Charged Amino Acids: Study with Video Lessons, Practice Problems & Examples

The five charged amino acids are categorized into negatively charged acidic amino acids (aspartic acid and glutamic acid) and positively charged basic amino acids (lysine, arginine, and histidine). At physiological pH (around 7), acidic amino acids donate hydrogen ions, resulting in a negative charge, while basic amino acids accept hydrogen ions, leading to a positive charge. Understanding these behaviors is crucial for grasping acid-base interactions in biochemistry, particularly in protein structure and function.

Charged Amino Acids

In this video, we're going to begin our discussion on our fourth and final amino acid group, the charged amino acids. So the charged amino acids all have R groups that are electrically charged, but they're not always charged, and it turns out that their charges depend on the pH of the solution that they're sitting in. But we'll talk about the pHs and the charges of R groups later on in our course in another video. But for now, in this video, all I want you guys to know is that the charged amino acids are specifically charged at physiological pH, which recall is a pH of about 7. And so this group of charged amino acids can actually be broken up into two smaller subgroups, and so there are two groups of charged amino acids. We have the negatively charged acidic amino acids, as well as the positively charged basic amino acids. And so over here on the far right, we have a mnemonic to help you guys memorize not only the five charged amino acids but also how these five charged amino acids can be further categorized into their smaller subgroups. The negatively charged acidic amino acids, as well as the positively charged basic amino acids. And so this mnemonic is just dragons eat knights riding horses. And so we kind of already knew that. Right? Back in the medieval times, there were dragons, they were eating knights, riding horses, whatever. And so, what you'll see here is that our image is broken up into two parts. And so it's really breaking it up into our smaller categories of the negatively charged acidic amino acids on the left, and the positively charged basic amino acids on the right. And so each of these letters here, the capital letters of these words, so the D, the E, and the K, R, and H, are representing the one-letter amino acid codes for those five charged amino acids. And we have all five of those one-letter codes on the left over here. And so, essentially, what you'll see is that the D represents aspartic acid, the E is for glutamic acid, K is for lysine, R is for arginine, and H is for histidine. And so, this means that aspartic acid and glutamic acid are the two negatively charged acidic amino acids. And so looking at this image right over here, notice that we have a dragon that is shooting fire and acid out of its mouth, and there's nothing positive about a dragon shooting fire and acid out of its mouth. It's clearly negative. And so up above, we have a negative sign, and so you can see that the dragons eating things and shooting fire and acid out of their mouth is a negative thing. And so that means that aspartic acid and glutamic acid are negatively charged acidic amino acids. Now, looking at our image over here on the right with the knights riding horses, you can see that we have a knight riding a horse, and that is something that is very noble and positive. And so these amino acids here, lysine, arginine, histidine, are the positively charged amino acids. And so, because horses do not shoot fire and acid out of their mouths, that means that they're not acidic, and that must mean that they are basic. And so if you want, you can even imagine our knight carrying a pumpkin spice latte and our horse marching into battle with some UGG boots. So super basic. And so hopefully, this will help you guys remember the five charged amino acids and how they can be broken up into the negatively charged amino acids and the positively charged amino acids. And so in our next lesson video, we're going to focus specifically on the

Charged Amino Acids

So now that we understand this mnemonic up here a little bit better to memorize the 5 charged amino acids, we're going to focus on the negatively charged acidic amino acids or this particular group that's boxed in up here. With these acidic amino acids, it turns out that all the R groups actually donate a hydrogen ion (H⁺). When the R groups donate an H⁺, that results in a negative charge. There are only 2 amino acids that fall into this group, negatively charged and acidic amino acids, and they are aspartic acid and glutamic acid. It's the presence of carboxylic acids that render these amino acids acidic in the first place. You really want to make this association between negatively charged and acidic amino acids.

