methionine, and proline. And so you might be wondering, Jason, how am I supposed to memorize all these amino acids are nonpolar? And we'll get to that very shortly once we get to our example and our mnemonic, which is going to help us memorize these amino acids as being nonpolar. But before we get to the mnemonic, I want to remind you guys of 2 important things. The first is that hydrocarbon structures, or structures that only have hydrogens and carbons, are very nonpolar and hydrophobic. So the more hydrocarbons that there are in an r group, the more nonpolar and the more hydrophobic that r group will be. So that's something to keep in mind as we go through our nonpolar amino acids. The second thing I want to remind you guys of is that the groupings of these amino acids is actually relative and they can vary from textbook to textbook. And so, although your textbook or your professor might group other amino acids into the nonpolar group, we've specifically decided to group them elsewhere in a different group just to maximize the effectiveness of remembering, not only the groupings of the amino acids but also for remembering the structures of the r groups themselves.

And we'll be able to see how that works as we go through our examples in our lesson videos. So now let's move on to our example down below of all the nonpolar amino acids, all 7. And so first, we're going to start with this mnemonic. And so the mnemonic for the nonpolar amino acids is just GAVLIMP. And again, recall that the nonpolar amino acids are all largely hydrophobic or water-fearing. And so here's the story behind GAVLIMP. I've got this friend, his name is Gavin, but really he just goes by Gav. That's his nickname, Gav. So everybody calls him Gav. And so, Gav actually broke his ankle a couple of weeks ago. He was trying to do a backflip off a trampoline or something crazy, whatever. Anyways, he broke his leg and he can't run anymore. So now, Gav can only limp everywhere. He's got a cast on his leg and he's always got these crutches and he's just limping around. And Gav has always been super afraid of water ever since he was 2 years old. So Gav is super afraid of water and he's super hydrophobic. And whenever Gav is just chilling on the beach lately, and a big wave starts to come, you'll see Gav get up real quick and he's got it. He grabs his crutches and he tries to go as fast as he can, but he can't run, he can only limp. And so you can see Gav right here limping away on his crutches, away from the water because he's hydrophobic. So if you can remember GAVLIMP, you can remember that GAV limps away from the water and that's why those are the hydrophobic nonpolar amino acids.

So hopefully, that'll help you guys remember GAVLIMP, the nonpolar amino acids. So now let's go in and talk about each of the nonpolar amino acids 1 by 1. The first on our list is g, and g remembers the one-letter code for glycine. And so in these blanks, what we're going to do is we're going to put the 3-letter code in the first blank and the one-letter code in the second blank. And so glycine's 3-letter code is actually GLY, glycine, and its one-letter code is G. And it turns out that glycine's r group is one of the easiest to remember. It's literally just a single hydrogen atom. And so it actually has the smallest r group, and glycine's just so easy that its r group literally just glides right on through, figuratively because it's just so easy, but also literally because its r group is so small, it can just glide into pretty much any place it wants to without encountering a lot of steric resistance. It can go into small spaces that other r groups can't fit into. So glycine's r group is the smallest, and it's one of the easiest to remember. It's just a single hydrogen. Alright. The next on our list is alanine or A. And so A, the 3-letter code for alanine is just ALA, and its one-letter code is A. And just like A is the first letter of the alphabet, just like A is the first letter of the alphabet, A and alanine are both really easy to remember because the R group of alanine is just a simple methyl group. It's just a CH3 methyl group. And also, just like A is the first letter of the alphabet, A, the letter A, is the leader for all of the other letters in the alphabet. And in a similar way, alanine is also a leader for all of the other chiral amino acids that are in not only this group but also in other groups. And remember that glycine over here, the one that we already talked about, is the only amino acid that is not chiral. Glycine is achiral, so that's one of our learning objectives from our amino acid configuration lesson. Alright. So now that we've known alanine, let's now talk about valine, and again, like we just said, valine alanine is a leader, and so valine kind of copies off of alanine.

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

So this practice problem wants us to draw in the R groups from memory for each of the nonpolar amino acids. And whenever you hear nonpolar, you want to think about our mnemonic, GAV LIMP, where each of these letters here represents the one-letter symbol for each of the nonpolar amino acids. The first is glycine, G. So glycine is so easy that it just glides right on through. It's literally just a hydrogen and it's the easiest amino acid to remember. Now the next is actually A, and A represents alanine. And just like A is the first letter of the alphabet, and it's easy to remember, alanine is also easy to remember because it's just a methyl group. So it's just a CH3. And also, just like A is the leader of the alphabet, alanine is also the leader of all of the chiral amino acids. And so you can always start the R group of any of the chiral amino acids just by drawing a simple CH. And we know the hydrogens are going to change, but you can always just start by drawing a CH. Now, the next is valine, which is V, and valine's R group literally looks like a V. So what we could do is just draw our alanine, which is the leader, and then just draw in our V. And so that's the R group of valine. Now leucine is our next one, and leucine is L. And so leucine's R group literally is just a loose extension of valine. You can see how they're all building off of the previous one. So remembering it in this order is helpful for remembering the structures. Leucine is an extension, a loose extension of valine. And so what we need to do is draw in an extra CH2 and extend the valine just a little bit more. You can see this part here is the valine, but then it's extended by this extra CH2 here. So leucine is just a loose extension of valine. Now, the next one on our list is I, and I is for isoleucine. Isoleucine is an isomer of leucine, so we know that it's going to have the same number of atoms and the same types of atoms, but it's going to be arranged differently and the arrangement is actually going to come from being a lopsided valine. So what we can do is just draw valine. Now we've got valine and then all we got to do make it lopsided by drawing an extra methyl group in there. And now we've got isoleucine. Pretty easy, right? Next on our list is methionine, or M. And just like its one-letter symbol is an M, methionine's R group kind of looks like an M. So what we can do is we can draw in the M, so it goes up, it goes down, and it goes up. And at the very end of our M, it needs to be a methylthiol. So for methionine, it has both a methyl group, a meth, but it also has the thio in there. So that means that it's got to have both a methyl, which is a CH3, and then a thiol would be the sulfur atom. We'll put our sulfur here, and then we'll fill in our M with this methyl group. So now we've got our methylthiol, we've got our sideways M shape, and that is the R group of methionine. Lastly, what we've got is P, and P is for proline. And just like a P has a backbone and a loop, the R group has pretty much looks like the loop of a P. So again, if this is the backbone of the amino acid, proline's R group is the loop of a flipped P. So you can also think P for pentagon because, again, it's going to look like a pentagon when you finish drawing the R group. Go on in and fill this in. And so its R group is actually going to consist of all methylenes, which are just all CH2's. So we'll put in a CH2 here, then we'll put in a CH2 here, we'll put in a CH2 up here, and then finally, it connects back to the nitrogen, it loops back. And again, the number of hydrogens is different than the number of hydrogens on the amino group for all the other amino acids. And that is because it's the only one that has a cyclic R group that connects right back to the amino group. And so, this concludes our practice problem, and I'll see you guys in the next one.

