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 - Online Tutor, Practice Problems & Exam Prep
Charged Amino Acids
Video transcript
Charged Amino Acids
Video transcript
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
Video transcript
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
Video transcript
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.
Problem Transcript
Fill-in the missing R-groups for the following peptide from memory: H-E-K. Circle the acidic amino acids.
Problem Transcript
Which of the following amino acids does not have a basic R-group?
Circle all the following amino acids with a basic R-group?