Let's jump into our example where we can look at the actual structure, the R group structure, of these two amino acids. Our first amino acid is aspartic acid, and aspartic acid's three-letter code is ASP, and its one-letter code is D because it fits in phonetically at this position for aspartic acid. It turns out that alanine, once again, is a leader for this amino acid, and aspartic acid is really just alanine with, you guessed it, a carboxyl group. All we need to do to get the aspartic acid structure is to start off drawing alanine, so a CH, and we know the hydrogen numbers are going to change, it's going to end up being 2, and then we just draw a carboxyl group, so a C double bond O and an OH group. The H on this OH group is going to be acidic. Now we've got our carboxyl group, and it's alanine with a carboxyl group. That's aspartic acid structure.

Aspartic acid is an acid, so it can donate its hydrogen, and that's what we're seeing in this process. When aspartic acid donates its hydrogen ion, it becomes aspartate. All we need to do to draw aspartate structure is to redraw aspartic acid without this acidic hydrogen. So it's going to be CH₂, C double bond O, and then it's just going to have an oxygen there without that acidic hydrogen, so the oxygen is going to have a negative charge. That's why aspartic acid is a negatively charged acidic amino acid, and it's aspartate that holds that negative charge.

Our next amino acid in this group is glutamic acid, and glutamic acid's three-letter code is GLU, and its one-letter code is E because it fits in phonetically at gluten MEK acid. We've seen this GLUT here before with glutamine. You can associate these GLUTs with one another. We've got glutamine and glutamic acid. Glutamic acid is really just glutamine with, you guessed it, a carboxyl group instead of an amide group. So it's going to be the same thing, so it'll have 2 CH₂s and it'll also have a carboxyl group, so it'll be C double bond O and an O, and then I'll put the acidic H in a different color. What we've got is a carboxyl group here instead of an amide group for glutamic acid's R group.

There's a second way also to memorize glutamic acid, and that's just going back to the GLUTE idea. Imagine if you're missing your entire butt area, you're just going to be really short. But if you have the glutes, which glutamic acid obviously has, it's going to be a little bit longer. Notice that glutamic acid just has a little bit more, has a CH₂, an extra CH₂ that aspartic acid doesn't have. That's how you memorize glutamic acid structure. Glutamic acid is an acid, so it can donate a hydrogen, and when it does that, glutamic acid becomes glutamate. For glutamate, all we need to do is redraw glutamic acid, so we'll have the CH₂, the CH₂ again, the C double bond O, and then all we need to do is just put in the O without the hydrogen, and now it's got a negative charge. It's glutamate that holds that negative charge.

This concludes our lesson on the negatively charged acidic amino acids. We've got some memory tools here, and we'll be able to apply those memory tools once we finish with the positively charged amino acids, which is in our next video. So I'll see you guys in that video.

Charged Amino Acids

So now that we've talked about the negatively charged acidic amino acids, we're going to focus on the positively charged basic amino acids. With these basic amino acids, they all contain R groups that are capable of accepting a positively charged hydrogen ion. Included are lysine, arginine, and histidine. Our mnemonic for remembering these three amino acids is "knights riding horses." Lysine's one-letter code is K for knights, arginine's is R for riding, and histidine's is H for horses. It's the presence of these ionizable nitrogen atoms in the R groups that render these amino acids basic. You want to make this association between positively charged and basic amino acids.

In our example, we're going to talk about the structures of these R groups themselves. Our first amino acid is lysine, with its three-letter code just being LYS, and again, its one-letter code is K. In our mnemonic, lysine is represented by a knight, and lysine's R group resembles a knight's sword. There are 4 pointy ends that you could try to poke someone with. The letter K has 4 pointy ends; there's also a 4 carbon start to the chain of lysine's R group. Let's draw in our first carbon, so CH₂, our second CH₂, third, and fourth. At the very end of the sword, there's an amino group, NH₃⁺, which is positively charged because of these nitrogens in the R groups.

