Amino Acid Classifications: Study with Video Lessons, Practice Problems & Examples

Amino acids are categorized into three groups based on their side chains: nonpolar, polar, and charged. Nonpolar amino acids, such as glycine and alanine, are hydrophobic and avoid water. Polar amino acids, like serine and threonine, are hydrophilic and can form hydrogen bonds with water. Charged amino acids, including aspartate and lysine, possess positive or negative charges at physiological pH, influencing their interactions in biological systems. Understanding these classifications is crucial for grasping protein structure and function.

Nonpolar Amino Acids Concept 1

Now, here we're going to say that amino acids are classified by the functionality of the r-groups attached to their alpha carbon. And we're going to say they can be classified as either being non-polar, polar, or charged amino acids. And when we say charged, we mean that they have a positive or negative charge overall. Now here we're going to take a look at our first grouping of amino acids, which are non-polar amino acids. Now, remember, when we say non-polar, we're going to say these amino acids contain r-groups that are non-polar, and because of that they are hydrophobic. Remember, hydrophobic means that you are water-fearing and do not interact with water. Now, our memory tool here will help us remember which of the amino acids can be classified as being non-polar in origin. Our memory tool is Gav Tripp Fell Limping. So, here I have this friend, his name is Gavin. We call him Gav for short. He's pretty crazy and extreme in the sense that he likes to do. Last week, he tried to jump off the roof and do a backflip. He landed in a pile of leaves, but somehow broke his ankle. Now he's on crutches. And for the longest time, Gavin has had this fear of the ocean, so he always tries to stay away from water. But here there's a large tsunami that's coming towards him, and Gav in his crutches with his broken ankle is trying to get away. Now Gav Tripp fell limping, running away from the water that he fears. Alright. So remember, we're going to use this memory tool to help us remember those amino acids that fall under this memory tool. So when we say GAV, GAV stands for glycine, alanine, and valine. Remember, their 3-letter codes here. We'd have glycine—gly, and then g; alanine—we'd have ala, and then a; valine will be val and v. Now, let's talk about these. When it comes to Glycine, its r-group is just a hydrogen. Pretty simple. It's the smallest of the 20 amino acids. Alanine is a little bit bigger. Instead of a hydrogen, it's a methyl. And then we can say that Valine is an isopropyl group instead of a methyl, so yet a little bit bigger. Now, he tripped. Tripped stands for tryptophan here though. So here we're going to say tryptophan, remember it's trp. It is phonetic in origin, its one-letter code is w. Tryptophan is much larger. We have this large ring as its r-group. Even though it has this NH group here, it's greatly outnumbered by all the carbons within this benzene ring and within this ring portion here, as well as the CH2 group. So overall, it's non-polar in origin. Then we're going to say that it fell. Fell with f. F here stands for Phenylalanine. Now remember, Phenylalanine here, its 3-letter code is phe. It is phonetic in origin, phenyl f. Phenylalanine is very similar to tryptophan, where we have a CH2 in both. But instead of being connected to these two massive rings like tryptophan, we just have it connected to a benzene ring. Then we have what's left. We have limping. Limping, trying to limp away from the tsunami. So limp is Leucine, Isoleucine, Methionine, and Proline. Alright. So here if we look at Leucine, it’d be leu and then l. We’re going to say Leucine is very similar here to Valine, that it has this isopropyl part right here, but in addition, it has a little bit extra, it has a CH2 group there. Isoleucine is ile. Isoleucine is I. Isoleucine has these 4 carbons that make up its r-group, which is a sec-butyl. Methionine, which is met and uses m. It has this chain, and near the end of it is this sulfur group right here. So, this is a sulfide group right here. So Methionine is a little bit different. And then Proline is unique because its r-group is part of a ring that's with the amino group. Here, Proline will be pro, and its one letter would be p. If you were to draw this out straighter, you could kind of see that it forms a letter p. The way the ring loops around. So that's another way of looking at this structure to remember that, oh, Proline almost looks like a p. It it loops around itself to make that ring. So remember, you have these non-polar amino acids. My buddy Gavin broke his ankle trying to escape the tsunami, can't quite move fast enough, trip fell limping. Okay. So just keep this in mind when you have to look at your different types of nonpolar amino acids.

