Amino Acid Groups - Online Tutor, Practice Problems & Exam Prep

In biochemistry, proteins are constructed from 20 standard amino acids, which differ by their R groups. These amino acids can be categorized based on their chemical properties into functional groupings (non-polar, polar, charged) and structural groupings (aromatic). Understanding these classifications aids in grasping protein structure and function. Each amino acid has a unique three-letter and one-letter abbreviation, essential for protein analysis. The backbone of amino acids exists as a dipolar ion at physiological pH, influencing their behavior in biochemical reactions.

Amino Acid Groups

Hey, guys. Congratulations. You finally made it through all unit 1 topics, which was mostly a refresher and review from your previous courses. But now we're transitioning into a new unit on proteins. This is a heavy unit in biochemistry where we're going to be learning a lot of things that you may not have learned in your previous courses. The plan is, I'm first going to tell you the most important things you need to know about amino acids, then we'll transition into protein structure and protein techniques, and then we'll finally finish off talking about enzymes and regulation.

So, to get this unit started, let's first talk about amino acid groups. As you guys already know, there are 20 different amino acids that are called standard amino acids collectively, but they're also referred to as alpha amino acids from time to time. These 20 standard amino acids differ by their R groups. We know that the amino acids are the monomers or the Lego pieces that are used to build proteins. These amino acids can be grouped together based on the chemical characteristics that their R groups display, at physiological pH specifically. Recall that physiological pH is a pH of about 7. What you may not have known is that the common backbone of every amino acid is what's known as a dipolar ion at physiological pH. All that means is that there are 2 different charge species or 2 different charge groups on the backbone of every amino acid.

Let's take a look at our example which shows the typical structure of an amino acid. Recall that there's a central carbon atom, which is also known as the alpha carbon, branching off is a central hydrogen atom, and then on either side we have 2 functional groups. We have an amino group, which notice is one of the charged species. Notice it has a positive charge at physiological pH. And then we have a carboxyl group on the other side of the alpha carbon, which is the second charged species making it a dipolar ion at physiological pH. But we'll talk more about these ions and charges in another topic later on in our course.

Now again, what makes each amino acid different from one another is the R group that it has. These amino acids can be grouped together based on the chemical characteristics of their R groups. So we'll talk more about those groupings in our next video. So I'll see you guys then.

Amino Acid Groups

So in our last lesson video, we talked about how amino acids can be grouped together based on the chemical properties and characteristics of their R groups. It turns out that amino acid R groups can actually be grouped in many different ways and some textbooks group the amino acids slightly differently and your professor might even group the amino acids in a slightly different way than the way that we group them here at Clutch. But that's totally okay, because there are actually multiple correct ways to group the amino acids and there is not a single universal correct way to group the amino acids. That is because the groupings of the amino acids are actually relative to the way that you're trying to group the amino acids, and we will be able to talk about that down below. Now, you might be wondering, what do I do if my professor uses a different grouping system? Maybe hearing that there are multiple grouping systems gave you a little bit of anxiety.

First of all, I want you to know that here at Clutch Prep, we've chosen the most popular grouping system and so it's very likely that your professor uses the same exact one as us. Now, even if your professor does use a different system, it's likely that it will only vary just by a little bit and so using Clutch's system is still going to be very helpful and very effective for you. I promise I'm going to get you to the point where you know amino acids inside and out and then you can slightly tweak what you've learned based on what your professor wants. So, if you're a little bit anxious, hang on tight and trust me on this one. That being said, I want you to know that there are really 2 major ways to group the amino acids.

The first major way is a functional grouping. The functional grouping has to do with the biological function of the R groups or the specific behavior that the R groups display under certain settings. For example, we have basic and acidic, the behavior of the R group to either accept the hydrogen or donate a hydrogen atom, or polar and nonpolar, the behavior of the covalent bonds between specific atoms. The second major way of grouping the amino acids is a structural grouping. Of course, the structural grouping has less to do with the behavior and more to do with just the structures or the presence and arrangement of atoms in the R groups. Examples are like aromatic or aliphatic R groups. Because we have these 2 major different ways of grouping the amino acids, acids, it turns out that some amino acids can be categorized into multiple groups.