Next, we have histidine. Recall, histidine's three-letter code is just HIS, and its one-letter code is H. In our mnemonic, the H stands for a horse. Histidine is alanine with a 5-membered ring branching off of it. Histidine's R group has 2 parallel lines and two nitrogen atoms. The 5-membered ring in histidine's R group resembles a sideways horse. The nitrogen is positively charged, resembling the neck of the horse.

We shall now address arginine. Arginine's three-letter code is ARG, and its one-letter code is R for riding. Arginine's R group is a mixture of features from both lysine's and histidine's R groups. The R has three pointy ends and there is a 3 carbon start to the chain. There is also a triangular nitrogen structure at the bottom, resembling a triangular sword tip. This structure includes a double bond to a nitrogen, which is positively charged, and it has a NH₂ group. The double bonds in the triangular structure are likened to a rider's legs straddling a horse, aiding in the mnemonic of R for riding. This concludes the structure for arginine. We will utilize all these memory tools in our practice problems.

Charged Amino Acids

So in our previous videos, we've said that the 5 charged amino acids can be further split into 2 smaller subcategories: the negatively charged acidic amino acids and the positively charged basic amino acids. In this video, we're going to try to clear up any confusion that you might have about how to group the charged amino acids as either acids or bases. Recall from our previous lesson videos that acids are substances capable of donating hydrogen ions to the environment. Bases, on the other hand, are substances that accept hydrogen ions from the environment.

Below, you'll see we have our mnemonic for memorizing the charged amino acids, and that is "dragons eat knights riding horses," where aspartic acid and glutamic acid are the negatively charged acidic amino acids, and lysine, arginine, and histidine are the positively charged basic amino acids. They are positively charged because they have these extra hydrogen ions that are positive.

Here we have a question: Why aren't the positively charged amino acids, lysine, arginine, and histidine, grouped as being acidic if they have these extra hydrogens? If they have extra hydrogens, shouldn't they be able to donate those hydrogens and be categorized as acids and not as bases? It turns out that this confusion has to do with the fact that we're not realizing that acid-base groupings of amino acids are defined by the behaviors of the R groups under physiological conditions.

Recall that physiological pH is right around a pH of 7, and it's the pH of the solution that is going to determine whether the amino acids are going to have a positive or a negative charge. We'll focus more on pHs later in our course.

For now, let's take a look at our example below. What I want you to realize is on the left image over here, we have aspartic acid and aspartate, which we know is a negatively charged acidic amino acid from our mnemonic. The one-letter codes are just D for both aspartic acid and aspartate. Aspartate has this extra hydrogen here that is capable of being acidic, and aspartate, notice, is missing that hydrogen, and it has a negative charge on it. At lower pHs, this amino acid exists as aspartic acid with this extra hydrogen on its carboxyl group. But as the pH changes from low pHs up to physiological pH, right around a pH of 7, this reaction occurs, where the hydrogen is released. This hydrogen is released to the environment, making it an acidic hydrogen. Notice over here what we have is a carboxylate group. Essentially, this structure here for aspartate is the structure that exists at physiological pH. Under physiological conditions, notice that aspartic acid is acting as an acid, and that's why we group it as an acid, because of its behavior, specifically at physiological pH.

Now over here on the right-hand image, we have lysine. Lysine, we know, is a positively charged amino acid that is basic. Lysine's one-letter code is just a K. Notice that even when its R group does not have a charge, its name doesn't change; it's still lysine, and its one-letter code is still K. Essentially, the reaction that you see over here is the one that takes place under physiological conditions, and notice that the arrow is reversed from the arrow that we see over here. Essentially, what we're seeing is that a hydrogen ion from the environment is being accepted and incorporated into the R group. That means it's acting as a base, and this is a basic reaction here. It accepts the hydrogen ion, and notice that its amino group becomes charged after it accepts the hydrogen ion. Essentially, what we're seeing is that this is the structure that exists at physiological pH, and this is the reaction that occurs under physiological pH. This structure over here that is uncharged is the structure that would only occur at higher pHs. But again, we are specifically grouping the amino acids under physiological conditions, and how they behave under physiological conditions.