Amino Acid Classifications Example 1

Here it says, identify all hydrophobic amino acids. Now remember, when they say hydrophobic, they're really just saying, which one represents non-polar amino acids? Remember, our memory tool here will be GAV, TRIP, FOUL, LIMPING. So here, if we take a look at these images, A would be a hydrophobic amino acid, because it represents the P within our memory tool. This is proline. Remember, it can be identified by its R group, which is this cyclic portion that actually encompasses the amino group.

Then what else do we have? We don't recall seeing one which is CH₂OH, so that's out. We don't recall seeing one with just one ring with an NH group in it, so that's also out. Now this last one here, its R group is a hydrocarbon, so for sure it's non-polar because hydrocarbons are non-polar. This one here will represent isoleucine. So, isoleucine. So out of the options here, it's both A and D. Both of them have non-polar R groups, and therefore, represent hydrophobic amino acids. So those would be our final answers.

Polar Amino Acids Concept 2

In this video, we take a look at our polar amino acids. Now here, they contain our groups that are polar and therefore hydrophilic. When we say hydrophilic, it means water-loving, and therefore interacts with water. We're going to say all except cysteine form hydrogen bonds with water. And the memory tool that we can utilize to remember which ones are classified as polar amino acids is that Santa's team tirelessly crafts new quilts. So here we have our first letters as well as TYR tirelessly to help us remember our amino acids here. So, Santa stands for serine, which is SER and S. Serine here has a CH₂OH as its R group. The presence of that OH group, we know that it can form hydrogen bonds with water.

Next, we have threonine, which is going to be THR and then T. It too also has an OH group. And that OH group is then going to be overpowered by an abundance of carbons within this R group. There's only 2 other carbons within this R group. Then we have Tyrosine here, TYR, Tyrosly. Here, Tyrosine is phonetic in origin, so it's Y. This one is larger; it's a CH₂ group connected to Benzene connected to an OH group. We can see a theme developing here for these first three. All of them have OH groups on their R group.

Then we have here is cysteine, which is unique because here it is a thiol. Remember a thiol is an SH group. So CH₂SH forms its R group here. Its 3 letter code would be CYS, and then here it is unique that it's the only of the 20 amino acids that begins with C. Asparagine, remember this is phonetic in origin, so it'd be ASN, and then N. Here we have an amide group to complete the last two of these amino acids.

Let's talk about glutamine as well, so GLN and then Q. This is also phonetic in origin. Both of these have this nitrogen here, an amide end. The difference between them is that asparagine only has 1 CH₂ connected to it, but then glutamine is a little bit bigger, so it has 2 CH₂ groups connected to that amide group.

So this is a way of looking at their similarities and their slight differences from one another. So remember, these are classified as all the polar amino acids out of the 20 that you need to remember. And the memory tool you can utilize here is that Santa's team tirelessly crafts new quilts. Okay? So just remember that memory tool to help you organize these different types of polar amino acids.

Amino Acid Classifications Example 2

Which of the following amino acids will have R groups mostly on the outside of the protein? So they're saying here is outside of the protein, and you're assuming that you're in an aqueous environment, a water environment. If you're on the outside, you're going to interact with the water molecules. So you have to love the water. So you have to be hydrophilic. So, what type of amino acids are hydrophilic? The polar ones. So this question is really asking which one of the amino acids is classified as polar amino acids? So if we take a look here, we see that our memory tool here is that Santa's team, so ST, tirelessly crafts new quilts. If we take a look here, which one of these fit with our 4 options? Well, here we have threonine, which has one letter code T. So this is a polar amino acid and therefore would be found on the outside of the protein. And then who else do we have? We have GLN. GLN is Glutamine. Its one letter code is Q. Here, we have Methionine which is not classified as a polar one, so this is out. Then we have TRP, which is not Tyrosine here. So this would be out. Okay. So here, we talk about tryptophan in terms of this TRP, but tryptophan is not one of our amino acids that are polar in nature, so it would not be found on the outside of the protein. So here the options B and C as our final final selections.