The good thing is that here at Clutch Prep with the system that we've chosen, each amino acid only gets categorized into 1 group, so that's very helpful and good to know. What you'll see is that down below in this chart in our example, we have all 20 amino acids listed out and all of their R groups are highlighted. I don't want you to focus on these R group structures just yet. We'll eventually get to memorizing those, but in another video later in our course. For now, what I want you to focus on are the major groupings that we have on the side here. Notice that we have 4 major groupings. We have nonpolar, aromatic, polar, and charged. These are the groupings that are going to help us, not only group the amino acids, but also help us memorize the individual structures for each amino acid R group, and we'll be able to see how that works, again, later in our course when we get there.

But first, what I want to do is to group, associate these groupings that we have with the major groupings that we have above. So, we already said that nonpolar and polar are types of functional groupings. To color code things, we’ll circle these as functional groupings, so nonpolar and polar are both functional. Now it turns out that charge is also a type of functional grouping, and that's because it has not a lot to do with the structure of the R group, but more so to do with its behavior under certain conditions. Now, what that means is that our only group that we have here that is a structural grouping is the aromatic grouping, and so this is the one that is our structural grouping for the ones that we're going to be focused on, and what you'll see is that the amino acids that fall in here all have aromatic rings.

Again, we’re going to talk about each of these individual R groups later on in our course, but before we actually talk about those R groups, what I want you to notice is that each of these amino acids has an abbreviation at the bottom. It has a 3-letter abbreviation and a one-letter abbreviation. And so you'll see that at the bottom of all of our amino acids. First, we’re going to talk about these amino acid abbreviations before we get to the structures. So that concludes our lesson on amino acid groupings, and I'll see you guys in our practice videos.

Which of the following is not an example of a structural grouping of amino acids?

• A. Hydrophobic
• B. Sulfur-containing
• C. Aromatic
• D. Aliphatic

Here’s what students ask on this topic:

What are the different ways to group amino acids based on their R groups?

Amino acids can be grouped based on the chemical properties and characteristics of their R groups. There are two major ways to group them: functional grouping and structural grouping. Functional grouping is based on the biological function or behavior of the R groups, such as non-polar, polar, and charged. Structural grouping, on the other hand, focuses on the presence and arrangement of atoms in the R groups, such as aromatic and aliphatic. These groupings help in understanding the behavior and structure of proteins.

Why is the physiological pH important for amino acids?

Physiological pH, approximately 7, is crucial for amino acids because it influences their ionization state. At this pH, the amino group (NH₃⁺) and the carboxyl group (COO^-) on the amino acid backbone exist as a dipolar ion, or zwitterion. This dipolar nature affects the amino acid's solubility, reactivity, and interaction with other molecules, which are essential for protein structure and function.

What are the abbreviations used for amino acids?

Each amino acid has a unique three-letter and one-letter abbreviation. For example, Glycine is abbreviated as Gly (three-letter) and G (one-letter). These abbreviations are essential for protein analysis and communication in biochemistry, as they provide a standardized way to represent amino acids in sequences and structures.

How do the R groups of amino acids affect protein structure?

The R groups of amino acids determine the chemical properties and interactions of the amino acids, which in turn affect protein structure. Non-polar R groups tend to be hydrophobic and cluster inside the protein, while polar and charged R groups are hydrophilic and often found on the protein surface. Aromatic R groups can participate in stacking interactions. These interactions contribute to the folding and stability of the protein's three-dimensional structure.

What is the significance of amino acids being dipolar ions at physiological pH?

At physiological pH, amino acids exist as dipolar ions (zwitterions), with both a positively charged amino group (NH₃⁺) and a negatively charged carboxyl group (COO^-). This dipolar nature is significant because it affects the solubility, reactivity, and overall behavior of amino acids in biological systems. It also plays a crucial role in the formation of peptide bonds and the overall structure and function of proteins.