Essentially, the main takeaway here is that the negatively charged acidic amino acids are grouped as acids because of this reaction that occurs to create negative charges at physiological pH. The basic amino acids are positively charged, or the positively charged amino acids are basic because they undergo basic reactions to accept hydrogens under physiological conditions. That is it for this lesson, and we'll be able to move on and get some more practice in our next couple of videos. I'll see you guys there.

Draw in the R-groups from memory for each of the charged amino acids at physiological pH.

So this practice problem wants us to draw in the R groups from memory for each of the charged amino acids at physiological pH. And remember that at physiological pH, the R groups of these charged amino acids are going to tend to have a charge on them. So, we're going to want to draw them in their charged state. And so recall that the mnemonic that we have for memorizing these charged amino acids is "dragons eat knights riding horses." The 'D' and the 'E' here stand for aspartic acid and glutamic acid, which are going to have a negative charge on them. Then the "knights riding horses" is going to stand for lysine, arginine, and histidine, which have a positive charge in their R groups. The first one that we have here is 'D', and 'D' again stands for aspartic acid or just aspartate. Remember, aspartic acid and aspartate are basically the same amino acid, they just differ by a single hydrogen atom, and so that makes them a conjugate acid-base pair. And so because we're drawing the charged amino acids at physiological pH, we're going to want to draw the one with a charge on it. So we're going to want to draw aspartate. And so remember that aspartate or aspartic acid here, it's just going to be alanine with a carboxylate group coming off of it. It's going to have a carbon with a hydrogen, and we know the hydrogens are going to change, so it's going to be two actually. And then it's going to be a carboxylate group coming off, so it's going to be C double bond O with an O minus charge on there. That's the structure of aspartate. We've got our carboxylate group here, and it's alanine with a carboxylate group. Now, the next one is 'E', and remember 'E' stands for glutamic acid or just glutamate because we want it in the charged state. And so, glutamate has the GLUT in it and so, we know that one way that we can think about this is by remembering that it feeds off of the previous structure in our system. So, the 'E' feeds off of the 'D' here like that. And so, we know that because it has the glute, it's going to have the extra butt area, right? The extra glute area, and the extra glute area is represented by an extra CH2. And so all we need to do is draw the same exact structure, except with an additional CH2. So we go and draw that in, so we know we're going to have our first CH2, we're going to have our second CH2, and then we're going to have our carboxylate group, so it's going to be our C double bond O with an O minus. We've got our carboxylate group at the bottom and the only thing that's really changed from the previous is that we've just got an additional CH2 that's been added in. That's the structure of glutamic acid, or glutamate. Now, for the next one that we have is 'K', and 'K' is lysine. And remember, 'K' here stands for the knight and, really, lysine's R group looks like a knight's sword. Just like the 'K' has four pointy ends on it that you could try to poke somebody with, there's going to be a four-carbon start to this chain. So we can draw in our four carbon start, so it'll be CH2, CH2, there's CH2, that's three, and CH2, that's four. Perfect. Now we've got our four-carbon start to the chain. We know that there's going to be an amino group at the very end, and so we can go ahead and draw in our amino group. Since we know that all of these positively charged basic amino acids here have amino groups in them, and so it's going to have a positive charge. That's the structure of lysine. Next, we have arginine. And remember, arginine has features of both the knight and the horse. So let's skip arginine for now and let's go to histidine, just like we did in our concept. And so with histidine, remember its three-letter code is HIS, so one letter code is H, and it stands for the horse. And so really all histidine is is alanine with a five-membered ring. So we can draw an alanine CH2 and it's going to have a five-membered ring, 1, 2, 3, 4, 5. And then just like the 'H' has two parallel lines, there's going to be two double bonds and two nitrogens. And so it also looks like a sideways horse. We know that it's going to look like a sideways horse. So we can draw in our back legs, our front legs, we know that there's going to be a neck here, and then we know that the neck is going to be positively charged. So it's going to have an H and this is going to be positively charged. And then if we wanted to, we could even draw in a little back tail for our horse if we wanted to. And then, the last part is to realize where does that second nitrogen go? Now that we have two double bonds and one nitrogen, where does the second nitrogen go? And you have to remember that the horse here is not stepping on the second nitrogen. And so that means that the only place that the second nitrogen can go is right here. So all we need to do is add in its hydrogen and that's it. That's the structure of histidine. Lastly, what we have is arginine, which again has features of both the sword and the horse. And so what we're going to do because it has features of the sword is we're going to count how many pointy ends you could try to poke somebody with an R. And so, really, you could try with these three points here. And so what that means is that there's going to be a three-carbon start to our chain. So it will be CH2, CH2, and CH2. Perfect. So there's our three-carbon start, and also just like there are three pointy ends on our R, there's going to be a triangular nitrogen structure at the bottom that's somewhat going to resemble a person riding a horse. And so, the way that that works is we get our nitrogen, our carbon, and our nitrogen, and then our double bond N. And so, again, here is our Oops. I'm sorry. Not a double bond O, a double bond N. And so, you can see that we've got our triangular nitrogen structure in here. And so now, all we need to do is fill in the hydrogens. We know that this one's only going to have one, this one will have two, and the one that's up top is going to be the one that is positively charged. So this is the structure of arginine. And so, hopefully, utilizing those memory tools help you guys remember the structures and I'll see you guys in our next practice problem.