Charged Amino Acids Concept 3

Now when it comes to charged amino acids, we're going to say that they represent polar amino acids, and they contain groups that are electrically charged at physiological pH, meaning they possess a positive or negative charge. Now here if we have an acidic group, we're gonna say it contains a negative carboxylate anion group. And if you have a basic group, then you're gonna be a positive amino group. Our memory tool here is dragons eat knights riding horses. So if we take a look at this image, remember dragons tend to have a negative connotation attached to them. So the dragons, which are negative, are connected to our carboxylate groups. Knights, on the other hand, are positive because they defend us against the dragons. They're positive. They represent the basic groups.

O- The first letters here represent the one-letter code for each of these amino acids. Now, here if we take a look, for our acidic ones, it's a giveaway when we're talking about their acid forms; aspartic acid, glutamic acid, acids in the name, we know that they're acidic amino acids. So here, as carboxylic acid, they'd be O. But, if we're talking about them being in physiological pH, the pH will be high enough that they're gonna lose their H⁺. So they actually will exist as O- . In these forms, their names are now aspartate and glutamate. Here we're gonna say the one-letter code, for both of them are the same it would be ASP, and then here would be D, D for dragon. Here, glutamic acid now becomes glutamate. We're gonna say here this is GLU, and this is E, dragons eat.

Lysine here is LY, arginine, and histidine are our basic ones. We're gonna say lysine here is LYS. It is different from all the other amino acids because it doesn't use the first letter of its name, and it is not phonetic in origin. It uses K as its one-letter code. So here, knights. And so here, it has to be positive. For it to be positive, the nitrogen has to make 4 bonds, so it has to be an NH₃ group here and positive. Arginine or arginine, depending on how you want to pronounce it. We're gonna say ARG. It is phonetic in origin, so R is its one-letter code. Again, remember, nitrogen needs to make 4 bonds in order to be positive. So this nitrogen, which is already making 2, would have to more hydrogens. NH ₃ ⁺ Histidine, his, it is unique. It's one-letter code is H. Nitrogen, here we see it making 3 bonds already, so it only has one H on it to make it positive. Now going back to the acidic ones, we're going to say that they exist as the conjugate forms of their acidic and basic forms. So aspartic acid, physiological pH, pH is too high, can exist as an O- group. Same thing with glutamic acid. Physiological pH, the pH is actually too low for the basic ones. So instead of these nitrogens being neutral, they're gonna have a positive charge. So that's why we have to make sure they're each making 4 bonds by adding enough hydrogens so they gain a positive charge on the nitrogen. So remember, these would represent our charged polar amino acids, which can be further broken down into acidic and basic amino acids.

Amino Acid Classifications Example 3

Which amino acid would have a net charge of plus 1 at physiological pH? Now remember, having a charge around physiological pH is indicative of a charged amino acid. Charged amino acids can be either acidic or basic in nature. If we take a look at all these options, we would say that this is out because this is asparagine, which is not one of our acidic or basic amino acids. This one here, it has an R group that is a hydrocarbon, so it can't be acidic or basic; it's nonpolar. And then here we have proline. Proline does not classify as a charged amino acid either. If we take a look here, the only answer that works is option A. This molecule represents Histidine. Histidine is one of our charged amino acids. Remember, our memory tool here is "dragons eat knights riding horses." The H here is for histidine. At physiological pH, we say that the pH is low enough that the nitrogen here is going to gain an H⁺ and become positive. So this negative one is canceled out by this positive one, but then there's nothing to cancel out this positive one. So overall, the charge of this amino acid would be positive 1 in physiological pH. So here, the answer would be option A, histidine.

Classify each amino acid as polar, nonpolar, acidic, or basic.