Fill-in the missing R-groups for the following tripeptide: L-A-M. 

So this practice problem wants us to fill in the missing R groups for the following tripeptide LAM. Recall from Chapter 1 that peptides are small proteins. "Tri" here means 3, so there are 3 amino acids, L, A, and M, all of which are nonpolar amino acids. Remember the mnemonic for recalling the nonpolar amino acids is GAVLYMP. The first one on our list that we need to draw is L, for leucine. Notice the R groups here are numbered 1, 2, 3, so we're going to put our first one, L, into the first spot. Leucine is just a loose extension of valine. All we need to do is make it a little bit longer, so we can draw our CH2 and then draw our CH. That's our valine with a looser extension because it's just a little bit longer and it's got a bit more flexibility. Now, L is done. The next is A, for alanine. Just like the first letter of the alphabet is A, the R group of alanine is also simple. It’s literally just a methyl group, CH3. So, that is alanine right there. Alanine is done. The next is methionine, or M. Methionine, when you write it out, you'll realize that it has really two parts to it: it's got the methyl group part, and also the thio part. The methyl, remember, is CH3 and the thio part is the sulfur atom. Because its one-letter symbol is an M, its R group also kind of looks like a sideways M. We can draw our sideways M and then we know that the methyl thiol has to fit into this position. So what we can do is put our methyl methylthiol at the end here. Now we've got our M looking R group. Again, it goes up, down, up, and then connect back to the backbone. This concludes this practice problem, and I'll see you guys in our next video.

Here’s what students ask on this topic:

What are nonpolar amino acids and why are they hydrophobic?

Nonpolar amino acids are a group of amino acids characterized by their hydrophobic R groups, which means they do not mix well with water. This hydrophobic nature arises because their R groups lack terminal electronegative atoms, such as oxygen or nitrogen, that can interact with water molecules. Instead, their R groups are primarily composed of hydrocarbons (carbon and hydrogen atoms), which are nonpolar and do not form hydrogen bonds with water. The seven nonpolar amino acids are glycine (Gly), alanine (Ala), valine (Val), leucine (Leu), isoleucine (Ile), methionine (Met), and proline (Pro).

How can I memorize the nonpolar amino acids?

A useful mnemonic to memorize the nonpolar amino acids is "GAV LIMP." This stands for Glycine (Gly), Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), and Proline (Pro). The story behind the mnemonic involves a character named Gav who is hydrophobic and limps away from water, helping you remember that these amino acids are hydrophobic. Visualizing this story can make it easier to recall the specific amino acids in this group.

What is the structure of glycine and why is it unique among nonpolar amino acids?

Glycine (Gly) is unique among nonpolar amino acids because it has the smallest R group, which is just a single hydrogen atom. This simplicity makes glycine achiral, meaning it does not have a stereoisomer. The structure of glycine is:

${\mathrm{NH}}_2-\mathrm{CH}-{\mathrm{COOH}}_2$

Because of its small size, glycine can fit into tight spaces within protein structures, providing flexibility and reducing steric hindrance.

What is the difference between leucine and isoleucine?

Leucine (Leu) and isoleucine (Ile) are both nonpolar amino acids, but they differ in their structure. Leucine has a straight-chain R group, while isoleucine is an isomer of leucine, meaning it has the same molecular formula but a different arrangement of atoms. Specifically, isoleucine has a lopsided structure compared to leucine. The R group of leucine is:

$\mathrm{CH}-{\mathrm{CH}}_2-\mathrm{CH}-{\mathrm{CH}}_3-{\mathrm{CH}}_3$

While the R group of isoleucine is:

$\mathrm{CH}-{\mathrm{CH}}_3-\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$

This difference in structure affects their properties and how they interact within proteins.

Why is proline unique among the nonpolar amino acids?

Proline (Pro) is unique among nonpolar amino acids because its R group forms a cyclic structure that connects back to the amino group. This cyclic nature restricts the flexibility of proline and introduces a kink in the protein chain where it is incorporated. The structure of proline is:

${\mathrm{NH}}_2-\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{CH}-{\mathrm{COOH}}_2$

This cyclic structure makes proline a unique amino acid that can influence the folding and stability of proteins.