Fill-in the missing R-groups for the following peptide from memory: H-E-K. Circle the acidic amino acids.

So this practice problem wants us to fill in the missing R groups for the following peptide from memory: HEK, so histidine, glutamic acid, and lysine. And then it says to circle all of the acidic amino acids. Alright. So first what we're going to do is recall our mnemonic for remembering the charged amino acids. And of course, that's going to be dragons eat knights riding horses. And so the first amino acid that we have listed here is H, and remember that H stands for the horse. And of course, histidine is just going to be alanine, our leader, with a 5 membered ring. And so let's go ahead and draw that in. We can draw an alanine here, so CH, and then we know the hydrogens are going to change, so CH2, with a 5 membered ring. So 1, 2, 3, 4, 5. And then what we need to remember is that just like the letter H has two parallel lines, there's going to be two hydrogen, two double bonds, and two nitrogen atoms. And so it's also going to look like a sideways horse. And so we can draw in our back legs, our front legs, we can put in our nitrogen atom here, and then we can draw in a little tail if we'd like for the back. We know that the nitrogen here, the neck of the horse here is going to be positively charged, so we can draw in a hydrogen here. And then we can make this a positive charge. And then the last thing to remember, now that we have two double bonds and one nitrogen, we know there's going to need to be a second nitrogen. And the thing to remember is that the horse is not stepping on the second nitrogen, and so that means that the only position that it can go in is here, because if it were in either this position or this position, then the horse would be stepping on it, so it can't go into those positions. And so, all we need to do is fill in our last hydrogen and that's the structure of histidine. Now, for the next one, what we have is E, and E here is for glutamic acid or just for glutamate. And, of course, glutamate or glutamic acid is just going to be building off of aspartic acid, and aspartic acid is just alanine with a carboxyl group. And so because glutamate, has the GLUT in it, it's just going to have an extra CH2. And here, we're kind of drawing it the opposite direction. So here's our carbon. What we're going to have to do is do a CH2 here. We're going to need an extra CH2, so that's our second CH2 for the glute part. And then we can draw in our carboxylate group, so C double bond O, O-. And so, again, what we have is our carboxylate group here, extending off of our, 2 CH2s. And so that's it. That's the structure of glutamic acid. And then last, what we have is K, and K is for lysine. And just like lysine stands for the knight, its R group actually looks like a knight's sword. And we want to count how many pointy ends do you have that you can try to poke somebody with in the letter K. And so there are 4 pointy ends, 1, 2, 3, and 4, that you could really try to poke somebody with. So that helps us remember that there's a 4 carbon, start to the chain. So we know that there's going to be a CH2, there's going to be another CH2, there be another CH2, and one more, CH2. Perfect. So there's our 4 carbons start to our chain. And then, of course, at the very end of our sword, there's going to be an amino group. And so what we can do is draw in our amino group, NH3+, and we know that it's going to be positively charged. And so there it is, that's the structure of lysine. And so, finally, it asks us to circle the acidic amino acids. And recall here that with our mnemonic, that it actually groups our amino acids into acidic amino acids and basic amino acids. And remember that the dragons eat or the d and the e, they are negatively charged, and dragons, they spit fire and acid out of their mouth. So the d and the e here are the acidic ones. So these are the acidic ones, whereas these over here are the basic ones. And so, all we need to do here is circle E, so we circle E here as being acidic. We can also circle it here. Then we've circled the acidic amino acids. And that concludes this practice problem. I'll see you guys in the next one.