a) Glu

b) Val

c) Pro

d) Lys

e) Gln

f) Arg 

Here it says, classify each amino acid as polar, non-polar, acidic, or basic. Alright. So if we look through all of this, we have 6 amino acids that we need to investigate. If we start off by telling what they are, so Glu is glutamate, Val is valine, Pro is proline, Lys is lysine, Gln is glutamine, and Arg is arginine. Alright. Next, what we're going to say is let's look to see which ones are non-polar. The non-polar ones, we just remember that the memory tool that we can utilize is 'GAV, trip, fell, limping'. So here, we look for these one-letter codes to see which one of these amino acids falls under it. So here, Val for valine, this will be non-polar. Pro in 'limping' is proline, that's also non-polar. And it looks like there's nothing else left in terms of non-polar. Next, let's go to polar. 'Polar is Santa's team tirelessly crafts new quilts.' Okay. So here, we don't need to write out the whole sentence; we're looking at the one-letter code, or in this case, three-letter code to see if they show up anywhere here. So if we look here, we're looking for polar, we're going to say Gln. Q is the one-letter code for glutamine. So this one here would be polar. And doesn't look like there's anything else that fits under this. So then finally, let's look at charged. 'Charged, we're gonna say it is Dragons eat knights riding horses.' Alright. So let's see. Who here would fall under that? Okay. So glutamate, we'd say here its one-letter code is E. Remember these first two of this memory tool for charge is acidic, and the last three are basic. So E here is glutamate, so it would be acidic. Lysine, remember, is a unique one. Its one-letter code is K. So it stands for knight, which is part of the basic crew. And then arginine is phonetic in origin, arginine, R. So it's also basic. So here, this is how we classify each one of these amino acids. They being polar, non-polar, acidic, or basic. Remember, we utilized our memory tools to help us group them into these different types of classifications.

Provide one letter codes for amino acids with neutral hydrophilic R groups that participate in H bonding.

• A. S, T, Y, C, G
• B. S, T, Y, N, Q
• C. S, T, Y, C, N, Q
• D. S, T, Y, A, Q

Here’s what students ask on this topic:

What are the classifications of amino acids based on their side chains?

Amino acids are classified into three main groups based on their side chains: nonpolar, polar, and charged. Nonpolar amino acids, such as glycine and alanine, have hydrophobic side chains that avoid water. Polar amino acids, like serine and threonine, have hydrophilic side chains that can form hydrogen bonds with water. Charged amino acids, including aspartate and lysine, possess side chains that are either positively or negatively charged at physiological pH. These classifications are crucial for understanding protein structure and function, as the side chain properties influence how amino acids interact within proteins and with other molecules.

How do nonpolar amino acids differ from polar amino acids?

Nonpolar amino acids have side chains that are hydrophobic, meaning they do not interact with water. Examples include glycine, alanine, and valine. These amino acids tend to be found in the interior of proteins, away from the aqueous environment. In contrast, polar amino acids have hydrophilic side chains that can form hydrogen bonds with water. Examples include serine, threonine, and tyrosine. These amino acids are often found on the exterior of proteins, interacting with the aqueous environment. The difference in their side chain properties significantly influences their roles in protein structure and function.

What is the significance of charged amino acids in proteins?

Charged amino acids, such as aspartate and lysine, play crucial roles in protein structure and function due to their ability to form ionic bonds and interact with other charged molecules. At physiological pH, these amino acids possess either a positive or negative charge. Aspartate and glutamate are negatively charged (acidic), while lysine, arginine, and histidine are positively charged (basic). These interactions are essential for the stability of protein structures, enzyme activity, and the binding of substrates and cofactors. Understanding the behavior of charged amino acids helps in studying protein folding, function, and interactions.

What mnemonic devices can help remember the classifications of amino acids?

Mnemonic devices are useful for remembering the classifications of amino acids. For nonpolar amino acids, the mnemonic 'Gav Tripp Fell Limping' helps recall glycine, alanine, valine, tryptophan, phenylalanine, leucine, isoleucine, methionine, and proline. For polar amino acids, 'Santa's Team Tirelessly Crafts New Quilts' aids in remembering serine, threonine, tyrosine, cysteine, asparagine, and glutamine. For charged amino acids, 'Dragons Eat Knights Riding Horses' helps recall aspartate, glutamate, lysine, arginine, and histidine. These mnemonics simplify the memorization process, making it easier to recall the amino acids and their properties.

Why are polar amino acids important in protein interactions?

Polar amino acids are crucial in protein interactions because their hydrophilic side chains can form hydrogen bonds with water and other polar molecules. This property allows them to participate in various biochemical interactions, such as enzyme-substrate binding, signal transduction, and molecular recognition. Examples of polar amino acids include serine, threonine, and tyrosine. These amino acids often reside on the protein surface, facilitating interactions with the aqueous environment and other biomolecules. Their ability to form hydrogen bonds and interact with water is essential for maintaining protein structure and function.