Which of the following amino acids does not have a basic R-group?

• A. H
• B. S
• C. R
• D. K

Circle all the following amino acids with a basic R-group?

• A. Asp
• B. Lys
• C. A
• D. E
• E. Asn

Here’s what students ask on this topic:

What are the five charged amino acids and how are they categorized?

The five charged amino acids are categorized into two groups: negatively charged acidic amino acids and positively charged basic amino acids. The negatively charged acidic amino acids are aspartic acid (Asp, D) and glutamic acid (Glu, E). These amino acids donate hydrogen ions (H⁺) at physiological pH (around 7), resulting in a negative charge. The positively charged basic amino acids are lysine (Lys, K), arginine (Arg, R), and histidine (His, H). These amino acids accept hydrogen ions at physiological pH, leading to a positive charge. Understanding these categories is crucial for studying protein structure and function in biochemistry.

How does pH affect the charge of amino acids?

The charge of amino acids is influenced by the pH of the surrounding environment. At physiological pH (around 7), acidic amino acids like aspartic acid and glutamic acid donate hydrogen ions (H⁺), resulting in a negative charge. Conversely, basic amino acids like lysine, arginine, and histidine accept hydrogen ions, leading to a positive charge. The pH determines whether the amino acids will donate or accept hydrogen ions, thus affecting their overall charge. This behavior is essential for understanding acid-base interactions in proteins and other biochemical processes.

What is the mnemonic for remembering the charged amino acids?

The mnemonic for remembering the five charged amino acids is "Dragons Eat Knights Riding Horses." This helps categorize them into their respective groups: aspartic acid (D) and glutamic acid (E) are the negatively charged acidic amino acids, while lysine (K), arginine (R), and histidine (H) are the positively charged basic amino acids. This mnemonic aids in memorizing both the amino acids and their one-letter codes, making it easier to recall their properties and functions in biochemistry.

Why are lysine, arginine, and histidine considered basic amino acids?

Lysine, arginine, and histidine are considered basic amino acids because their R groups contain ionizable nitrogen atoms that can accept hydrogen ions (H⁺) from the environment. At physiological pH (around 7), these amino acids accept hydrogen ions, resulting in a positive charge. This behavior classifies them as basic amino acids. The presence of these ionizable nitrogen atoms is what makes their R groups capable of accepting hydrogen ions, thus rendering them basic and positively charged under physiological conditions.

What is the difference between aspartic acid and aspartate?

Aspartic acid and aspartate are related but differ in their protonation state. Aspartic acid (Asp, D) is the protonated form, containing a carboxyl group (-COOH) that can donate a hydrogen ion (H⁺). When aspartic acid donates this hydrogen ion, it becomes aspartate, which has a carboxylate group (-COO^-) and carries a negative charge. At physiological pH (around 7), aspartate is the predominant form due to the loss of the hydrogen ion, making it a negatively charged acidic amino acid.